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Diversity, classification and origin of fossil-Moho from the Oman ophiolite
Shoji ARAI (Depart. Earth Sci., Kanazawa Univ., Japan)
Well-exposed, well-preserved ophiolites, such as the Oman ophiolite, will give us petrological
nature of Moho (Mohorovičić discontinuity), which is otherwise inaccessible until the
21st-Century Mohole in the near future. To conduct the Mohole successfully, we should have
possible figures of Moho to prepare for the Mohole. We have investigated the northern part of
the Oman ophiolite to understand the petrological nature of Moho, i.e. petrogtaphical and
petrogological characteristics of rocks involved in the Moho formation.
Hess Model to the Oman ophiolite
In the Hess Model oceanic lithosphere, the lower crust is required to mainly comprise
antigorite (high-T serpentine species) serpentinite. The peridotite section of the Oman
ophiolite has been extensively serpentinized to form chrysotile/lizardite serpentines.
Antigorite is very limited in occurrence, only around diopsidites, a product of strong
hydrothermalism, within peridotite. Thus, the Moho as the serpentinization front in the
peridotite section is not recognized in the Oman ophiolite. Moho is represented by
gabbro/peridotite transitions in the Oman ophiolite.
Gabbro/peridotite boundaries; diversity and classification
Layered gabbros grade downward to peridotites in various ways in the Oman ophiolite.
1. Gabbro-in-dunite boundaries. Layered gabbros change to peridotites (mainly dunite) via
gabbro layers with wehrlitic screen and dunite/wehrlite with gabbroic bands. In this transition
zone (= Moho transition zone; MTZ in short), gabbros are apparently intrusive to wehrlites
and dunites. Some of the gabbroic bands form flatted networks. This transition typically takes
place within a few tens of meters. Wehrlites are common near the gabbro bands. The dunites
further change downward to harzburgite.
2. Dunite-in-gabbro boundaries. So-called late-intrusive wehrlites or dunites are sometimes in
contact with (= intrusive to) gabbros, forming a complicated MTZ. Both gabbros and the
late-intrusive gabbros are sometimes rich in clinopyroxene around or along the contact. Due
to the intrusive character of the dunites/wehrlites, the gabbro/peridotite boundaries are
complicated and neither planar nor parallel with the ophiolite stratification.
3. Involvement of “within-crust wehrlites”. Masses of wehrlites or dunites are found within
the layered gabbros above the peridotite section. The wehrlites or dunites are apparently
layered with gabbroic rocks, forming a prominent layered structure. The relationship between
the wehrlites/dunites and gabbros is, however, complicated. Some of gabbro layers seem to be
intrusive to the peridotites, but others are seemingly intruded by the latter. The contact
between the overlying gabbros and the wehrlites/dunites is similar in appearance to the
boundary of category (1) above.
Implications for the sharpness of Moho
The variety of gabbor/peridotite transitions will give us implications for the sharpness of
Moho seen by seismicity. The gabbro-in-dunite boundaries (category 1) will be representative
of the sharpest Moho. The Moho is obscured by involvement of the within-crust
wehrlites/dunites. The dunite-in-gabbro boundaries possibly provide us with various Mohos
from quite sharp to diffuse ones depending on the way of intrusion of the wehrlites/dunites
into gabbros.
Origin of Moho (gabbro/peridotite boundaries)
The dunites or wehrlites from all the transition zones described above share clinopyroxenes
with the same trace-element characteristics, which can be in equilibrium with a MORB-like
magma. Some of them (especially late-intrusive wehrlites) contain primary pargasites,
indicating hydrous nature of the involved magma. This indicates that all the gabbro/peridotite
transitions from the Oman ophiolite were formed at one tectonic setting, back-arc basin,
where hydrous MORB –like melts are available.
Some of the dunites/wehrlites are a product between the mantle peridotite (harzburgite)
and melt (precipitation of olivine coupled with pyroxene dissolution) based on field
observations. The transitions possibly started to form along a melt conduit system beneath a
spreading center. The diversity of the Moho is, therefore, controlled by the complexity of the
melt conduit system. The MTZ dunites (and wehrlites) possibly represent the most outside
conduit, and the within-crust wehrlites (and dunites), trapped wall peridotites within the main
conduit. With an increase of involvement of the melt, the reaction product (olivine + melt) can
be mobile to intrude, as the late-intrusive wehrlites (and dunites) into gabbros already
consolidating upsection.
Proposal for the Stage-1 Mohole
I would like to propose an ultra-deep drilling on one of oceanic core complexes with shallow
water depths, such as Atlantis Bank, SWIR, to seek the Hess Model. This also contributes to
recovering fresh abyssal peridotites for the first time.
Potential drilling target of solid earth sciences and IODP-SP (2013-2023)
Is enriched MORB a fundamental component of a ridge subduction zone? (or Testing stability of the
Mantle circulation system) -
age transect of crust formed at the subducting Chile ridge
By Ryo Anma (University of Tsukuba), Hikaru Iwamori, Yuji Orihashi, Shiki Machida (U. Tokyo),
Natsue Abe (JAMSTEC), Naoto Hirano (U. Tohoku)
Aim of the proposed study:
The proposed study aim to reveal stability or rate of the mantle circulation system using composition
of MORB, and how any composition of MORBs near ridge subduction zones is influenced by the deep
mantle source or sub-ridge processes near the ridge subduction zone.
Testable hypothesis:
MORB near the Chile ridge subduction zone has variously enriched composition (Klein and Karsten,
1995). A series of drill hole of the crust formed at the Segment 1 would reveal whether enrichment of the
mantle is an essential phenomenon in ridge subduction zone or it is characteristic of the corresponding
ridge segment, following change in composition of the basement rocks through time.
If distribution of enriched MORB (what-so-ever) was controlled by the patterns of global mantle
compositions (Iwamori, personal com.) then we can test the stability of global mantle circulation system
(or how fast the front of the enriched MORB migrated through time) using the same strategy and using
the samples of the basement rocks from the Segment 3 and/or 4 profile. Since the Chile ridge system is
approaching to the South American continent through time, the change in composition is expected even if
the enriched MORB front was not moving at all. Because of the same reason, and enriched MORB front
likely to move outward, the migration rate (if any) would be determined with better resolution at the Chile
ridge system than other possible ridges within a fixed time interval. If there was no compositional change
through time, then enriched MORB is characteristic of the corresponding ridge segment.
Holes along the Tres Montes fracture zone focus on the comparison with the CMU of the Taitao
ophiolite to understand processes of alteration or contamination along a transform fault zone. This is to
test whether a transform fault has significant influence on metasomatism of the sub-arc mantle.
Drilling strategy:
Drill a series of shallow holes to 200 m-deep of the basement penetration through sediments, along a
profile of crust that were formed in the Segment 1 (subducting segment), Segment 4 and Segment 5 of the
Chile ridge at 3, 5.6, 9, 12 and 15 Ma. Also drill a shallow hole through 3 Ma and 5.7 to 5.2 Ma crusts
along the Tres Montes fracture zone. Data of 5.7 Ma holes are directly comparable with the data from the
5.7 Ma Taitao ophiolite.
Requirement:
There is no data available from the Segment 5. It is most important to fill this gap to put this potential
proposal forth. Also seismic profiles that across crusts formed at the Segments 1, 4 and 5 are required.
Figure 1: Tectono-magmatic map near the Chile ridge subduction zone. MORBs from the Segments 1 ~ 3 have variously enriched
or contaminated compositions (Klein and Karsten, 1995) and those from the Segment 4 have compositions influenced by fluid
(Iwamori, unpublished data). Segment 6 has N-MORB composition. There is no recovery from the Segment 5. Whether front of the
enriched mantle extends to NS direction (green line) or EW direction (yellow line) is not certain. Compositional variation of the
rocks from the Andean volcanoes and/or the Patagonian volcanic field may provide a clue to understand this (study by Orihashi).
The igneous piercement/ pipe investigation on the Ontong Java Plateau
Akira Ishikawa, Kenji Shimizu, Katsuhiko Suzuki (IFREE/JAMSTEC)
The nature and origin of the Large igneous Provinces (LIPs) have been intensely debated
because of widespread interest to link major dynamics in the Earth’s deep mantle with a global
environmental impact on the Earth’s surface (e.g. Coffin and Eldholm, 1994). The Cretaceous Ontong
Java Plateau (OJP) in the western Pacific, which is the largest of the world’s LIPs with an area of ~2 x
106 km2 and a maximum crustal thickness >30 km, is one of the most attractive targets because of its
huge magma flux and the temporal coincidence with Cretaceous magnetic superchron and greenhouse
climate change (e.g. Larson 1991). With the aim of constraining the cause of the OJP magmatism,
particularly for testing a widely accepted the plume-head hypothesis (e.g. Richards et al 1989), several
DSDP and ODP cruises have sampled the igneous basement in the widely distributed sites (Figure 1).
Geochronological, petrological and geochemical data of the recovered basalts, together with the data
from obducted southwestern margin of the OJP crust in the eastern Solomon Islands (Santa Isabel,
Malaita and San Cristobal) revealed that they have an extremely limited variations in age (122±3 and
90±4 Ma), compositions (low-K tholeiites with flat primitive-mantle-normalized incompatible element
patterns and OIB-like isotopic signature) and inferred eruptive environments (emplacement beneath
the calcite compensation depth). This information does not allow us to approach a singular working
model. Conversely, a considerable debate over the plume-head hypothesis has been grown, especially
with questioning of the existence of mantle plumes (Fitton and Godard 2004; Tejada et al., 2004, and
references therein).
One great difficulty with research to date can be attributed to the insufficient knowledge of the
deep crust-mantle structure beneath the OJP, where the drilling hole cannot access. However, we
recently demonstrated that xenoliths entrained by late-stage magmatism, such as 34-Ma alnöite
intruding crust of the southern OJP on Malaita, offer the opportunity to reconstruct the lithospheric
stratigraphy (vertical distribution of rock types) beneath the OJP, analogous to long-established
investigations of continental lithosphere by the studies of kimberlite-borne xenoliths (Ishikawa et al.,
2004 and references therein). Our on-going research on geochemistry and geochronology of the
Malaitan xenoliths documents possible mechanisms and timing for creating the observed lithospheric
structure in the context of OJP formation (Ishikawa et al., 2005: 2007). Thus, further investigation
with the aim to discover other xenoliths localities over the OJP would be desirable for obtaining a
more complete picture of the sub-plateau lithosphere in terms of the lateral variation.
Seismic reflection profiles of the OJP obtained during Research Vessel traverses reveal a number
of what have been termed ‘Igneous Piercements’ (Kroenke, 1972). The size and structure of the
‘piercements’ show them to be volcanic vents (pipes) up to 3 km in diameter and with internal
post-volcanic collapse features, although images on the microfilms vary from sharp to diffuse
depending presumably on the proximity of the seismic traverse to the ‘piercements’ (Figure 2). Ocean
floor sedimentation has taken place within the collapsed craters and is preserved by post-volcanic
settlement. These vent features are typical of central volcanoclastic eruptions rather than highly
voluminous fissure type effusions. They could be alnõites from their abundance, size and
volcaniclastic nature. Undoubtedly if some were located off shore to Malaita they are likely to be
alnõites. But the nature of those in the central parts of OJP several hundred km away would be
dependent upon the thickness of the lithosphere – likely to be thicker than the 120 km determined
petrologically for the peripheral thinner lithosphere beneath Malaita.
The potential existence of a thick lithospheric keel has been inferred by previous seismological
studies. A three-dimensional tomographic model of the S-velocity structure beneath the OJP indicated
the presence of a low-velocity root reaching a depth of 300 km (Figure 3), and the seismic
characteristics of this root suggest that they represent a chemical anomaly because the observed
deficiency of shear velocity is too large to attribute to a thermal perturbation (e.g. Richardson et al.,
2000). Although size and shape of this root is not well constrained due to poor resolution, the mantle
root can be interpreted as a remnant of the OJP residuum, which has traveled with the OJP since its
formation. If this hypothesis is valid, the igneous piercements (volcanic vent) penetrated where the
lithosphere is inferred to be of maximum thickness i.e. a structurally central ‘cratonic’ location
(continental terminology) would be kimberlitic, that might have brought abundant xenolithic material
from the depth interval of ~300 km to the surface. Therefore we propose, complement to further
seismic work, a future drilling into the igneous piercements in order to obtain the xenolithic samples
from the thickest part of the OJP. The studies of such material may provide new insights to the nature
and origin of the slow velocity root and the OJP itself
Coffin, M. F. and O. Eldholm (1994) Reviews of Geophysics 32: 1-36.
Fitton, J. G. and M. Godard (2004) Origin and evolution of the Ontong Java Plateau 229: 151-178.
Ishikawa, A. et al. (2004) Journal of Petrology 45: 2011-2044.
Ishikawa, A. et al. (2005) Geology 33: 393-396.
Ishikawa, A. et al. (2007) Earth and Planetary Sciences 259: 134-148.
Kroenke L. (1972) Rep. HIG-72-5 (Hawaii Institute of Geophysics).
Larson, R. L. (1991) Geology 19: 547-550.
Nixon P. H. (1980) Nature 287: 718-720.
Richards, M. A. (1989) Science 246: 103-107.
Richardson, W. P. et al. (2000) Physics of the Earth and Planetary Interiors 118: 29-51.
Tejada, M. L. G. et al. (2004) Origin and evolution of the Ontong Java Plateau 229: 133-150.
Figure 1. Predicted bathymetry of the Ontong Java Plateau showing the location of sites drilled on
previous DSDP and ODP cruises. Solid stars = sites penetrating lava sections..Open star = Site 1184,
where a volcaniclastic sequence was recovered. Solid circles = ODP and DSDP drill sites before Leg
192) that reached basement. Open circles = Site 288, which did not reach basement but bottomed in
Aptian limestone, and Site OJ-7, which was proposed for Leg 192 but not drilled (see text). The
bathymetric contour interval is 1000 m.
Figure 2. Seismic reflection profiles of plug-like features on Ontong Java Plateau (from microfilm
supplied to P.H. Nixon by Hawaiian Institute of Geophysics). Vertical scale given by bar (= 250m) on
bottom-right side of each picture; vertical exaggeration about x 12.
Figure 3. Results of Reayleigh-wave tomographic images of the OJP root (Richardson et al. 2002).
White paper for the domestic INVEST meeting: a proposal for drilling tectonic
windows
Dec. 12, 2008
Yasuhiko Ohara
Hydrographic and Oceanographic Department of Japan
Also at IFREE-JAMSTEC
Formation and evolution of the oceanic lithosphere is an important dominant process in
the chemical and physical evolution of the Earth. Since the end of the 1960’s, scientific
ocean drillings (DSDP, ODP and IODP) have drilled and cored series of holes in
oceanic lithosphere. These led to major improvements in our understanding of the
oceanic lithosphere architecture and of mid-ocean ridge processes.
Recent studies including ODP/IODP expeditions at tectonic windows (e.g. IODP
X304/305) and numerous mapping and dredging cruises, however, have provided the
new perspectives of architecture of oceanic lithosphere, of which we had not imagined
just several years ago. One important example is the fact that oceanic core complexes
drilled so far all yield long continuous section of gabbros, not peridotites (Ildefonse et
al., 2007). This example is in contrast to our popular view that oceanic core complex is
composed mainly of peridotite because of the magma-starved formation mechanism at
an end of a spreading axis. The fact that oceanic core complex is composed mainly of
gabbros has led us to a new numerical modeling of formation of oceanic detachment
faults (Tucholke et al., 2008). The new modeling indicates that oceanic detachments
(hence oceanic core complexes) most develop when magmatic flux at ridge axis is
moderate (~40-50 %) (Tucholke et al., 2008), being totally opposite to the previous
notion that oceanic core complex most develops at a magma-starved environment
(Tucholke et al., 1998).
This example clearly shows that the architecture and evolution of oceanic lithosphere
still remain largely unknown and still a lot of works (on geophysics, geology,
petrology, microbiology etc.) are necessary. The goals of deep drilling in oceanic
lithosphere are therefore to efficiently characterize the spatial and temporal variability
in crustal and upper mantle architecture, and to identify and constrain the key forcing
functions that control this variability.
One clear way to achieve these goals is to have a Mohole to characterize an in situ
oceanic lithosphere at fast-spreading environment as a reference site. However, drilling
at tectonic windows will also give us important data and information to achieve these
goals. In particular, the key objectives for the tectonic-window-targeted drilling that
was not noted in the current IODP Science Plan may include:
• Investigating the lithosphere formed at intermediate-spreading environment.
Scientific ocean drilling thus far only have drilled slow- (Atlantic Massif, Kane,
15-45N, Atlantis Bank) and fast-spreading (Hess Deep) tectonic windows. We do
not know the architecture of intermediate-spreading ridges and the result will be a
necessary bridge to compare/understand the difference between slow- and
fast-spreading lithosphere.
• Investigating the lithosphere formed at backarc environment. Although backarc
basin is an important fraction of the total oceanic lithosphere, however, only limited
number of scientific ocean drilling has been performed (e.g., Lau Basin). This
objective will help us understand the relationships between crustal architecture and
tectonic setting.
In summary, the architecture and evolution of oceanic lithosphere still remain largely
unknown and IODP can provide important data/information to us.
References
Ildefonse, J., Blackman, D. K., John, B. E., Ohara, Y., Miller, D. J., MacLeod, C. J.,
and Integrated Ocean Drilling Program Expeditions 304/305 Science Party,
2007, Oceanic core complexes and crustal accretion at slow-spreading ridges.
Geology, v. 35, no. 7, p. 623–626, doi: 10.1130/G23531A.
Tucholke, B. E., Lin, J., and Kleinrock, M. C., 1998, Megamullions and mullion
structure defining oceanic metamorphic core complexes on the Mid-Atlantic
Ridge. Journal of Geophysical Research, v. 103, p. 9857-9866.
Tucholke, B. E., Behn, M. D., Buck, W. R., and Lin, J., 2008, Role of melt supply in
oceanic detachment faulting and formation of megamullions: Geology, v. 36,
no. 6, p. 455-458, doi: 10.1130/G2463A.
Paleomagnetic study in high latitude oceans for understanding of geomagnetic behavior
within the tangent cylinder.
Toshiya Kanamatsu (IFREE JAMSTEC)
Spaciotemporal paleogeomagnetic records have been detailed to understand the origin
of the Earth’s magnetic field. This information for the last 2-3 million years has been
rapidly accumulated by marine sediment studies in the last two decades. This type
approach is more steady-going and necessary for further understanding. However, those
global data acquisitions have been concentrated in the lower latitude areas (~ 40° N or
S). It is mainly due to difficult accessibilities (e.g. sea ice coverage or high sea
condition) to high-latitude areas. But IODP shed light on the high latitude research by
ACEX successful challenging
Available paleomagnetic data from the high latitude (> ca. 70°N) reveal more frequent
geomagnetic excursions (e.g. Nowaczyk and Frederichs, 1999) than general global
observations at the lower latitude areas. Paleomagnetic records in the Lomonosov Ridge
show a unique pattern that no excursion in the upper duration of Brunhes interval, and
the longer and more frequent excursion-like events were reported in the lower duration,
although these excursion-like events are not fully interpreted in ACEX record (O’Regon
et al., 2008). These paleomagnetic properties in the high latitude seem to be
characterized by distinct excursions (in duration and frequency). If it is true, it is
possible that the geomagnetism in the higher latitude is strongly affected by the
undocumented factor such as fluid dynamics within the inner core tangent cylinder,
although the major geomagnetic field of the Earth is produced by the fluid flow in the
outer core. Because unfortunately we don’t have enough data in the high latitude
(within the tangent cylinder) to verify this idea so far, data acquisition through IODP
drilling around the area is proposed.
Detail age control on paleomagnetic records is crucial for understanding of
spaciotemporal variations. Because ACEX core was difficult to obtain precise age data
due to lack of carbonate components and microfossils, we propose a traverse coring
from the lower site (ca. 60°N) to inside the tangent cylinder sites. The lower latitude site
is designed to obtain more reliable age model. Site by site correlation among traversed
sites will bring age information to the higher latitude paleomagnetic record from the
lower. Additionally transecting records will provide the opportunity to document the
magnetic field variation around outside and inside the tangent cylinder. It is considered
that these approaches can be applied to both Arctic and Antarctic areas around the
tangent cylinder (e.g Chukchi Sea, and Ross Sea).
A Proposal for INVEST Earth Interior WS
Early diagenetic effects on magnetic properties of marine sediments
Noriko Kawamura (Geological Survey of Japan, AIST)
Rock magnetic properties of marine sediments change with variations in the
abundance, type, and grain size of magnetic minerals. Magnetic iron oxides, such as
magnetite (Fe3O4), maghemite (γFe2O3), hematite (αFe2O3), are common magnetic
minerals in marine sediments. These magnetic minerals are supplied as grains of detrital or
biogenic origin. Consequently, magnetic properties have been used as proxies of
detritus-supply changes for recording paleoenvironmental and paleoclimate changes (e.g.,
Evans and Heller, 2003). In general, organic matter in sediments undergoes decomposition
by bacterial activity during burial, which changes the oxidation-reduction conditions in the
sediments. Bacterial decomposition of organic matter is an example of early diagenesis.
During the early diagenetic regime, iron oxides are dissolved, causing changes in the grain
size distribution of magnetic minerals. Ferrimagnetic iron sulfides are also crystallized
under anoxic conditions as early diagenesis proceeds. The diagenetic changes and
authigenic formation of magnetic minerals alter the magnetic properties of the sediments.
As a result, the original magnetic information for paleoenvironmental and paleoclimate
conditions are masked by the early diagenetic effects. It is thus essential to clarify the
original magnetic mineral assemblage from the alteration products in the marine sediments.
Early diagenetic effects on magnetic properties are controlled by various
chemical factors during the sedimentation and post-depositional processes, such as, for
example, the supply rate of organic matter, temperature, and diffusion of oxygen from oxic
bottom water into the sediments (e.g., Froelich et al., 1979; Berner, 1980; Carman and
Rahm, 1997; Rey et al., 2005). Such factors are also expected to affect magnetic mineral
composition in marine sediments. Kawamura et al. (2007; 2008) suggested that the
dissolution of magnetic minerals in hemipelagic sediments from the southern Okhotsk Sea
and north Pacific was significantly accelerated due to the high supply of organic matter and
low concentration of dissolved oxygen in interstitial water. Thus I would suggest that
concentration of oxygen and iron ions in interstitial water must be measured on board in the
routine work. In order to avoid early diagenetic effects on magnetic properties, oxic zone in
sediments must be selected for paleomagnetic study. Drillings in oxic area (for example,
north Atlantic, Atrantic Sea, and southern Ocean) are expected.
Reference
Berner, R. A., Early diagenesis: A theoretical approach, 241 pp., Princeton University Press,
Princeton, 1980.
Carman, R. and L. Rahm, Early diagenesis and chemical characteristics of interstitial water and
sediments in the deep deposition bottoms of the Baltic proper, J. Sea Res., 37, 25–47,
1997.
Evans, M. E. and F. Heller, Environmental Magnetism, Principles and applications of
enviromagnetics, 299 pp., Elsevier Science, USA, 2003.
Froelich, P. N., G. P. Klinkhammer, M. L. Bender, N. A. Luedtke, G. R. Feath, D. Cullen, P.
Daphin, D. Hammond, B. Hartman, and V. Maynard, Early oxidation of organic matter
in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis, Geochim.
Cosmochim.Acta, 34, 1075–1090, 1979.
Kawamura, N., H. Oda, K. Ikehara, T. Yamazaki, K. Shioi, S. Taga, S. Hatakeyama, and M. Torii,
Diagenetic effect on magnetic propeties of marine core sediments from the southern
Okhotsk Sea, Earth Planets Space, 59, 83–93, 2007.
Kawamura, N., K. Kawamura, and N. Ishikawa, Rock magnetic and geochemical analyses of
surface sediment characteristics in deep ocean environments: A case study across the
Ryukyu Trench, Earth Planets Space, 60, 179–189, 2008.
Rey, D., K. J. Mohamed, A. Bernabeu, B. Rubio, and F. Vilas, Early digenesis of magnetic minerals
in marine transitional environments: geochemical signatures of hydrodynamic forcing,
Mar. Geol., 215, 215–236, 2005.
MASS BALANCE of the ARC: Subduction zone input-output toward the whole earth
material recycling
Jun-Ichi Kimura (IFREE/JAMSTEC)
Convergent margin magmas have typical geochemical signatures including elevated
concentrations of large ion lithophile elements, depleted heavy rare earth elements and
high field strength elements, and variously radiogenic Sr, Pb, and Nd isotopes. Origin of
these characteristics have been interpreted as melting of depleted mantle peridotite by
fluxing of fluids or melts derived from subducted oceanic plate consisting of altered
oceanic crust and sediments. High Mg# (Mg/(Mg+Fe) molar ratio) basalts and high Mg#
andesites are inferred majority to make up the bulk of subduction-related primary
magmas and may be generated by fluid- or melt-fluxing of mantle peridotite,
respectively. Recent development in experimental studies and thermodynamic models
better constrain the phase petrology of the slab components during prograde
dehydration metamorphism and slab melting and of the wedge mantle melting and
mantle–slab melt reaction. Experimental results also constrain behaviors of many
elements in these processes. In addition, pressure (P)–temperature (T) structure of
subduction zones has also been investigated via geodynamic models, allowing
increasingly realistic, quantitative P-T models to be developed for subducted slab and
wedge mantle. These developments together enable generation of forward models to
explain arc magma geochemical compositions.
Island arc lavas exhibit across and along arc geochemical variations and some
arcs, e.g., Sunda-Banda, Central America, and Aleutian arcs, show geochemical
correlation between input slab and output magmas particularly in fluid mobile
elements or isotopes. This indicates strong link between subduction materials and
generated magmas beneath the arcs. Notwithstanding the existing very interesting
observations, geochemical database for both subduction inventory and output magma in
the arcs are not sufficient or even far from complete. Pioneering works by Plank and
Langmuir (Plank, T., and C. H. Langmuir (1993), Tracing trace elements from sediment
input to volcanic output at subduction zones, Nature, 362, 739-742; Plank, T., and C. H.
Langmuir (1998), The chemical composition of subducting sediment and its consequence
for the crust and mantle, Chemical Geology, 145, 325-394) on the ocean sediment
inventory has elucidated key chemical and isotopical compositions of average global sea
sediment (GLOSS) and variability of the seafloor sediments from ocean drilling projects.
Some altered mid ocean ridge basalt (AMORB) have also been analyzed for isotopes and
trace elements by high precision analytical methods (Hauff, F., et al. (2003), Sr-Nd-Pb
composition of Mesozoic Pacific oceanic crust (Site 1149 and 801, ODP Leg 185):
Implications for alteration of ocean crust and the input into the Izu-Bonin-Mariana
subduction
system,
Geochemistry
Geophysics
Geosystems,
4,
doi:10.1029/2002GC000421; Kelley, K. A., et al. (2003), Composition of altered oceanic
crust at ODP Sites 801 and 1149, Geochemistry Geophysics Geosystems, 4,
doi:10.1029/2002GC000435.) Arc lava database has also been developed and growing
database of high-precision analytical results on trace element and isotopes allow use at
ease.
Notwithstanding the recent developments, geochemical database on
subduction inventories are still far immature to solve the mass balance problems in the
subduction zone systems. For example, Central America has quite matured database on
along arc variability of the lavas input data are only available from one sediment core
(DSDP495) and volcanic sections on Cocos Ridge(Carr M.J., et al. (2007) Element fluxes
from the volcanic front of Nicaragua and Costa Rica Seismology and Volcanology, G3,
doi:10.1029/2006GC001396). The same situations are true for Aleutians (three sediment
cores) and Izu-Bonin-Mariana arcs (only ten boreholes for > 4000 km arc length) (Stern,
R. J., et al. (2003), An overview of the Izu-Bonin-Mariana subduction factory, in Inside
of Subduction Factory, edited, pp. 175-222, American Geopgysical Union.). Such the
poor coverage in subduction inventories (sediments, AMORB, etc.) prevent precise
discussions on how and why subduction input return back to the arcs via magmatic
processes. Dense ocean drilling on the subducting sediments and AMORB in front of the
magmatic subduction zones can only provide the database of subduction slab
inventories that are able to correlate to magmatic output spatially. And thus the project
provides opportunities examining quantitatively the subduction zone mass balance.
What are the inputs, how these are processed, how much of them return to the surface
or return back into the mantle, and why are the key questions of this subduction zone
mass balance. Along arc variability of inputs and magmatic outputs provide the best
ruler rather than modeling mass balance in a single trench-arc system. Because how
much the difference in input chemistry are reflected to output magma to what extent is
an immediate test of mass balance between neighboring arc transects. It can be
achieved by observing along arc variations in one arc with similar kinematic subduction
geometry.
The key to success this proposal is to obtain borehole core samples of the
oceanic crust Layer 1 (sediment) and 2 (AMORB) dug densely along the arc in front of
an arc trench system that has sufficiently precise geochemical database of the
Quaternary volcanoes. Potential arcs are Izu-Bonin-Mariana arc, Aleutian arc, and
Central America. The IBM has a length over 4000 km and subducted sediments varying
from volcaniclastics from Cretaceous seamounts to the south to the pelagic sediments to
the north both lie on the Pacific Plate oceanic crust with very particular DMM
geochemical characteristics. The Central America, in contrast, can depict the effect of
subduction of an aseismic ridge (Cocos Ridge) with HIMU geochemical characteristics.
Aleutian arc also has subduction of a fracture zone of the Pacific Plate that may
increase water budget beneath the arc. Chemistries of the Quaternary lavas have been
or are going to be published soon and thus these arcs are well suited for the purpose. At
least 10 drill holes in an arc with penetration of >100 m into AMORB basement is
crucial. When these cores are successfully recovered, geochemical community of
subduction zone are ready to analyze the samples and interpret the data with a
cutting-edge mass balance model. The success of mass balance calculation in the
subduction zone systems eventually provide the quantity of material input through the
subduction zones back into the mantle, thus providing a strong constraint to the
material recycling in the mantle.
Potential of Scientific Gas Monitoring at the wellsides
H Kumagai (IFREE, JAMSTEC)
Developments of analytical equipment make us possible to advanced
geochemical monitoring at well sites. The most popular application of the method is the
safety monitoring. Recently, scientific application has been tried (Wiersberg and
Erzinger, 2008 and references therein).
Although real time scientific monitoring is almost performed by only one group
in Germany, its potential is very large judging from their study (Wiersberg and Erznger,
2008). Helium profiles well corresponds to the lithological changes of wall rock of drill
holes (see Figure 3 in Erzinger et al. Geofluids, 2006). Particularly, volatile inflow at the
fractured zones, or faults were well documented. To realize this scientific advancement,
they developed comprehensive gas extraction-analyze system composed of three types of
analytical equipment: mass spectrometer, gas chromatograph and α-detector. They also
developed original gas extraction apparatus equipped for the mud circulation system.
From different aspect, potential of mud-gas monitoring is very large. The
popular volatile extraction for gas mass spectrometry is “crushing” method. In this
method, the sample specimens are stored into the ultra high vacuum apparatus and
mechanically crushed for gas release. This situation is very similar to the riser drilling
system; the rocks at the bottom of the hole are crushed by drill bits and gases contained
in the crushed rocks will be released in the circulated muds. Thus, drill mud is potential
carrier of the volatiles released at the drilling. For example, sudden change of helium
concentration found at the fracture zone of KTB drill hole (Erzinger et al., 2006).
However, following problems should be solved to obtain reliable geochemical
data. 1) Elemental and isotopic fractionation thorough the path of flow line; from gas
release to measurement. 2) Contamination from mud or environment, 3) Accuracy of
sampled position. Thus firstly, basic technical investigation by comparison between core,
mud and extracted volatiles should be considered. Once such fundamental information
is obtained,
References
J. Erzinger, T. Wiersberg and M. Zimmmer (2006) Real-time mud gas logging and sampling during
drilling, Geofluids, 6, 225 – 233.
T. Wiersberg and J. Erzinger (2008) Origin and spatial distribution of gas at seismogenic depths of
the San Andreas Fault from drill-mud gas analysis, Applied Geochemistry, 23, 1670 – 1695.
Ocean bottom seismological observation for exploring crust and mantle structure
beneath the Ontong-Java Plateau
D. Suetsugu and S. Kodaira (IFREE/JAMSTEC)
1. Introduction
The Ontong Java Plateau (OJP) is the largest Large Igneous Province (LIP), which arises an
average of 2000 m above the seafloor over an area of 1.6 x 106 km2 in the western Pacific. Because
of its size and instantaneous emplacement, the formation of the OJP had a large impact on the global
environment. There has been an intensive discussion on origin of the OJP. Some studies propose that
a large head of mantle plume rising from the deep mantle causes rapid episodes of massive basaltic
flooding (e.g., Larson, 1991) and others favor a “shallow origin” model in which the OJP formed
near a fast-spreading ride by passive asthenospheric upwelling (e.g., Korenaga, 2005). All such
models still remain speculative, mainly because crust and mantle structure beneath the OJP is not
well understood. We introduce an ocean bottom observation project with active and passive OBS
experiments to determine the crust and mantle structure beneath the OJP, which is essential to design
a future drilling plan in the OJP.
2. Previous seismological studies
The active crust survey for the OJP has been performed since the early 1970s, although none of
them are based on a long survey line to cover the entire OJP. Gladczenko et al. (1997) compiled and
re-interpret data obtained in 1970s with recent gravity and refraction experiment data involving
ocean bottom seismographs (Miura et al., 1997) and concluded that a thick crust (about 32 km) near
the central part of the OJP with a 6.1 km/s middle crust. Surface wave tomographic studies
(Richardson et al., 2000) enabled two-dimensional distribution of the Moho depth, showing a 30-40
km thick crust in the OJP, while its spatial resolution is much less than that of refraction survey.
Systematic refraction and multi-channel seismic reflection survey (MCS) on long survey lines
traversing the entire OJP is desired.
There have been only a few passive seismological experiments targeting mantle structure beneath
the OJP, mostly because of its oceanic and remote environment. Richardson et al. (2000)
investigated shear wave velocity structure using long-period surface waves recorded on oceanic
islands near the northern rim of the OJP. Their upper mantle model is characterized by 300 km thick
slow velocity anomalies beneath the entire OJP region. The slow velocity anomalies have an
amplitude of about -5 % in average and are interpreted as the OJP keel by Richardson et al. (2000).
If the keel is caused by thermal anomalies, it should accompany a strong seismic attenuation.
However, Gomer and Okal (2003) showed that the seismic attenuation measured by multiple ScS
wave spectra is rather weak, suggesting that the slow velocity anomalies may have a non-thermal
origin. The pioneering studies by the Okal’s group are important because they showed that the OJP
has an indeed peculiar crust and upper mantle structure. However, the spatial resolution of their
studies is insufficient to understand the origin the OJP. Their slow velocity anomalies are spatially
blurred by limited ray path coverage as shown in Richardson et al. (2000). There is a possibility that
the actual slow anomaly keel is localized somewhere in the OJP region, while it is hard to seek only
from seismic data recorded at the northern rim of the OJP. The weak attenuation found by ScS wave
analysis (Gomer and Okal, 2003) represents an average attenuation of the entire mantle beneath the
OJP, not of the OJP keel. The poor resolution is caused by a lack of seismic stations due to a lack of
oceanic islands in the OJP region itself. It is highly desired to perform a systematic ocean bottom
seismic observation to improve the spatial resolution for the OJP crust and mantle structure.
3. Ocean bottom seismological observation plan
We plan to perform an active seismic experiment for crust and uppermost mantle structure and a
passive experiment for upper mantle structure for the next five years.
The active experiment will be performed with 100 short-period ocean bottom seismographs on
four survey lines traversing the entire OJP (Fig. 1). A refraction seismic tomography and MCS will
be made to obtain detailed two-dimensional P-wave velocity structure and the lateral variation of the
Moho depth along each survey line. We will start the experiment with a north-south transect in the
middle part of the OJP in 2009 (Fig. 2).
The passive experiment will be performed with 10-15 broadband ocean bottom seismographs in a
few years (Fig. 3). The aim of the experiment is to obtain a fine three-dimensional image of the
upper mantle and transition zone structure beneath the OJP than the previous study. The main
interest is whether the slow velocity keel really exists, and, if any, the lateral and vertical sizes, and
the origin: thermal or chemical (or both). P and S wave velocity structure will be obtained by body
wave and surface wave tomography, respectively, which will delineate the slow velocity keel with a
lateral resolution of 300 km and a vertical resolution of 50 km (Fig. 4). The receiver function method
will be used to obtain a depth distribution of the Moho, the Lid, and 410-km and 660-km
discontinuities. The attenuation and Vp/Vs ratio of the keel should be useful to identify the keel’s
origin.
References
Gladczenko, T.P., M. Coffin, O., Eldholm, Crustal structure of the Ontong-Java Plateau: modeling of
new gravity and existing data, J. Geophys. Res., 102, 22711-22729, 1997.
Gomer, B.M., and E.A. Okal, Multiple-ScS probing of the Ontong-Java Plateau, Phys. Earth Planet.
Inter., 138, 317-331, 2003.
Korenaga, J., Why did not the Ontong Java Plateau form subaerially? Earth and Planet. Sci. Lett.,
234, 385-399, 2005.
Larson, R.L., Latest pulse of Earth: Evidence for a mid Cretaceous superplume, Geology, 19,
547-550, 1991.
Miura, S., E. Araki, M. Shinohara, Taira, A., K. Suyehiro, N. Takahashi, M. Ciffin, T. Shipley, P.
Mann, P-wave seismic velocity structure of the Solomon double trench arc system by ocean
bottom seismograph observations, EOS Trans. Am. Geophys. Union 78 (46), F469, 1997.
Richardson, W.P., E.A. Okal, and S. Van der Lee, Rayleigh-wave tomography of the Ontong Java
Plateau, Phys. Earth Planet. Inter., 118, 29-51, 2000.
Figure 1
Figure 2
Proposed transects of refraction and MCS survey for
Planned transect of refraction and MCS
the next five years.
survey in 2009.
Figure 3
Proposed stations of broadband ocean bottom seismographs (red). The green triangles are
broadband stations on oceanic islands whose data are available.
(a)
(b)
(c)
Figure 4
Checkerboard resolution test for surface wave tomography. Earthquakes greater than 6 in 2005
are used to compute ray paths. (a) Input structure; (b) Reconstructed structure with oceanic island
stations; (c) Reconstructed structure with broadband ocean bottom seismograph stations and
oceanic island stations.
Explosive magma-water interaction in oceanic areas:
A proposal for the 2 n d stage of IODP
Hiroaki Sato (Kobe University)
More than 90 % of volcanic eruptions on earth occur under water. Ocean drillings
have the privilege of finding the section of volcanic deposits, and elucidate the
processes of magma-water interaction during eruptions in oceanic areas. In island arcs
and continental margins, there are a lot of large calderas undersea or near the coast,
eruptions of which potentially cause disastrous effects to the nearby cities. Examples
of such gigantic eruptions include the A.D.1883 Krakatau, A.D.1815 Tambora, 3600
b.p. Santorini, and 7300 b.p. Kikai caldera. Koyaguchi and Woods (1996, JGR)
analyzed the magma-water interaction in terms of thermodynamics and fluid mechanics,
and verified that addition and mixing of 10-20 wt% of water to magma maximize the
explosive energies and forms high eruption columns. Sato and Taniguchi(1997, GRL)
examined the crater size of both magmatic and phreatomagmatic explosions and
showed that interaction of external water with magma causes 10 to 100 times larger
explosive energies compared with the same magnitude magmatic explosions. Therefore
caldera-forming eruptions in shallow water may cause unexpectedly large disaster,
although the processes of phreatomagmatic explosions are still to be scrutinized for
more adequate prediction of the posed hazard in the surrounding areas.
The 7300 b.p. eruption of Kikai caldera ca. 50 km south of Kyushu island,
southern Japan, produced ca. 150 km 3 of rhyolitic deposits, and part of the low-aspect
ratio pyroclastic flow covered the southernmost part of Kyushu island, and destroyed
the pre-historic culture in the area. There still remains main question about this
eruption; i.e., the co-ignimbrite ash is too voluminous (more than 100 km 3 ) compared
with the main pyroclastic flow deposit (20-45 km 3 : Machida and Arai, 1990; Maeno
and Taniguchi, 2007).
Volume ratio of pyroclastic flow deposit to co-ignimbrite ash
generally ranges from 40:60 to 50:50 (Taupo: Walker, 1980; Aira: Ueno and Adachi,
2004 etc). If these ratios are applied to the Kikai-Akahoya eruption, most of the main
pyroclastic flows should lie underwater in the surrounding areas of the Kikai caldera.
The estimated thickness ranges from 20 meters to 200 meters depending on the local
topography. It is well known that if high temperature pumice interact with cool water,
pumice is rapidly cooled and sucks water and sink (Whitham and Sparks, 1986 Bull
Volcanol). Therefore we expect thick water-quenched pyroclastic deposits if we drill
sites nearby the Kikai caldera. In the case of Krakatau eruption in 1883, large
explosion noises were heard 6000 km from the volcano, which may be related stron
magma-water interaction during the emplacement of pyroclastic flows. Mandeville et
al. (1994 JGR, 1996, Sedimentology) showed that the underwater pyroclastic deposit
mainly consists of high-temperature massive deposit with minor amounts of
low-temperature stratified deposits. Comparisons of the underwater deposits of
large-scale eruptions are requisite for understanding the processes of magma-water
interaction and should help for mitigation of posed large-scale caldera forming
eruptions in near future.
Mud logging -its significance for ultra-deep ocean drilling
Hiroshi SATO (Senshu University)
Non-coring operation is divided into two method, physical logging and mud
logging. The former has been used for scientific ocean drilling, the latter, on the other
hand, will be a new tool for scientific ocean riser drilling.
The mud logging consists of cuttings analysis and mud gas analysis. "Cuttings" is
a piece of drilled material with a few mm in size. It is crushed at the tip of drilling bit,
and returns with upwelling of drilling mud fluid. Then it is collected at the "shale
shaker".
The logging by cuttings has some problems particularly for determination depth
of each piece of cuttings due to, for examples,
1. mixing of materials from upper lithologies fall into mud fluid
2. mixing of cuttings from different depth in upwelling mud fluid
3. circulation of mud fluid, and
4. chemical compositions of mud fluid.
Therefore, it is not suitable for high resolution analysis. However, it is available
for low- to middle-resolution analysis, for example, determinations of hard-rock
lithology, last occurrence of fossil spices, and so on. Nakata et al. (2007) showed that
similar geocgemical characteristics of volcanic rocks between cuttings and core
during Unzen Volcanic drilling project.
A weak point for cuttings will be covered by 3rd party logging tool, for example,
Mechanical Coring Tool (Schlumberger). More depth-precise samples will be
recovered with MSCT. The borehole imaging tool is also powerful tool helping
non-coring operation.
The reason why we should consider non-coring operation is the time required to
reach to objective depth particularly for ultra-deep drilling, e.g. 21st century Mohole
project. When we will aim toward Moho (approximately 6 to 7 km deep beneath
ocean floor), it will take more than 400 days with only a small amount of coring. If we
will have complete core section from ocean floor to Moho, it will take more than 500
days to reach Moho. Therefore, we should understand mechanism, merit and
demerit, and additional tools of non-coring operations in order to reduce time and
money to reach Moho.
As a matter of fact, pilot hole with coring and physical logging should be drilled
before mail-hole drilling. Then at the main hole, shallower portion will be drilled
without coring, and geological characteristics will be checked by both physical and
mud logging. At more deeper portion, physical and mud logging with some spot
coring and sidewall coring for significant portion will be operated. Geolgical section
near Moho will be drilled with core as long as possible.
Expedition 324：シャツキー海台におけるプルームモデルとプレートモデルの検証
航海期間：2009 年 9 月 4 日（横浜）～11 月 4 日（豪州・タウンスビル）
共同主席研究者：William W Sager・佐野貴司
航海の目的
巨大火成岩岩石区（Large Igneous Provinces: LIPs）は、巨大海台や大陸洪水玄武
岩の総称であり，大規模なマグマ活動が短時間に起こった結果形成されたものであ
る．LIPs は巨大プルームに起源があると提案されてきた．これはマントル深部から
上昇してきたプルームがリソスフェアの下で広がり，大規模なマントル溶融がおき
て火成活動が起こるというものである．このメカニズムは広く受け入れられている
が，かならずしも全ての LIPs 形成を説明できるものではない．むしろ 1990 年代後
半から 2000 年代前半に掘削された 1 番目と 2 番目に巨大な LIPs のオントンジャワ
海台とケルゲレン海台は，互いに全く異なった特徴をもち，共にプリュームモデル
で説明できない可能性が指摘されている．そのために更なる検証が必要であるが，
研究はあまり進んでいない．検証が困難となっている原因の 1 つとして，巨大海台
の多くは形成時の中央海嶺や海洋プレートとの位置関係が明らかになっていないと
いう問題がある．この理由は巨大海台の多くは地磁気の逆転が起きていない中期白
亜紀に形成されているため，昔の中央海嶺の位置を示す地磁気の縞模様が得られな
いという欠点である．
シャツキー海台は、地磁気の逆転が起きていたジュラ紀と白亜紀境界に形成された
唯一の巨大海台である．このためテクトニックセッティングがよく分かっている．
地磁気の縞模様を基に，シャツキー海台は海嶺の 3 重会合点のトレースに沿って形
成されており、海嶺のテクトニクスと密接に関係していることが分かっている．1990
年代には 3 重会合点付近に上昇してきたプリュームに原因があるというモデルが提
案されている．しかし一方，3 重会合点のある場所にちょうどプリュームが上昇し
てくる確率は低いこと，ホットスポットの特徴を示さない同位体組成が数少ない火
成岩試料から得られていることなどから，2000 年代になってからプルームよりもむ
しろプレート活動に関連した形成メカニズムが示唆されている．このような特徴か
ら，シャツキー海台は LIPs の形成メカニズムとして「プルームモデルとプレートモ
デルのどちらが重要か」を評価するのに最適の場所である．
本研究では，シャツキー海台の 5 サイトでそれぞれ基盤岩を 100-300m，合計 800m
掘削・回収する．これにより，シャツキー海台の年代，マグマの起源および進化を
検証する．得られた結果から，LIPs がマントルプルームによってできたのか，それ
ともプレート境界における特異な火成活動なのかという疑問を解決できると期待し
ている．この他にも，シャツキー海台の噴火にともなう表層環境への影響を評価す
ること，古地磁気情報からプレートの動きや火山の構造を明らかにすること，そし
てシャツキー海台の噴火とジュラ紀-白亜紀境界イベントとの関係を明らかにする
ことも研究目的とする．
必要な乗船者の分野
本航海は基盤岩である火成岩採取が主目的のため，火成岩岩石学者（Igneous
Petlogist），無機化学者（Inorganic Geochemist），火山学者（Volcanologist），変
質岩岩石学者（Metamorphic Petrologist）が主な乗船研究員となります．しかし，
噴出時に海台上部は浅海に存在または陸地化していた可能性が高いため，噴出物は
火山砕屑物として産出する可能性があります．このため火山砕屑物の記載を得意と
する堆積学者（Sedimentologist）の乗船も歓迎します．さらに，全てのサイトで基
盤岩上位 50m の堆積層（ジュラ紀〜白亜紀）も採取するため，海台形成以降の古環
境（年代・深度）を調査する古生物学者（Paleontologist）にも乗船いただきたい
と願っています．
Primary volatile content in magma from large igneous provinces
(LIPs): potential trigger for environmental perturbation and
Cretaceous LIP eruptive events.
Kenji Shimizu, Katsuhiko Suzuki and Akira Ishikawa (Institute for Research on Earth
Evolution, Japan Agency for Marine-Earth Science and Technology)
The timing of emplacement of many large igneous provinces (LIPs) coincides with
oceanic anoxic events (OAEs), which are recorded in coeval pelagic sedimentary layers (i.
e. black shale occurrences). Large igneous events somehow caused subsequent
environmental responses. Larson and co-workers (e.g. Larson and Erba, 1999; Self et al.,
2006) proposed that excesses of volcanic gasses (especially carbon dioxide and sulfur) to
the surface are the trigger for the global environmental changes in the oceanic and
atmospheric systems. Volatiles in the source mantle of the LIP also strongly affect the
conditions of magma genesis and global material circulation.
Two major Cretaceous OAEs occurred at ~120Ma (OAE-1a) and 94Ma (OAE-2).
These two events were proposed to be related to the 120Ma eruption of the Ontong Java
Plateau (OJP) and the ~90Ma OJP and Caribbean-Colombian Oceanic Plateau (CCOP).
Recently, these LIP-OAE correlations have been verified by osmium and carbon
isotopic profiles of sedimentary sequence from ODP drilling sites of OAE-2 (Turgeon
and Creaser, 2008). However, these correlations have not yet been proven from LIPs side,
because of the shortage of valid volatile data and accurate age data from LIPs’ volcanic
rocks. Volatiles in the melt easily degas during eruption and emplacement of magma, thus
it is difficult to evaluate primary volatile content. Moreover, most volcanic rocks of LIPs
have undergone hydrothermal alteration, which make it more difficult to evaluate their
original volatile content. In addition, CO2, the most effective greenhouse gas, is much
less soluble in magma at low pressure than H2O and other volatiles (Dixon et al., 1995),
thus evaluation of primary melt in CO2 is much more challenging. Melt inclusions hosted
by igneous minerals preserve various important pieces of information regarding the
history of magmas. One of the biggest advantages for studying melt inclusions is that we
can quantitatively determine the volatile contents (e.g. H2O, CO2, F, S and Cl) of magma
before eruption.
OJP is the ideal LIPs to verify LIPs-OAE correlation from LIPs’ aspect, because
OJP formed during various ages (~120Ma, ~90Ma, ~70Ma and ~40Ma: Tejada et al.,
2002), which include the ages of two major Cretaceous OAEs. Although ODP and DSDP
have succeeded to drill the OJP basement at seven sites and recovered unaltered basaltic
samples, those samples are only from the uppermost OJP crust (less than 200m out of
~33km total thickness of OJP crust). We need least altered igneous rock samples from the
earlier stage of the OJP formations to estimate the primary volatile content and the
eruptive history of the world-largest igneous province.
References
Dixon, J. E., Stolper, E. M., and Holloway, J. R., 1995. An experimental study of water
and carbon dioxide solubilities in mid-ocean ridge basaltic liquids 1. Calibration
and solubility models. J. Petrol. 36, 1607-1631.
Larson, M. S. and Erba, E., 1999. Onset of the mid-Cretaceous greenhouse in wwfwthe
Barremian-Aptian: Igneous events and the biological, sedimentary, and
geochemical responses. Paleoceanography 14, 663-678.
Self, S., Widdowson, M., Thordarson, T., and Jay, A. E., 2006. Volatile fluxes during
flood basalt eruptions and potential effects on the global environment: a Deccan
perspective. Earth Planet. Sci. Lett. 248, 518-532.
Tejada, M. L. G., Mahoney, J. J., Neal, C. R., Duncan, R. A., and Petterson, M. G., 2002.
Basement geochemistry and geochronology of central Malaita, Solomon islands,
with implications for the origin and evolution of the Ontong Java Plateau. J.
Petrol. 43, 449-484.
Turgeon, S. C. and Creaser, R. A., 2008. Cretaceous oceanic anoxic event 2 triggered by a
massive magmatic episode. Nature 454, 323-326.
<INVEST domestic proposal>
To verify LIPs – OAE linkage
Katsuhiko Suzuki
Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology
The massive submarine volcanism that formed Large Igneous Provinces (LIPs) such as the
Ontong Java Plateau (OJP) has been suggested as the driving force for widespread oceanographic
changes (Larson and Erba, 1999). These changes led to the deposition of thick, laminated,
organic-rich black shales. Such event in Cretaceous is called oceanic anoxic event (OAE). Sinton
and Duncan (1997) and Snow et al. (2005) demonstrated in studies of OAE1a and OAE2 (~93.5 Ma)
sediments, respectively, that large-volume, submarine plateau volcanism could have caused
environmental changes by fertilizing the ocean with trace metals, in addition to CO2 release and
consequent global warming. Recently, the Os isotopic records of seawater preserved in marine
sedimentary rocks have been used to detect large volcanic inputs into the ocean; e.g., from the
predominantly subaerial eruptions of the ~65 Ma Deccan Traps (Ravizza and Peucker-Ehrenbrink,
2003) and ~200 Ma Central Atlantic Magmatic Province (Cohen and Coe, 2002; 2007) to the ~90
Ma submarine Caribbean and OJP eruptions (Turgeon and Creaser, 2008). The Pb isotope excursion
of black shales during OAE2 was also found (Kuroda et al., 2007).
The Early Cretaceous Ontong Java Plateau (OJP) was emplaced at the same time as marine
biotic changes that culminated in Oceanic Anoxic Event 1a (OAE1a). A linkage between these
events has been suggested. We found based on new Os isotope data across the Lower Aptian black
shale deposited during OAE1a in central Italy two negative excursions in marine 187Os/188Os within
a period of 2 million years starting above the Barremian-Aptian boundary and ending just above the
Selli Level horizon (Tejada et al., submitted). It suggests an order-of-magnitude increase in the
global flux of unradiogenic Os. The results are consistent with early and major phases of eruption of
the OJP. The latter phase is estimated to have been as short as ~1 million years and may have
induced widespread oceanic stratification that triggered OAE 1a.
To verify LIPs – OAE linkage, we propose the following research:
1. Direct measurement of volatile species, especially green-house effect gases such as CO2, of
primitive magma of LIPs, which may have significantly affected the earth’s surface
environment.
2. Estimates of volatiles and chemical compositions of palaeo- seawater through analyses of such
species in quartz formed in lava halos.
3. Estimates of amounts of Os and Pb supply from alteration of LIP rocks to check whether enough
Os and Pb to vary the seawater Os excursion in the time of LIPs activities was provided.
4. Estimates of the direct measurements of amounts of elements provided to the surface by
volcanic gases.
[References]
Cohen, A.S., and Coe, A.L. (2002) New geochemical evidence for the onset of volcanism in the
Central Atlantic Magmatic Province and environmental change at the Triassic–Jurassic
boundary. Geology, 30, 267–270.
Cohen, A.S., and Coe, A.L. (2007) The impact of the Central Atlantic Magmatic Province on climate
and on the Sr- and Os-isotope evolution of seawater. Palaeogeography, Palaeoclimatology,
Palaeoecology, 244, 374–390.
Duncan, R.A., Tiraboschi, D., Erba, E., Walczak, P.S. and Clarke, L.J., (2007) The Cretaceous OAE
1a-submarine plateau link: Additional geochemical evidence from marine sedimentary sections.
Eos, Transactions, American Geophysical Union, Fall Meeting, T13A-1125.
Kuroda, J., Ogawa, N., Tanimizu, M., Coffin M. F., Tokuyama, H., Kitazato, H., Ohkouchi, N.
(2007) Contemporaneous massive subaerial volcanism and Late Cretaceous oceanic anoxic
event 2. Earth Planet. Sci. Lett. 256, 211-223.
Larson, R. L. and Erba, E. (1999) Onset of the mid-Cretaceous greenhouse in the Barremian-Aptian:
igneous events and the biological, sedimentary, and geochemical responses. Paleoceanography,
14, 663–678.
Ravizza, G. and Peucker-Ehrenbrink, B. (2003) Chemostratigraphic evidence of Deccan volcanism
from the marine osmium isotope record. Science, 302, 1392–1395.
Snow, L. J., Duncan, R. A. and Bralower, T. J. (2005) Trace element abundances in the Rock Canyon
Anticline, Pueblo, Colorado marine sedimentary section and their relationship to ocean plateau
construction and OAE2. Paleoceanography, . 20, PA3005, doi: 10.1029/2004PA001093.
Turgeon, S. C. and Creaser, R. A. (2008) Cretaceous oceanic anoxic event 2 triggered by a massive
magmatic episode. Nature, 454, 323-327.
<INVEST domestic proposal>
Solid earth – life interaction: Introduction to TAIGA and Precambrian Eco-system
Projects
Katsuhiko Suzuki
Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology
Here I introduce the on-going “TAIGA” and Precambrian Eco-system projects. Both
projects are interdisciplinary research on origin and evolution of life.
“TAIGA” is abbreviated from Trans-crustal Advection and In-situ reaction of Global
sub-seafloor Aquifer. The amount of flowing water in the hydrothermal and low-temperature spring
system in the sub-seafloor aquifer is considered to be equivalent to that of all the rivers on land.
Supply of various elements by mid-ocean ridge hydrothermal solutions is less than that of rivers
(Elderfield and Schultz, 1996). Recently, Wheat & Mottl (2004) claimed that the sub-seafloor system
including both the hydrothermal and low-temperature spring systems provides some elements such
as phosphorus whose amounts are equivalent to those of rivers on land. We propose that the other
elements are possibly provided by the sub-seafloor system. These sub-seafloor systems give not only
those elements required to maintain the eco-system, but also the solution enriched in reducing agents,
which contacts the oxidizing seawater. Such area is a suitable habitat for chemolithoautotrophy such
as methanogenesis and sulfur-reduction, which supports the various types of the ocean eco-systems.
Therefore, we consider that “TAIGA” is very important system for evolution of life in the ocean. To
verify the TAIGA hypothesis, we are developing the new laboratory hydrothermal experiment
system to reproduce the sub-seafloor conditions.
The Precambrian Eco-system project involves evolution of the earliest life on the earth,
which have attracted interests of many people. However, actual origin of life and its evolution in the
early Earth is poorly constrained. Since their discovery in the late 1970s, deep-sea hydrothermal
systems have been considered as likely candidates for the origin and early evolution of life on the
Earth. Multiple lines of evidences from different fields such as phylogenetic, biochemical and
geochemical
clues
all
seem
to
point
to
the
early
evolution
of
hydrogenotrophic
chemolithoautotrophy such as methanogenesis and sulfur-reduction, thus pinpointing the availability
of hydrogen as one of the key elements needed for the early evolution of earthly life. Though
hydrogen-driven, photosynthesis-independent communities are very rare on the contemporary Earth,
they were unambiguously found only in subsurface environments of H2-dominated hydrothermal
systems. Such systems have been termed hyperthermophilic subsurface lithoautotrophic microbial
ecosystems (HyperSLiMEs) (e.g., Takai et al., 2004). The supply of abundant hydrogen and
available inorganic carbon (CO2) sources to sustain such communities is the most likely coupled to
hydrothermal serpentinization of ultramafic rocks and input of magmatic volatiles, both of which are
related to specific geological settings. We proposed, on the basis of findings in the modern Earth and
implications for the deep-sea hydrothermal systems in the Archean Earth, that "Ultramafics Hydrothermalism - Hydrogenesis - HyperSLiME", a linkage we refer to as Ultra-H3 provided a
suitable habitat for the early microbial ecosystem on the Archean Earth (Takai et al., 2006). To verify
this hypothesis, we have been conducting laboratory experiments as well as field observation.
We have developed two types of apparatuses for high-temperature hydrothermal
experiments; a batch-type and a flow-type systems. In the batch-type, we use a flexible gold tube for
reaction cell equipped with a titanium head (e.g., Seyfried et al., 1987). This reaction cell was
confined within a stainless steel or nickel-alloy autoclave with H2O as a pressure buffer. First, we
performed an experiment for hydrogen production during serpentinization of natural olivine in
Na-Mg-Cl aqueous solution (artificial seawater) at 350 degree-C, and 50 MPa. We could detect a
significant amount of H2 using the direct measurement system we developed for dissolved gases in
seawater (Suzuki et al., 2007). Detailed microscopic observation, electron microprobe, and Raman
spectroscopic analyses of the run products show that olivine grains were partially serpentinized, and
production of brucite and magnetite were confirmed as products as well as serpentine mineral. The
produced serpentine is identified as chrysotile by Raman spectroscopy.
In contrast to batch-type experiments, there have been few examples of the flow-type
experiments for hydrothermal reactions because of their operational difficulties. However, we
succeeded an experiment of serpentinization of harzburgite in pure water at 350 degree-C and 50
MPa for 700 hours. Now we are upgrading the experimental system to use seawater, instead of pure
water, for serpentinization of various mafic minerals.
To fix the experimental condition in the hydrothermal reactions such as temperature,
pressure, rock or mineral type and reacting fluid chemical compositions, the field observation is very
important. Especially, seafloor drilling possibly gives crucial information on the in-situ reaction in
the sub-seafloor hydrothermal system.
[References]
H. Elderfield, and A. Schultz, (1996) Mid-ocean ridge hydrothermal fluxes and the chemical
composition of the ocean. Ann. Rev. Earth Planet. Sci., 24, 191-224.
W. E. Seyfried Jr., D. R. Janecky, and M. E. Berndt (1987) Rocking autoclaves for hydrothermal
experiments. II. The flexible reaction – cell system in G. C. Ulmer and H. L. Barnes, eds.,
Hydrothermal Experimental Techniques: John Wiley and Sons, New York, p. 216-239.
K. Takai, T. Gamo, U. Tsunogai, N. Nakayama, H. Hirayama, K. H. Nealson, and K. Horikoshi,
(2004) Geochemical and microbiological evidence for a hydrogen-based, hyperthermophilic
subsurface lithoautotrophic microbial ecosystem (HyperSLiME) beneath an active deep-sea
hydrothermal field. Extremophiles 8, 269-282.
K. Takai, K. Nakamura, K. Suzuki, F. Inagaki, Kenneth H. Nealson and H. Kumagai (2006)
Ultramafics-Hydrothermalism-Hydrogenesis-HyperSLiME (UltraH3) linkage: a key insight into
early microbial ecosystem in the Archean deep-sea hydrothermal systems, Paleontol. Res, 10 (4 ),
269-282.
The northern Oman ophiolite as an analog for the study of juvenile oceanic arc
Eiichi Takazawa
Department of Geology, Faculty of Science, Niigata University, 2-8050, Ikarashi, Niigata,
950-2181, Japan. [email protected]
Oman ophiolite has been considered as an on-land analog for oceanic
lithosphere formed at mid-ocean ridge (e.g., Nicolas, 1989). However, the studies of
northern Oman ophiolite show significant influence from subsequent subduction zone
processes (e.g., Tamura and Arai, 2006; Arai et al., 2006; Dare et al., 2008). We have
investigated spatial compositional variability in the mantle section of Fizh block,
northern Oman ophiolite (Fig. 1; Takazawa et al., 2008). The southern part of Fizh block
mainly consists of relatively homogeneous harzburgites while the northern part consists
of both less-depleted harzburgites and highly-refractory harzburgites such that Cr# [=100
x Cr/(Cr+Al)] ratio of spinel widely ranges from 24.2 to 77.6 contrasting to the range of
spinel Cr# in the south from 55.6 to 63.2. These results indicate that the uppermost mantle
in the north where a segment end was located is more heterogeneous in basaltic
components relative to the south where a segment center was located. The localized
highly-refractory zone in the north Fizh indicates that the residues after partial melting at
mid ocean ridge were subjected to hydrous remelting during detachment of oceanic
lithosphere. Our results combined with previous studies demonstrate that the Oman
ophiolite is an ideal place to study both crust and mantle processes in a juvenile oceanic
arc system. Moreover, the results from Oman ophiolite should be directly compared with
a reference crust and mantle section in young oceanic arc such as the Izu-Bonin-Mariana
forearc.
Forearc peridotites have been sampled from serpentine mud volcanoes (e.g.,
ODP Leg 125 in Izu-Bonin-Mariana forearc; Fig.2). Parkinson and Pearce (1998)
demonstrated that harzburgites from Conical Seamount in Mariana forearc were
considered as residual MORB mantle which had subsequently been modified by
interaction with boninitic melt within the mantle wedge. The evolution of oceanic
lithosphere beneath Conical seamount seems similar to the northern Oman ophiolite.
Although peridotite clasts have severely serpentinized samplings at multiple mud
volcanoes along trench will give us spatial variability in forearc mantle wedge.
Comparison between peridotite clasts from Mariana mud volcanoes and Oman mantle
peridotites have a potential to resolve a question how oceanic lithosphere was modified to
become a juvenile arc. Thus the drillings at summits of multiple serpentine mud
volcanoes are strongly anticipated.
Drilling and coring of crust and mantle rocks are more attractive strategy for
obtaining intact forearc lithosphere. In this moment I have no clear idea where is an
appropriate place for drilling. However, recent observations of southern Mariana forearc
by submersible Shinkai 6500 have advanced our knowledge of forearc mantle wedge
(e.g., Ohara and Ishii, 1998). The petrogenesis of peridotites seems very similar to the
northern Oman peridotites: i.e., they were residues after partial melting at mid-ocean
ridge and later influenced by fluid at forearc environment (Ohara and Ishii, 1998).
Although the drilling of deep sea floor in the southern Mariana forearc may be associated
with technological difficulties I would like to stress a high potentiality in drilling an entire
section of forearc crust and mantle wedge in this area. The southern Mariana forearc
could be one of the targets for the 21 century Mohole project.
References
Nicolas, A. (1989) Structures in ophiolites and dynamics of oceanic lithosphere, 367pp, Kluwer Acad.
Tamura and Arai (2006) Lithos, 90, 43-56.
Arai et al. (2006) J. Geol. Soc. London, 163, 869-879.
Dare et al. (2008) Chem. Geol., in press.
Parkinson and Pearce (1998) J. Petrol., 39, 1577-1618
Ohara and Ishii (1998) Island Arc, 7, 541-558.
Fig. 1 (right) Geological map of the Fizh block, northern Oman ophiolite. (left) spatial
variations of spinel Cr# in the northern Fizh block.
Fig. 2 Mariana forearc and locations of
serpentine mud volcanoes (Fryer et al., 2000).
Fig. 3 Topography in the vicinity of the southern part of the Mariana Trench. Fig. 1 of
Ohara and Ishii (1998).
Paleomagnetic data expected from perspectives of dynamo theory and simulation
Futoshi Takahashi (Tokyo Institute of Technology)
Recent advances of computer technology enable us to numerically simulate magnetic field
generation processes (dynamo action) operating in the Earth’s core (Christensen and Aubert,
2006; Kageyama et al., 2008; Takahashi et al., 2005, 2008a). These advanced numerical dynamo
models are characterized by the fact that they attain the Ekman number, E, which is a
non-dimensional parameter representing the relative effect of viscosity to planetary rotation in
force balance, close to the regime of the real Earth. Although the Ekman number in the Earth’s
core is extremely small, E = O(10-9), the most advanced models can reach E = O(10-7). Such
low-E dynamo models are found to be dynamically similar to the real Earth (Takahashi et al.,
2005). Therefore, numerical dynamo model allows us to make detailed comparison with the
geomagnetic field, which would strongly enhance our knowledge on the geodynamo. For this
purpose, however, reliable paleomagnetic data is necessary. From this point of view, I suggest
three fundamental unresolved topics regarding relationships between core dynamics and
paleomagnetic data, which we believe, should be treated in the IODP Initial Science Plan
2013-2023.
The radial magnetic field at the CMB (core-mantle boundary) is characterized by two pairs of
high-latitude magnetic flux patches located beneath North-America and Siberia in the northern
hemisphere and two counterparts in southern hemisphere symmetric with respect to the equator.
The high-latitude flux patches are understood in terms of flux concentration by equator-ward
down-welling flows from the CMB in cyclonic convection rolls. Since such a structure is
temporally and spatially stable at least from the last 400 years (Jackson et al., 2000), it is
important to examine whether the high-latitude flux patches are stable on longer timescale to
understand spatial stability of core convection and its dynamics. In order to examine it,
construction of global paleomagnetic field model is expected.
Let us imagine a virtual cylinder aligned with the rotation axis in touch with the inner core at
the equator, which is called the tangent cylinder (TC). The TC geometrically and dynamically
divides the Earth’s core into three independent portions: regime of convection roll outside the
TC, northern and southern parts of the TC isolated by the solid inner core. Numerical dynamo
model tells us that convection and dynamo action occur differently in such regimes. Convection
roll outside the TC is responsible for magnetic field generation, whereas helical plume and
thermal wind dominate inside the TC (e.g. Kono and Roberts, 2002). Therefore, paleomagnetic
data collected from high-latitude region would reflect such differences, especially, in the secular
variation. Successive high-latitudes paleomagnetic records would appreciably improve our
understanding of the TC.
Strength of the geomagnetic field is an indicator of dynamo activity. A relationship between
the field strength and polarity stability has long been suggested. One of the longest polarity
epochs is known as the Cretaceous Normal Superchron (CNS). Although available absolute
paleointensity data does not give any conclusive evidence, strong paleointensity is expected
from a numerical dynamo model (Takahashi et al., 2008b) and geodynamical point of view
(Larson and Olson, 1991). Both studies focus on effects of thermal core-mantle coupling on
dynamo action, which can enhance the generated magnetic field. Hence, reliable paleointensity
data during the CNS provide us with a clue to understand the effects of mantle dynamics on
core dynamics and dynamo action.
References
U. R. Christensen, J. Aubert, Scaling properties of convection-driven dynamos in rotating
spherical shells and application to planetary magnetic fields, Geophys. J. Int. 166, 97-114, 2006.
A. Jackson, A. R. T. Jonkers, M. R. Walker, Four centuries of geomagnetic secular variation
from historical records, Phil. Trans. R. Soc. Lond. A 358, 957-990, 2000.
A. Kageyama, T. Miyagoshi, T. Sato, Formation of current coils in geodynamo simulations,
Nature, 454, 1106-1109, 2008.
M. Kono, P. H. Roberts, Recent geodynamo simulations and observations of the geomagnetic
field, Rev. Geophys. 40, 1013, doi:10.1029/2000RG000102, 2002.
R. L. Larson, P. Olson, Mantle plumes control magnetic reversal frequency, Earth Planet. Sci.
Lett. 107, 437-447, 1991.
F. Takahashi, M. Matsushima, Y. Honkura, Simulations of a quasi-Taylor state geomagnetic
field including polarity reversals on the Earth Simulator, Science, 309, 459-461, 2005.
F. Takahashi, M. Matsushima, Y. Honkura, Scale variability in convection-driven MHD
dynamos at low Ekman number, Phys. Earth Planet. Inter., 167, 168-178, 2008a.
F. Takahashi, H. Tsunakawa, M. Matsushima, N. Mochizuki, Y. Honkura, Effects of thermally
heterogeneous structure in the lowermost mantle on the geomagnetic field strength, Earth
Planet. Sci. Lett., 272, 738-746, 2008b.
Possible applications of in situ geochemical & geochronological micro-analyses to the
cuttings obtained from the IODP drilling: Effective tools for IBM crustal drilling & Mohole
Kenichiro Tani (IFREE, JAMSTEC)
Obtaining geochemical, geochronological, and petrological information from the drilling
slimes/cuttings during the future IODP riser-drilling will be an important and effective method
to minimize the time and cost for ultra-deep drilling required for IBM middle crust drilling and
Mohole.
Geochemical and petrological analyses of the drilled cuttings have been a common tool
used in the oil industries and subaerial scientific drillings, although they are mainly focused
on whole-rock analyses of the cuttings. Since the cuttings are derived from a mixture of
multiple lithofacies encountered during the drilling, whole-rock geochemical analyses of the
cuttings can obscure the detailed geological and geochemical information which the cuttings
may contain.
One of the possible methods to obtain high-resolution geological and geochemical
information from the cuttings is application of in situ geochemical and geochronological
micro-analyses such as a Laser abrasion-Inductively Coupled Plasma-Mass Spectrometry
(LA-ICP-MS) or a Secondary Ion Mass Spectrometry (SIMS) to the separated minerals and
volcanic glasses. Recent improvements in the micro-analyses techniques allow us to obtain
trace element and isotopic compositions from minerals included in the igneous rocks such
as olivine, pyroxene, plagioclase, quartz, apatite, zircon and etc. These in situ trace element
compositions can be used to estimate the detailed geochemical characteristics of the
magma which these minerals were crystallized and isotopic compositions can be used to
recognize different magma sources and also for geochronology. If the volcanic glass was
present in the cuttings, we can directly obtain the geochemical information of the magma
which the glass has derived from. Another advantage of using the in situ mineral and glass
micro-analyses is that they are less affected by alteration and contamination compared to
the whole-rock analyses. We can selectively analyze unaltered minerals and glasses, and
the minerals are commonly robust than the matrix. In case of the whole-rock analyses, it is
difficult to remove the effect from the altered matrix, and the contaminations from the
chemical and mineral addictives added to the drilling mud are not negligible.
Even if we are able to accomplish a full-core ultra-deep drilling such as IBM middle crust
drilling and Mohole, the upper volcaniclastic layers which consist most of IBM crust and
oceanic crust are expected to be highly affected by hydrothermal alterations. With the
current geochemical and geochronology techniques, it is inevitable to crush the obtained
core sample to separate unaltered mineral and glasses if the rocks have experienced
hydrothermal alteration. So if we can develop a technique to separate unaltered mineral and
glass from the cuttings, it can minimize the time and cost required for the drilling while
obtaining same scientific results as to the full-core drilling.
However, careful test and verification are required before we can obtain detailed
high-resolution geochemical and geochronological information from the cuttings. First,
comparison between the logging data and the petrographic features of the cuttings are
required to precisely correspond drilling depth and obtained cuttings. Grain size distribution
of the cuttings should also require careful examination to avoid cross-contamination
between the cuttings derived from the different depths. Second, contamination from the
chemical addictives added to the drilling mud must be carefully checked for the minerals and
glasses separated from the cuttings though the effect should be small compared to the
whole-rock analyses. Such tests and verifications are essential and must be accomplished
before the ultra-deep riser drilling becomes real in the near future.
Crust and Mantle Evolution
Along- and across-arc (AAA) variations in magmas, arc crust, mantle wedge and
subduction input
Yoshihiko Tamura (IFREE, JAMSTEC)
We propose to study along- and across-arc (AAA) variations in magmas, arc crust,
mantle wedge and subduction input in subduction zones by deep sea drilling. New
findings about along-arc periodic heterogeneity in the Izu-Bonin-Mariana arc are quite
intriguing. Isse et al. (in press) shows that the mantle convection and thermal structure
might not be uniform under the IBM arc, which stretches over 2800 km. The IBM arc
could be divided into three geographic segments based on the isotopic values and the
degree of isotopic variability. Moreover, the segmentation seen in the isotope ratios
matches the segmentation in shear wave speed anomalies. Isse et al. (in press) suggest
that upper mantle enters the wedge of the IBM subduction zone at the three slow
anomalies spaced 500 km apart. They also suggest that the flows from the rear arc to the
volcanic front along the IBM arc are not chemically uniform and the differences could
be resulted from the different materials subducted along the 2800 km-long IBM arc.
Recent crustal studies have revealed that the crustal thickness varies
substantially along the IBM arc (Kodaira et al., 2007a, b). The crustal variation has
wavelengths of 80 and 1000 km, which correspond to the spacing of basaltic volcanoes
and the differences between the relatively thick Izu arc crust and the extremely thin
Bonin arc crust, respectively.
Tamura et al. (2002) showed that Quaternary volcanoes in the NE Japan arc
can be grouped into 10 elongate volcanic groups striking transverse to the arc and
concluded that mantle melting and the production of magmas in NE Japan may be
controlled by locally developed hot regions within the mantle wedge that have the form
of inclined, 50 km-wide fingers, which can be seen clearly as inclined slow-speed zone
in the high resolution body wave tomography of Hasegawa and Nakajima (2004).
These multidisciplinary endeavors involving geochemistry, petrology,
geophysics and marine science resulted in many brand-new findings. We propose to use
ocean drillings to study triple A variation in subduction zones to get insights into the
construction of island arcs and the manufacture of new crust.
Recommendation: an additional target site ‘Mariana Trough’ for the Mohole
Kiyoaki NIIDA
Hokkaido University
The Mission Moho Workshop proposed a possible drilling site in fast-spread crust and designed to
achieve full penetration of fast-spread crust through the Moho and then into the uppermost mantle
at a single site near the fast-spreading ridge (Workshop Reports, 2007). Many sites in various
settings were also discussed as potentially relevant to Mission Moho objectives such as several sites
in slow-spread crust near Mid-Atlantic Ridge, some sites in oceanic core complexes as rather
shorter holes, and a site in ultraslow-spread crust. Here I recommend an additional target site in the
northern Mariana Trough of the most representative active back-arc basin. The original plan of the
Mohole at Izu-Bonin-Mariana has been proposed in the Science Plan of the IODP domestic
committee of Japan (Japan Drilling Earth Science Consortium, 2004).
The best site to the Moho:
There are possible sites for deep drilling to penetrate the Moho in the northern Mariana Trough
around 144-145°E, 20-21°N. Alternatively, if we plan rather shorter drilling to touch the Moho and
into the upper mantle, there are some focused site in 3500 m water depth in the amagmatic deep of
Central Graben (144°05-10’E, 20°04-12’N).
Geologic setting:
The Mariana Trough is an actively spreading back-arc basin behind the Mariana volcanic arc. The
opening of the basin generated at 6 Ma near 18°N (Iwamoto et al., 2002). The range of spreading
rate at 18°N is 1.5-2.2 cm/year in half-rate (Hussong and Uyeda, 1981) and classifies the opening
axis as a slow-spreading ridge (Malinverno, 1993). The spreading system is asymmetric in the
trough (Yamazaki et al., 1993). The crustal thickness deduced from the seismic velocity structure is
approximately 5.5 km (ref., Fig. 5.3 of J-DESC 2004).
In the northern Mariana Trough, the opening of the basin generated at 4 Ma with a spreading
half-rate of 2-3 cm/year (Yamazaki et al., 2003). After 0.78 Ma, the opening became very slow, less
than 1 cm/year, and then still continues at present.
The background:
The target area in the northern Mariana Trough has been well surveyed by many workers (e. g.,
Chikyu Monthly, 2003, 2005, 2007).
Geochemical mapping of volcanic rocks from the Mariana arc-basin system, using the following
ICP-MS, XRF, INAA, and isotope dilution data, has been performed to detect new components of
entering asthenosphere into the arc-basin systems (Pearce et al., 2005).
Mriana Trough: Hawkins et al., 1987; Volpe et al., 1987; Stern et al., 1990; Gribble et al., 1996;
Gribble et al., 1998; Ikeda et al., 1998; Pearce et al., 2005.
Mariana Arc: Dixon and Stern, 1983; Stern and Bibee, 1984; Stern et al., 1989; Woodhead, 1989;
Elliott et al., 1997; Bloomer et al., 1998; Peate and Pearce, 1998; Sun and Stern, 2001; Woodheard
et al., 2001; Pearce et al., 2005.
Geochemistry of rather deeper crustal rocks such as tonalites, gabbros, and amphibolite from the
Central Graben has been done to complete a crustal evolution model for the Mariana Trough
(Masuda and Arima, 2003, 2005*).
Most recent accumulation of petrological and mineralogical reports for peridotitic samples derived
from the upper mantle should be listed here. The following publications include samples obtained
from the Mariana fore-arc, island arc, and the back-arc areas, and also from the other areas in IBM.
Fore-arc:
Torishima fore-arc seamount (Ishii et al., 1992)
Ohmachi seamount (Yasa et al., 1999*; Niida et al., 2001*, 2003*)
North-eastern Ogasawara Ridge (Tani et al., unpublished new data)
Hahajima seamount (Ishii, 1985; Ishii et al., 1992; Ishii et al., 2000*; Okamura et al., 2006;
Ishiwatari et al., 2006; Azuma, 2007*)
Mariana fore-arc seamounts (including onical smt., Pacman smt., Chamorro smt., and others)
(Ishii et al., 1992; Ishii et al., 2000*; Maekawa et al., 2001; Maekawa et al., unpublished new data)
Mariana Trench (Bloomer, 1983; Bloomer and Hawkins, 1983; Ohara and Ishii, 1998; Ohara et al.,
2002; Niida et al., 2005*Yanagida et al., 2007; Niida et al., unpublished new data; Ohara et al.,
unpublished new data)
Yap Trench (Ohara et al., 2002)
Mariana Trough (Stern et al., 1996; Arima et al., 2002*; Ohara et al., 2002, Chiba et al., 2008*)
Parece Vela back-arc basin (Ohara et al., 1996, 2003; unpublished new data)
A scientific challenge:
To understand the nature of lithospheric crust through the Moho and upper mantle.
To establish a reasonable model for initiation of active back-arc basin and the evolution of the
arc-basin system.
(To be continued)
Electrical conductivity of oceanic upper mantle: thermal and compositional structure
associated with lithospheric age
Kiyoshi Baba (Earthquake Research Institute, University of Tokyo)
Lithospheric cooling and transition with age has been discussed through bathymetry, crustal
heat flow, seismic low velocity zones, and so on. Seafloor magnetotelluircs (MT) also provides
information about the upper mantle in terms of electrical conductivity. This presentation
overviews the electrical conductivity structure of the oceanic upper mantle and its relation with
thermal structure and composition
The electrical conductivity of the mantle minerals depends strongly on the temperature,
composition (including the degree of mantle hydration), and the fraction and connectivity of
melt. The conductivity of dry hydrated subsolidus minerals and melt are determined in
laboratory studies. Using the laboratory results, the electrical conductivity model obtained from
the seafloor MT data can constrain the values of one or more of these parameters when
employed in conjunction with independent observations.
Since initiation of seafloor MT researches in 1970s, many of the experiments were conducted
in the tectonically active regions such as mid-ocean ridges, hotspots, and subduction zones.
Hence, there are only a few data sets in normal ocean basins. However, some literatures
describe a relation of the conductivity model with lithospheric age. For example, Oldenburg et
al. (1984) inverted seafloor MT data collected at three sites in northeastern Pacific Ocean and
found that high conductivity layer appears deeper for the older mantle. Seama et al. (2007)
reported the conductivity structure beneath Philippine Sea, which also indicates age-dependency
of the structure except for the Mariana Trough. These results suggest that the conductivity
structure is associated with thermal cooling of lithospheric mantle.
In the vicinity of the mid-ocean ridges, the conductivity structure is not dependent on thermal
structure but on compositional structure. Baba et al. (2006) reported two-dimensional
conductivity structure beneath East Pacific Rise at 17°S. The mantle is more conductive below
about 60 km, independently on the lithospheric age (0-3 Ma). The thermal boundary layer
thickens with the age but it must be much thinner than 60 km in the region. The flat
resistive-conductivity boundary at 60 km agrees well with the inferred depth of the dry solidus
of peridotite, suggesting that the uppermost 60 km represents the region of mantle that has
undergone melting at the ridge and has been depleted of water. By contrast, the underlying
mantle has retained a significant amount of water. Similar feature is detected in the central
Mariana Trough (Baba et al., 2005).
Taking account for the water re-distribution due to partial melting at mid-ocean ridges, the
conductivity of the upper mantle is controlled by mainly water distribution for young mantle.
The proton conduction (effect of water on conductivity) is also a function of temperature. The
conductivity is not high for lower temperature even if the mantle is hydrated. For matured
mantle that the thermal boundary layer becomes thicker than 60 km, the thermal structure
probably controls the depth of the high conductivity layer more dominantly than the water
distribution.
For very old (~130Ma) mantle in northwestern Pacific, the high conductivity appears at about
200 km depth (Baba et al., 2007), which is much deeper than the prediction based on plate
cooling thermal models such as GDH1 model (Stein and Stein, 1992). The inconsistency with
crustal heat flows in the region, which are well explained by the GDH1 model should be
elucidated through further observations and modeling. We may need to reconstruct a thermal
evolution model for old oceanic upper mantle in the future.
References:
Baba, K., M. Ichiki, N. Abe, and N. Hirano (2007), Upper mantle composition beneath the
petit-spot area in northwestern Pacific: Insights from electrical conductivity, AGU 2007 fall
meeting, Eos Trans. AGU, 88(52), Fall Meet. Suuple., Abstract T13A-1129.
Baba, K., A. D. Chave, R. L. Evans, G. Hirth, and R. L. Mackie (2006), Mantle dynamics
beneath the East Pacific Rise at 17°S: Insights from the Mantle Electromagnetic and
Tomography
(MELT)
experiment,
J.
Geophys.
Res.,
111,
B02101,
doi:10.1029/2004JB003598.
Baba, K., N. Seama, T. Goto, M. Ichiki, K. Schwalenberg, H. Utada, and K. Suyehiro (2005),
Electrical structure of the upper mantle in the Mariana subduction system, Frontier Research
on Earth Evolution, 2, IFREE Report 2003—2004.
Oldenburg, D. W., K. P. Whittall, and R. L. Parker (1984), Inversion of ocean bottom
magnetotelluric data revisited, J. Geophys. Res., 89, 1829—1833.
Seama, N., K. Baba, H. Utada, H. Toh, N. Tada, M. Ichiki, and T. Matsuno (2007), 1-D
electrical conductivity structure beneath the Philippine Sea: Results from an ocean bottom
magnetotelluric
survey,
Phys.
Earth
Planet.
Int.,
162,
1—2,
2—12,
doi:
10.1016/j.pepi.2007.02.014.
Stein, C. A. and S. Stein (1992), A model for the global variation in oceanic depth and heat flow
with lithospheric age, Nature, 359, 123—129.
Suggestions for the future drilling sites for the understanding global
dynamics of Earth, such as Hotspot Drift and TPW.
Yasushi Harada (Tokai University)
Recently hotspot drifts are suggested by several researches of both
simulations and paleomagnetic observations.
For instance, southward motion of the Hawaiian hotspot plume relative to
the paleomagnetic axis has been suggested by Tarduno et al.,2003.
Since 1970's, the paleolatitudinal shifts are already known for the
Pacific hotspot tracks, and paleolatitude of global hotspot tracks are
known to significantly change with time.
But the change of hotspot paleolatitude doesn't necessary deny the fixed
hotspot approximation. There is a case that the group of hotspots are moving
relative to the paleomagnetic axis (aka, True Polar Wander).
Jurdy,1981 compiled paleolatitude data of eight major hotspots and
concluded that the inter-hotspot motions are not significant but mantle
are rotating with respect to spin axis (aka, Mantle Roll).
The paleolatitude data of Tarduno et al.,2003 are quite in harmony with
the mantle roll theory.
One thing I would like to put stress on is that current standard model
of true polar wander path (Besse and Courtillot, 2003) depend highly on
the old model of absolute motion of the African plate (Morgan, 1983), and
the model is obsolete and significantly different from a new model which
is created recently using Poligonal Finite Rotation Method (Harada and
Hamano, 2000). The new African absolute motion is more consistent on both
location and ages of the African hotspots than any other model.
The new TPWP model (Figure 1) which is made from the new model of African
plate motion is so different from the one by Besse and Courtillot, 2003
especially in around 70-50 Ma poles. The new TPWP model is in quite harmony
with Mantle Roll by Jurdy,1981.
To solve the current controversy about whether hotspots drift or not, we
should clarify the real TPWP. For this purpose, we should drill hotspot
tracks which located in area around longitude 30W and 150E, because the
two models are most distinguishable with the area.
Super-Mohole project: New insights on understanding of oceanic
lithosphere from petit-spot volcanoes and the 21st century Mohole
Naoto Hirano (Center for Northeast Asian Studies, Tohoku University)
The petit-spot magmas escape to the surface along brittle fractures of the flexed upper lithosphere
and occur in the asthenosphere without the magma fed by mantle plume (Hirano et al., 2001; 2006).
Entrained xenoliths composed of all kind of the crust to mantle clearly show brittle fractures of upper
lithosphere (Hirano et al., 2006; Abe et al., 2006). Although we have already had the Cretaceous
oceanic basement on the Pacific Plate by some ODPs, all of them are only lavas. Therefore, only the
xenoliths from the petit-spot provide us with the information of an old lithosphere.
An outer-rise forms during subduction when old and cold lithosphere bends so that it can
sink into the interior of the Earth at the trenches. This flexing is mostly an elastic behavior, but it may
also cause brittle fracturing of the downgoing slabs. For example, the Japan, Chile and Tonga
trenches are possible sites to find good examples of monogenetic and fracture-related petit-spot
volcanoes since the Pacific Plate shows clear evidence of upward flexing at these subduction zone
locations based on the reflectivity data to be detected by multibeam bathymetry. Mofjeld et al.
(2004) also speculated that larger curvatures superimposed on the pre-flexed lithosphere might
induce brittle fracturing (e.g. bending-induced faults) of the upper lithosphere and the magma
extraction to the surface along such fissures at some irregular knolls on the bathymetry of oceanward
slope of the Tonga Trench. Therefore, we anticipate that petit-spot volcanic activity is an ubiquitous
phenomenon on all flexed parts of the subducting Pacific Plate and all subducting tectonic plates
globally. Fissures forming in the outer-rises seem to be the logical mechanism allowing
asthenospheric melts to escape to the surface and to form young petit-spot volcanoes
Here, we would comprehensively understand the oceanic lithosphere using both the
xenoliths of the petit-spots and the drill core of Mohole. Although the Mohole provide the sequential
information from oceanic crust to the mantle, the xenoliths are each rock-fragments of the upper
lithosphere. Although the xenoliths of the petit-spot provide the fragments of old and cold lithosphere,
the Mohole would be done at the young plate. Moreover, it might be difficult to know the “normal”
lithosphere by the one site Mohole. But, as mentioned before, the petit-spot magmas could occur
wherever the plate flexes and fractures and entrain xenoliths. The study of the xenoliths of petit-spot
would be most powerful tool to understand the oceanic lithospheres with the “21st century Mohole”.
Figure: Bathymetrical map of the
Pacific
Plate.
White
area
is
shallower than 2000 mbsl. Black
area show the deeper part than
8000 mbsl, which is correspond to
trenches. The data is provided by
Smith and Sandwell (1997). Black
stars show the petit-spot sites
reported by Hirano et al. (2006;
2008). Black circles are possible
Mohole sites around Pacific Plate
suggested in the mission proposal
of the 21st century Mohole.
Renfernces
Abe et al. (2006) Geocheim. Cosmochim. Acta, 70, A1-A1.
Hirano et al. (2008) Basin Res., 20, p. 543-553
Hirano et al. (2001) Geophys. Res. Lett., 28, 2719-2722.
Mofjeld et al. (2004) Oceanography, 17, 38-46.
Hirano et al. (2006) Science, 313, 1426-1428.
Smith & Sandwell (1997) Science, 277, 1957-1962.
INVEST domestic WS –Earth’s Interior- 2008/12/05
Fukashi Maeno (Earthquake Research Institute, Univ. Tokyo)
Suggestion for future IODP projects:
Drilling submarine pyroclastic deposits emplaced during 7.3 ka caldera-forming
eruption at Kikai caldera, Japan
[Introduction]
Caldera-forming eruptions, which may erupt up to a few thousands of km3 of magma, are
catastrophic volcanic events that pose one of the great natural hazards on earth. Such
eruptions are low frequent events, but their impacts are very severe. Even relatively small
caldera-forming eruptions may result in several thousands deaths, and alter the global climate.
While most recent work on recent eruptions is focused on on-land events, many
Quaternary caldera-forming eruptions have occurred in areas of deep/shallow seas, with the
production of voluminous pyroclastic flows. Such eruptions are therefore a crucial part of the
record of silicic magmatism.
The 1883 eruption of Krakatau in Indonesia and the 3.5 ka Minoan eruption of Santorini
in Greece are the most famous examples of recent marine caldera-forming eruptions. Other
Quaternary marine silicic calderas have been also discovered on subduction zones and near
ocean islands; the Shichito-Iwojima Ridge, Izu-Ogasawara (or Izu-Bonin) Arc, the Kermadec
Ridge north of Taupo volcanic zone, New Zealand, and the Hellenic Island Arc in Greece.
Although these eruptions must have significantly and devastatingly affected the
development of coastal human activities and environments around the volcanoes, they still
remain speculative and controversial, especially with respect to the effects of seawater on
dynamics and evolution of such large-scale silicic marine eruptions. The reasons for this
include the rare occurrence and violent nature of this type of eruption, the lack of direct
observations, and difficulties arising from global variations in sea levels and local tectonic or
volcano-tectonic effects.
The 7.3 ka eruption at Kikai caldera, Japan, occurred in a shallow sea. The volcanic
explosivity index (VEI) is 7, based on total volume of products (150-170 km3), which was
larger in scale than the celebrated Santorini and Krakatau eruptions (VEI is 6), and this
eruption was the largest in the last 10 ka in Japan.
Previous researchers characterized a major sequence of the deposits in the 7.3 ka Kikai
eruption. They showed that the eruption produced a climactic pyroclastic flow that traveled
over the sea and resulted in devastating damage to prehistoric human activities in southern
Kyushu. However, they did not carry out a precise analysis of the lithofacies in the eruptive
deposits especially in the sea.
[Potential works]
The detail of submarine deposits in and around the Kikai caldera is not well known. So, I
propose IODP to challenge to drill the intra- and outer- deposits from caldera-forming
eruptions at Kikai caldera. The deposits may include welded or non-welded thick pyroclastic
flow deposits and lava flows. Total thickness of deposits outside caldera is probably less than
a few hundred meters.
(1): Detail survey of submarine deposits around the Kikai caldera will enable us to reconstruct
the 7.3 ka caldera-forming eruption; especially, depositional processes of pyroclasts in
proximal area, dynamics of huge pyroclastic flows entering sea or traveling over sea and their
depositional/welding processes, and so on.
We can discuss eruption dynamics and magma-water interaction based on analyses of the
stratigraphy, textural, and lithofacies characteristics, and components of the deposits. How are
the distribution, thickness, degree of welding, and internal structures of huge marine
pyroclastic flow deposits? There are no such detail descriptions in previous researches for any
calderas; so, it will be very important part of ongoing studies of marine silicic eruptions.
(2): Detail survey of intra-caldera deposits and inner structure of the Kikai caldera will enable
us to evaluate the caldera-collapse and resurgent processes of the recent caldera-forming
eruption. The formation processes of ring-fault and intra-deposits are not well known, which
is classic problems on volcanology. The timing of resurgent domes and their characteristics
are also important for understanding the recent volcanic activities of Kikai caldera.
(3): Petrological and geochemical data of submarine rock samples in and around Kikai caldera
will be keys for understanding large silicic magmatic systems (LSMSs) belong to
Kyushu-Ryukyu Arc.
(4): Above three works will be linked with hazard assessments caused by huge pyroclastic
flows, ash-cloud, volcanic gases, and tsunamis, which will devastatively impact on very wide
areas. Such studies can provide constraints on predicted patterns for future volcanic activity.
Memo:
Recently, Japan Coast Guard got detail bathymetric data of Kikai caldera. It will be useful for
evaluating drilling sites.
The Dream team for the 21st Century MOHOLE project
Tomoaki MORISHITA
Frontier
Science
Organization,
Kanazawa
University,
Kakuma,
Kanazawa
920-1192,
Japan.
[email protected]
Sumiaki MACHI
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa 920-1192,
Japan. [email protected]
Introduction: What we need?
The 21st Century MOHOLE project is the only way to recover real mantle samples beneath ocean
floor, and is, therefore, should be a frontier science to understand an unsolved important question on the Earth
since its-finding at the beginning of 20th Century, “What is the MOHO?” Before we will show our scientific
proposal for other scientific groups in any scientific workshops, we would like to emphasize here the
significance of organization of the special analytical team for the 21st Century MOHOLE project. We must
clearly provide information on what kinds of data we will get from the samples by ourselves, even though only
cuttings through Raiser drilling will be available for analyses. Ideally we had better list up every scientist who
will be responsible person for each topic. We, therefore, listed up several scientific topics and candidates for
these topic as shown below although we have never discussed on the 21st Century MOHOLE project with all
scientists we listed up here. We also emphasize importance of outreaches and put senior level scientific leaders
to complete the Project.
Petrological descriptions with major elements analyses of the samples
This part is the most basic and is, therefore, the most important work for the following analytical
works. We will get basalts including dolerite dikes, gabbros and peridotites and their altered rocks during the
drilling. First of all, we must describe accurately petrological information of these samples. Prof. Umino of
Kanazawa Univ., Prof. Miyashita of Niigata Univ., and Prof. Arai of Kanazawa Univ., Dr. Takazwa of Niigata
Univ., Dr. Ohara of Hydrographic and Oceanographic Department of Japan, and Dr. Abe of JAMSTEC are
candidates for basalts, gabbros and peridotites, respectively. Basic analytical instruments for major element
analyses of minerals and whole-rock compositions, such as XRF and EPMA, are well organized in these
laboratories.
Trace element compositions
Trace element abundances in each phase in all lithologies will give us constraints on petrogenesis
and their histories. Laser-ablation ICPMS technique is now widely accepted to analyze multiple trace element
compositions in minerals in short time. We fist quick look at trace element signatures of minerals in each
sample and then we focus on a specific samples with careful analytical technique such as SIMS if we need.
Prof. Arai of Kanazawa Univ. and Prof. Hirata of Kyoto Univ., and Prof. Yurimoto of Hokkaido Univ., Prof.
Sano of Tokyo Univ. and Misasa-lab will be candidates for LAICPMS and SIMS, respectively. We also need
to consider whole-rock chemical compositions of representative samples. ICPMS in a well-organized clean lab.
is required for whole-rock trace element compositions for peridotite because of low abundances of trace
elements in peridotites. Dr. Takazawa of Niigata Univ., Dr. Nakamura of Tokyo University and Dr. Kimura of
JAMSTEC are candidates.
We will expect for the first time to get a fresh peridotite samples which allow to understand “grain
boundary effect”. There is an important question whether or not grain boundaries in peridotites are reservoirs
of trace elements (Hiraga et al., 2004 Nature 427, 699-703). It is worth to try to analyze element concentrations
at grain boundaries in the fresh peridotite samples using TEM. Dr. Hiraga of Tokyo Univ. should be a suitable
person for this.
Multiple Isotope compositions (Nd, Sr, Pb, Os, Hf, Li, B, C, O, and Noble gas)
Multiple isotope compositions give us useful information on geochemical evolution of the Earth,
age constrains and so on. Sr, Nd and Pb contents in peridotite samples are, however, usually very low for
isotopic analyses. We need a specific laboratory for each isotope for peridotite samples. Dr. Yoshikawa and Dr.
Shibata of Kyoto Univ. (Beppu branch) should be incorporated in the special team. In-situ isotope analyses on
minerals will be achieved by multi-collector ICPMS coupled with laser-ablation systems or micro-drilling
technique. Dr. Kimura of JAMSTEC and Prof. Hirata of Kyoto Univ. are candidate for these analyses. Dr.
Suzuki of JAMSTEC should be responsible person for Os isotope analyses on peridotites. Li and B isotopes
are useful to constraints on fluid mobilization. Dr. Nishio, and Dr. Ishikawa of JAMSTEC are candidates for Li
and B isotopes, respectively. Dr. Kumagai of JAMSTEC and Dr. Matsumoto of Misasa-lab. should be
candidates for noble gas analyses. We are not familiar with C-O-isotope communities in Japan.
Hydrogen (water) content in nominally anhydrous minerals
Hydrogen (water) in minerals will contribute to change the physical properties of minerals and,
therefore, controls global tectonics in the Earth. Water contents in nominally anhydrous minerals, such as
olivine, pyroxenes and garnet, have been reported in many mantle-derived samples (e.g., Bell & Rossman,
1992 Science 255, 1391-1397; Beran & Libowitzky, 2006 Rev. Min. Geochem., 62, 169-191; Skogby, 2006
Rev. Min. Geochem., 62, 155-167; Gose et al., 2008 Amer. Min., 93, 1613-1619) but have never been reported
from less altered peridotites beneath normal ocean floor. Water contents in minerals will be measured by FTIR
and SIMS. Candidate scientists are Dr. Katayama of Hiroshima Univ. and Prof. Nakashima of Osaka Univ.,
and Prof. Yurimoto of Hokkaido Univ, respectively.
Fluid inclusions
Fluid inclusions (and their relics) are commonly observed in mantle-derived rocks (e.g., Arai &
Hirai, 1985 Nature 318, 276-277; Yamamoto et al., 2002 EPSL 198, 511-519; Mizukami et al., 2008 Geology
36, 219-222) and give us direct information on flux in the mantle. Phases and conditions (gas or fluid) are
determined by Raman spectrometry. Dr. Kagi of Tokyo Univ., Dr. Yamamoto of Kyoto Univ. (Beppu branch)
and Dr. Mizukami of Kanazawa Univ. are candidates for Raman spectrometry on fluid inclusions.
Metamorphism/Alteration
The hydrothermal circulation of seawater into the oceanic lithosphere causes metamorphism and/or
alteration in constituent rocks and is, therefore, an important factor controlling seawater chemistry and
compositions of rocks where seawater pass through. In fact, hydrothermally altered rocks were widely
recovered from the ocean floor. Details of pressure-temperature conditions and processes for metamorphism
and/or alteration of rocks due to infiltration of seawater are, however, not clear. We need metamorphic
petrologist, alteration geochemist and specialists on clay minerals. Prof. Kawahata of Tokyo Univ., Dr.
Nakamura, D. of Okayama Univ., Dr. Nakamura, K. of Tokyo Univ. and Dr. Fukushi of Kanazawa Univ. are
candidates. High-resolution EPMA should be useful to explore textures and mineral assemblages in altered
portion. Dr. Shukuno of JAMSTEC is candidate for FE-SEM-WDS.
Dating
Zircons and apatite are minerals usually used for dating of their-bearing rocks. These minerals will
be observed as accessory minerals in a wide range of rocks, e.g., leucocratic veins in peridotites (Morishita et
al., 2004 Amer. Min., 89, 759-766) and highly evolved gabbros. There are several analytical methods for these
minerals. U-Pb ages by SHRIMP and/or LA-ICPMS and Fission track are very common methods. Dr. Tani of
JAMSTEC and Misasa-lab. on zircon dating by SHRIMP, Dr. Orihashi of Tokyo Univ and Prof. Hirata of
Kyoto Univ. on zircon by LAICPMS, Dr. Terada of Hiroshima Univ. on apatite by SHRIMP, and Dr. Hasebe
of Kanazawa Univ. on zircon and apatite by fission track dating are candidates.
Ar-Ar dating on basalts give us information on timing of the magmatism in the drilling hole. Dr.
Ishizuka of Geological Survey of Japan is candidate for Ar-Ar dating.
Structures and crystallographic orientations of minerals
Although it is difficult to get information on structures and crystallographic orientations of minerals
from cuttings, we need to prepare for getting data on structures. SEM-EBSD system is now widely accepted
for standard analytical technique for crystallographic orientations and grain size distributions in the samples.
Prof. Kanagawa of Chiba Univ., Prof. Ozawa of Tokyo Univ. and Dr. Michibayashi of Shizuoka Univ. should
be candidates for analyses. Nano-sacle observations by TEM in constituent minerals should be useful and will
be done by Dr. Ando of Hiroshima Univ.
Geodynamic models
We need to organize theoretical group who use geochemical data to reconstruct melting model and
tectonic model beneath the ocean floor. Prof. Ozawa of Tokyo Univ., and Dr. Iwamori and Prof. Honda of
Tokyo Univ. are candidates for melting model and geodynamic model, respectively.
Input from ophiolites and xenoliths in petit spot volcanoes
One drilling site is apparently not enough to understand what the MOHO is. We need input from
on-land ancient ocean floor, i.e., ophiolites, and new material information of normal ocean lithosphere from
xenoliths in petit spot volcanoes recently found by Hirano et al. (2006 Science 313, 1426-1428). In order to
support and to be widen our knowledge from one drilling site, we must continue to work on ophiolites and
xenoliths in petit spot volcanoes in the context of the 21st Century MOHOLE project. Prof. Ishiwatari of
Tohoku Univ. and Dr. Anma of Tsukuba Univ., and Dr. Hirano of Tohoku Univ. are candidates for ophiolites
and petit spot volcanoes, respectively.
Outreaches
Outreach activity of 21st Century MOHOLE project is very important. We must put a specific team
for outreaches on our scientific activities. If we got a new result, even though if we do not have new data, we
must explain the scientific target easily for any kinds of people, especially for kids. If we have a chance, we
must go to school, even in a small town meeting, and show what we found with materials recovered by
CHIKYU. We must organize a special volume on our results in popular scientific journal or news papers.
Concluding remarks
As we mentioned at the beginning of this statement, we did not have any agreements from scientist
we listed up above. We have understood that much more scientists and scientific groups work well and we
need to ask them to cooperate in the 21st Century MOHOLE project. Beyond these problems, we absolutely
need a strong senior scientific leader or strong scientific and financial supports from all Earth Scientific
communities in Japan for young scientific leader. The most important leaders for 21st Century MOHOLE
project, Prof. Arai of Kanazawa Univ. and Prof. Miyashita of Niigata Univ., are over 60 years old. They are
still active but may be retired from the position now when the drilling will start. Scientific leaders in other
countries are sometimes looking for a Japanese scientific partner, particularly senior level (personal
communications with H. J. B. Dick of WHOI). The 21st Century MOHOLE project will be completed by
CHIKYU. We, Japanese, must be a leader for the Project.
Joint Japan-Korea drilling programme in Okinawa Trough – a tentative plan
Takeshi Matsumoto
Faculty of Science, University of the Ryukyus
The Okinawa Trough is located along the south-eastern margin of the East China Sea. Besides the
extension-rifting tectonics, the trough includes some other key issues to solve the problems to understand its
evolution history. They are: (1) existence of hydrothermal venting, (2) channel-levee systems on the
westernmost part of the trough, and (3) across-arc normal faults which is located around Miyako and Yaeyama
districts.
Some scientists proposed different hypotheses about the evolution history of the Okinawa Trough.
These are:
(1) The extension of the Okinawa trough was triggered in Late Miocene due to the collision of the Luzon arc
and the subsequent clockwise rotation of the Ryukyu arc (Miki, et al., 1995; Sibuet and Hsu, 1997).
(2) Rifting of the volcanic zone along the arc took place in Late Pliocene - Early Quaternary (Letouzey and
Kimura, 1986).
(3) The convergence direction of the Philippine Sea Plate against the Eurasian Plate was changed from NNW
into ENE in Early Quaternary, which triggered the Ryukyu back-arc pull-apart motion along the across-arc
dextral faulting (Kong et al, 2000).
However, these may be the three steps of the evolution of the Okinawa Trough which may still be in the
immature stage of the breakup of the south-eastern margin of the Eurasian continent.
Other unknown factors about the Okinawa Trough rifting and the Ryukyu arc tectonics history are
listed as follows:
(1) Which was first to start rifting, north or south?
(2) Is (was) the Okinawa Trough in passive rifting (drifting) or active rifting (extensive magma intrusion)
stages?
(3) Is there any relationship between Okinawa Trough and IBM?
(4) East-west asymmetry of Okinawa Trough
(5) Formation of the NE of Taiwan-Sinzi Fold Zone
(6) Palaeozoic metamorphic rocks from Ryukyu arc – corresponding to Korea Peninsula metamorphic block
(7) Is there any relationship with the metamorphic belt in Japan?
To solve these problems, we decided to start the Joint Japan-Korea drilling programme in Okinawa
Trough.
The Okinawa Trough is also a very interesting place to study palaeoceanography/paleoclimatology.
Land bridge, or land-ocean linkage of the Ryukyu area may provide detailed information on East Asian
palaeoclimatology. The palaeoceanographical/paleoclimatological study is also essential to know more about
Kuroshio current and climate pattern in North Pacific: when it started and how it has evolved, Indonesian
gateway influences on the evolution of Kuroshio currents, etc. The hypotheses to be tested in this field are:
(1) Tectonics-related Younger-Dryas discharges
(2) Sedimentation history in Okinawa Trough
(3) Variability in the Kuroshio and subtropical ocean and atmosphere interaction
We will also solve these problems by using SST proxies, Kuroshio Current indicator microfossil
proxies, salinity proxies, nutrient proxies from the drilling samples which are to be collected in Milankovich or
more high resolution time scale.
The joint Japan-Korea team held a special scientific symposium and workshop on 21-22 September
2008 in Akita during the annual meeting of the Geological Society of Japan for the purpose of generalizing the
pre-existing results about the Okinawa Trough studies and the related works. The project team is aiming at
submitting a pre-proposal for IODP drilling before 01 October 2009. After compiling the pre-existing data, the
team will hold a two-days’ scientific meeting in late June 2009 for preparation of a pre-proposal submission.
Strategy of complete drilling through oceanic crust to mantle
Miyashita Sumio and Susumu Umino
Fundamental purpose: Comprehensive understanding for the architecture, substance and
its generation mechanism of oceanic crust
Geophysical models on the oceanic crust beneath fast-spreading ridges show thin melt
lens underlain by thick semi-consolidated crystal mush that extend to Moho
discontinuity. This is generally agreed with many scientists, however, it is controversial
that how to accrete the thick lower oceanic crust constituting about two third of the
crust from the thin melt lens.
Another important question is geometry of layering in gabbroic section; dipping toward
ocean ridge or opposite. This problem is related to two contrastive models in mantle
upwelling; active or passive upwelling. Origin of igneous layering is also problematic;
igneous origin or highly plastically deformed structure. Furthermore, two end-member
models for the genesis of the foliation in foliated gabbros interlay red between massive
upper gabbro and layered gabbro are proposed; downward flowage from the melt lens
or upward buoyant flow from depth.
Complete penetration through oceanic crust enable to solve above many fundamental
questions. However, the thickness of gabbro may attain about 4 km, so that, following
strategy is proposed.
1. From upper gabbro to layered gabbro (about 500 m thick?): all coring as possible as,
because, many unsolved questions are concentrated in this interval. This may be done
by pilot hole by JR or Chikyu.
2. Spot coring for thick-layered gabbros, at three to four places at each 500 to 800 m
apart. Each spot coring should be done by one drill bit for 50 to 70 m interval. Using
these cores, precise petrological, geochemical, and geophysical analyses may give new
insight for the genesis of igneous layering and layered gabbro mentioned above.
Coreless intervals are carefully monitored by cuttings and rogging. If some abnormal or
interesting phenomena were detected during the interval, side coring would be tried.
Spot coring is very significance to keep the motivation of petrologists and structural
geologists
3. 100% coring for the Moho transition zone to mantle peridotite, This is the final
purpose of this mission; nature and origin of the Moho discontinuity.
Paleointensity study of the Cretaceous Normal Superchron combining IODP drilling and
marine magnetic anomaly observation
Nobutatsu Mochizuki, Toshitsugu Yamazaki, Hirokuni Oda (Geological Survey of Japan, AIST)
Yuhji Yamamoto (Kochi University)
Futoshi Takahashi (Tokyo Institute of Technology)
The mean polarity intervals of the geomagnetic field for the last 160 million years are within the
order of 105 years, which is one of the intrinsic characters of the dynamo of the deep interior of
the Earth. The typical timescales of the core dynamics are estimated to be 10-104 years,
therefore, exceptionally long lasting polarities of the order of 107 years, which are called
“superchron”, are thought to be related to a change of the mantle. For example, a recent study of
numerical dynamo simulation shows that large heat-flux heterogeneity with equatorial
symmetry enhances strength of the dipolar magnetic field and suggests that thermally
heterogeneous structure of the lowermost mantle might give rise to an anomalously strong
geomagnetic field such as that during the Cretaceous Normal Superchron (CNS) (Takahashi et
al., 2008). From this point of view, recovering the paleomagnetic field intensities including CNS
have been an essential and fundamental topic in geodynamics, which should be noted in the
IODP Initial Science Plan 2013-2023, though this topic was mentioned in the Initial Science
Plan 2003-2013 (page 53).
Many paleointensity studies of CNS have been made using whole-rock basalts (e.g. Tanaka and
Kono, 2002; Zhu et al., 2008), single crystals (Tarduno et al., 2001; 2002) and submarine
basaltic glasses (e.g. Pick and Tauxe, 1993; Tauxe et al., 2006). The number of the absolute
paleointensity data has increased in the last decade. Those paleointensities during CNS appear
to be higher than those of before/after CNS. However, there are differences in distribution of
data between different rock materials and therefore further improvements of methods and/or
data analyses are required to obtain accurate paleointensity data. In order to clarify how
strong/weak the paleointensities of CNS is and how large/small the standard deviation
(variability) of paleointensities during CNS, we need to obtain a reliable paleointensity data
from another approach to clarify the paleointensity variation of CNS.
Recent studies reported that small-amplitude total-intensity magnetic anomalies, so called tiny
wiggles, can be correlated between different oceans and therefore these tiny wiggles contribute
to paleomagnetic intensity studies. In particular, in relatively long polarity chrons such as the
Brunhes chron and Chron 5A, tiny wiggles reflect paleointensity of the geomagnetic field
(Roberts and Lewin-Harris, 2000; Bowers et al., 2001), and have resolution comparable to the
relative paleointensity records obtained from sediments（e.g. Gee et al., 2000; Yamamoto et al.,
2005）. In addition to these studies, we recently measured marine magnetic anomalies of the
early Cretaceous oceanic crust at sea-surface and detected small-amplitude anomalies though
deep-sea observation is required for high-resolution detection of these magnetic anomalies.
Here we propose a paleointensity study of CNS combining marine magnetic anomaly
observation and IODP drilling. First we will obtain relative paleointensity variation of the order
of 104-105 years based on an inversion from deep-sea magnetic anomaly of the Cretaceous
oceanic crust. On the basis of the relative paleointensity curve, we can choose several drilling
sites covering one-cycle of paleointensity secular variation of the order of 105 year which is
retrieved from several tens km along the spreading direction of the crust. Several drilling sites
are located within a distance of several tens km and then collected basaltic glasses of the rim of
pillow basalts are used for absolute paleointensity determinations (e.g. Pick and Tauxe, 1993;
Tauxe, 2006). We can obtain a mean absolute paleointensity for each drilling site from many
units of pillow basalt. Using absolute paleointensities of each drilling site, we can calibrate the
relative paleointensity curve of one-cycle of secular variation of the order of 105 year into an
absolute paleointensity curve.
Assuming the Layer 2A reaches mature thickness within 3 km of the axis (e.g. Harding et al.,
1993) and the half-spreading rate is 95 mm/yr (Cognè and Humler, 2006), a vertical section of
the Layer 2A (~500m) is estimated to be formed within 30000 years. The 200-300 m drilling
depth, corresponding to an interval of 12000-18000 years, is enough for averaging out the
secular variation of less than 104 years. But the interval of 12000-18000 years may be shorter
than resolution of relative paleointensity obtained from deep-sea magnetic anomaly. Closely
spaced three drilling sites at 1.1-1.7 km interval, which correspond to 36000-54000 year record,
should be set some of the several sites.
This study combining marine magnetic anomaly and absolute paleointensity study of the
basaltic glasses is a new approach for a continuous absolute paleointensity variation of the
geomagnetic field during CNS and is expected to give a clear constraint on the dynamo of the
Earth.
References
Bowers et al., J. Geophys. Res., 26379-29396, 2001.
Cognè and Humler, Geochem. Geophys. Geosyst., 7, Q03011, doi:10.1029/2005GC001148,
2006.
Gee et al., Nature, 408, 827-832, 2000.
Harding et al., J. Geophys. Res., 98, 13,925–13,944, 1993.
Pick and Taxue, Nature, 366, 238–242, 1993.
Roberts and Lewin-Harris, EPSL, 375-388, 2000.
Takahashi et al., EPSL, 272, 738–746, 2008.
Tanaka and Kono, Phys. Earth Planet. Inter., 133, 147–157, 2002.
Tarduno et al., Science, 291, 1779–1783, 2001.
Tarduno et al., Proc. Nat. Acad. Sci. U.S.A., 99, 14020–14025, 2002.
Tauxe, Phys. Earth Planet. Inter., 156: 223–241, 2006.
Yamamoto et al., Earth Planets Space, 57, 465-470, 2005.
Zhu et al., Phy. Earth Planet. Inter., 169, 59–75, 2008.
Water-rock interaction on hydrothermal circulation in
the complete sequence of oceanic crust at mid-ocean
ridges by multiple isotopic analyses
Kyoko YAMAOKA
Ocean Research Institute, The Univ. of Tokyo
Submarine hydrothermal systems at mid-ocean ridges play important role to
transport heat and component since starting of plate tectonics on the earth. Seawater
circulation within the oceanic crust is strongly controlled by the thermal regime and the
permeability, both of which are strongly influenced by magmatic and tectonic processes.
On the other hand, hot volcanic rocks and magma chamber are cooled and changed their
physical properties by seawater circulation. It is essential to reveal the interaction
between hydrothermal circulation and magmatism for understanding the crustal
evolution. In addition, hydrothermal alteration of the oceanic crust changes the
chemistry of crustal material that returned to the mantle by subduction. These
water-rock interactions also constrain the compositions of seawater on geological time
scale.
A number of studies including geophysical exploration, geochemical analysis of
crustal material and hydrothermal fluid have attempted to construct the predictive model
of hydrothermal circulation. Recent studies suggest that seawater penetrate close to
magma chamber and react with hot rock at high temperature. Under such high P-T
condition, supercritical phase separation would take place and a small amount of dense
brine is formed. It is proposed that the brine accumulates and remains relatively stable
in the deep crust. However, we have not acquired enough samples to confirm such deep
hydrothermal circulation yet. Recovering of continuous drill core through whole
sequence of the oceanic crust will dramatically expand our knowledge on submarine
hydrothermal processes.
Fundamental questions to understand are depth and amount of seawater circulating
in the oceanic crust, reaction temperature, composition of hydrothermal fluid, and
water-rock reaction under supercritical condition. In order to reconstruct past
seawater-rock interaction from crustal materials, multi-isotope (strontium, oxygen,
hydrogen, sulfur and boron) geochemical technique is useful. Each isotope ratio is a
proxy, as follows;
1) Sr isotope ratio (87Sr/86Sr): water/rock ratio (temperature-independence)
2) Oxygen isotope ratio (δ18O): temperature, water/rock ratio
3) Hydrogen isotope ratio (δD): temperature, phase separation
4) Sulfur isotope ratio (δ34S): redox condition
5) Boron isotope ratio (δ11B): water/rock ratio.
The compile of information induced by each isotope lead us to comprehensive
understanding. Especially, boron isotope could be a new powerful tracer of seawater
circulation because of its high solubility and large isotopic fractionation (~40‰)
between fresh oceanic crust and seawater.
Ophiolite is appropriate to establish this technique because it is the best analogy of
the oceanic crust at present. It is important to make the complete profile of these
isotopes and elucidate these behaviors during water-rock interaction. This multi-isotope
approach is applicable to the direct drilling core of the oceanic crust and develops the
model of hydrothermal systems in the future.
Moreover, the upper mantle peridotite also should be investigated using this
technique to determine the depth of seawater circulation. The previous studies on
ophiolites suggest that seawater locally penetrate into the upper mantle. For
constraining the water and elemental flux from the earth’s surface to interior, it is
essential to estimate hydrothermal alteration of the upper mantle quantitatively.
Hydrothermal
circulation
is
more
active
at
fast-spreading
ridge
than
slow-spreading ridge because of the shallow and steady magma chamber at
fast-spreading, although large fractures and faults could cause deep penetration of
seawater at slow-spreading ridge. Considering that the fast-spreading ridges account for
>50% of total mid-ocean ridges, the oceanic crust formed at fast-spreading ridge is
favorable to evaluate the effect of hydrothermal circulation on both crust and seawater.
In addition, recharge zone where is close to a segment boundary is desirable to compare
the isotope profile of the in-situ oceanic crust with that of the ophiolite.
Construction of the continuous global paleomagnetic field model
for the last few million years
YAMAMOTO, Yuhji (Center for Advanced Marine Core Research, Kochi University)
Remanent magnetizations of igneous rocks and marine sediments reflect the past
geomagnetic field at their formation or post-deposition. We can retrieve ‘instantaneous’
paleomagnetic field from igneous rocks while ‘continuous’ variation from marine
sediments. In terms of the intensity of the paleomagnetic field (paleointensity), igneous
rocks give absolute paleointensities whereas marine sediments provide relative
paleointensities. Ideally, paleomagnetic records should be obtained both from igneous
rocks and marine sediments and be integrated for a complete understanding of the
ancient geomagnetic field. This understanding is important to investigate the Earth’s
core dynamics.
We have now known that the time-average of the ‘recent’ geomagnetic field is a
geocentric axial dipole field. Although its magnitude estimated from volcanic rocks is
still controversial (e.g. 7.26 x 1022 Am2 for 0-1 Ma, Ziegler et al., 2008; 3.64 x 1022 Am2
for 0.5-4.6 Ma, Yamamoto and Tsunakawa, 2005), continuous time variation of the
dipole moment (relative value) had been reported as stacked curves from marine
sediments: for example, Sint-800 curve for the last 800 kyr (Guyodo and Valet, 1999),
Sint-2000 curve for the last 2000 kyr (Valet et al., 2005), and EPAPIS-3Ma curve for the
last 800-3000 kyr (Yamazaki and Oda, 2005). Paleomagnetic records recovered from
ODP cores had played important roles on these stacked curves (the Sint-800 and
Sint-2000 curves involved eight and five ODP records, respectively). Further efforts
have been made to construct longer time series for an older era and high-resolution
stacks for a younger period.
As for a young period, we should also make effort to increase ‘spatial resolution’.
Geomagnetic field is not solely the dipolar field and there is a not negligible amount of
contributions from the non-dipole components. It is known that both the dipole and
non-dipole components are time-varying and the latter cause local variations. For a
complete understanding of the global paleomagnetic field variation, it is important to
construct global continuous models. There have been three such models reported so far.
One of the models is the CALS7K (Korte and Constable, 2005). This had been
constructed using both published paleodirection and paleointensity data: the former
data were from lake sediments of 41 areas, and archaeological material and volcanic
rocks of 23 regions; the latter data were from archaeological material and volcanic rocks
of 17 regions. From a view point of the Earth’s core dynamics, for example, Wardinski
and Korte (2008) analyzed the core-surface flow over the last 7 kyr by inversion
analyses of this model. Other two existing global continuous paleomagnetic models are
for the last geomagnetic reversal (around 765-795 ka; Leonhardt and Fabian, 2006) and
for the well-known Iceland Basin geomagnetic excursion event (178-202 ka; Lanci et al.,
2008).
None of the three models cover time scale more than ~ 105 years. Considering the last
five million years, a geologically recent period, average duration of a one geomagnetic
polarity interval is about 2x105 years, based on the geomagnetic polarity time scale by
Cande and Kent (1995). We think it is important to construct a global continuous
paleomagnetic field model covering more than a time scale of ~ 105 years, in order to
comprehensively investigate the Earth’s core dynamics. One of the target periods should
be the last few million years, because there have been several stacked relative
paleointensity records reported so far (e.g. Sint-2000 and EPAPIS-3Ma) which have
already revealed dipolar variations.
For a construction of the global continuous model, we should need (1) few hundreds of
volcanic paleomagnetic records and (2) few tens of sedimentary paleomagnetic records
from all over the world. There have been relatively abundant type (1) data but they are
mostly from the northern hemisphere. We should need to obtain new data mainly from
the southern hemisphere. As for type (2) data, only several ‘usable’ records seem to have
been reported and they are also mostly from the northern hemisphere. Future IODP
expeditions will collect many sediment cores from whole world: a part of them is
thought to be potentially contributed to the type (2) data and they will accumulate as
time advances. For a better construction of the model, we should independently collect
some ‘key’ sediment cores from the Pacific and the Atlantic, particularly from the
southern hemisphere.
Geo-dynamo simulations are considered to be one of the most rigorous approaches to
investigate the Earth’s core dynamics. Recent advances in these simulations (e.g.
Takahashi et al., 2005; Kageyama et al., 2008) need better continuous global
paleomagnetic field models for verifications of the simulations. The paleomagnetic
model covering the last few million years will play an important role.
INVEST DOMESTIC WS —Earth’s Interior—
“21st Century Mohole —Researches on Magmatic Processes of the Moho Transition Zone
beneath the fast-spreading ridges”
Susumu UMINO (Kanazawa Univ.)
The main objective of the 21st century Mohole is to understand the Moho below the
ocean floor. It is of prime interest not only to know the constituents of the Moho but also
to understand the formation processes of the Moho. I propose to understand the critical
role of the Moho transition zone (MTZ) in the magma plumbing system at fast-spreading
ridges in terms of the genetic link between the off-ridge volcanism and the MTZ.
Mid-ocean ridge system, the world largest volcanic zone, produces approximately 80% of
the magma on the whole Earth that forms oceanic plates covering the 70% of the Earth’s
surface. Fast-spreading (>8 cm/yr) ridges yield 50% of the entire oceanic plates and are
critical in understanding the mechanisms of plate construction and the magma plumbing
system at plate divergent boundaries. Therefore, the primary target of the 21st century
Mohole is to penetrate the entire oceanic crust formed at a fast-spreading rate through
the Moho to the uppermost mantle.
Lithospheric minerals obtained from the ocean floors show progressive depletion of
incompatible elements with increasing spreading rate, indicative of higher melting
degrees of uprising asthenospheric materials beneath faster-spreading ridges (Niu and
Hekinian, 1997). However, the degree of melting estimated from inversions of rare earth
element contents in mid-ocean ridge basalts (MORBs) show a positive correlation with
the spreading rate only for slow-spreading (3-4 cm/yr) ridges, but is almost constant for
faster-spreading ridges (Brown and White, 1994). This discrepancy may arise from the
interaction of the uprising magma with the surrounding mantle peridotite through the
crust/mantle boundary that determines the final primary magma compositions supplied
to the crust.
Uprising magma through the uppermost mantle reacts with the surrounding peridotite
and changes its chemical composition toward silica-enriched, olivine- saturated melts
that leaves dunite after peridotite (Arai, 1999; Kelemen et al., 2000; Ozawa, 2005, 2008).
Magma trapped within the dunite differentiates to form gabbroic veins and pods enriched
in volatile and incompatible elements, comprising the Moho Transition Zone (MTZ) with
the host dunite. The MTZ tends to be thicker beneath fast-spreading than slow-spreading
ridges. The primary MORB composition that forms the crust is more likely to be
controlled by the MTZ reaction, which obscured the spreading-rate dependence of the
MORB composition for faster-spreading ridges. However, the axial lava compositions
were modified from the primary MORB compositions through the crystallization
differentiation and magma mixing in the lower crust and do not retain their primitive
signatures obtained by the MTZ reaction.
Meanwhile, a number of young seamounts and lava flows are known to have erupted >4
km off the southern East Pacific Rise (Geshi et al., 2007; Kishimoto and Hilde, 2003; Macdonald
et al., 1989; Perfit et al., 1994; White et al., 1998, 2002). ODP Holes 1256C and D drilled into
superfast-spread oceanic crust and penetrated through a 100-m thick ponded lava, which
is considered to be a part of a single large flow field that erupted and were emplaced >10
km off the ridge axis (Crispini et al., 2006; Umino et al., 2008). These off-ridge volcanoes are
distributed symmetrically on the both sides of the ridge axis, suggesting that they
comprise the magma plumbing system of the East Pacific Rise. Large Lava Fields (LLFs)
are specifically important because they possess a volume of 7-30 km3, which amounts to
several times of the global average annual magma production of 4-5 km3 (Crisp, 1984),
and potentially have a large influence on the Earth’s environment. LLFs are also known
from slow-spreading ridges (Umino, 1994). Thus, LLFs are the manifest of common
fluctuation of magma production rates which are involved in the magma plumbing
system at plate divergent boundaries. The off-ridge magmas have much wider spectrum
of compositions than the axial magmas, and span from depleted Normal through
Transitional to enriched MORB compositions. Furthermore, erupted volumes of LLFs
largely exceed the volumes of the axial magma chambers (< 2 km3; Hooft et al. [1997]). Both
the large volumes and variable magma compositions of the off-ridge volcanoes strongly
suggest the existence of independent off-ridge magma conduits to the axial magma
plumbing system. Therefore, the variable off-ridge magmas are more likely to retain the
primary chemical signatures obtained through the MTZ magma processes than the axial
magmas.
The knowledge of the primitive pre-MORB compositions that pass through the MTZ to
become the primary MORB forming the crust is critical to understand the processes of
magma production within the uprising mantle asthenosphere. The fast-spread crust, the
prime target of the 21-century Mohole, is overlain by the off-ridge volcanoes, and will
consequently drill through the off-ridge eruptive products and their genetically related
MTZ materials. With the knowledge of the both products of the MTZ reaction (the MTZ
dunite and the primary MORBs represented by the off-ridge products) and one of the
reactants (host mantle peridotite), we are able to determine the rest of the reactants,
pre-MORB magma compositions that formed within the upwelling asthenospheric mantle
beneath the spreading ridge.
Can we see the Earth’s core from a seismic observation below see floor?
Satoru Tanaka
IFREE, JAMSTEC
Seismological studies have revealed many aspects of the Earth’s core such as
seismic heterogeneity and anisotropy as well as possible temporal changes [see review,
Souriau, 2007]. So far, all of the achievements are coming from the seismic observations
on lands (continents and islands). Unfortunately, we have no important results for the
Earth’s core from seismic observations on sea floor because small core phases are masked
by large noises. Furthermore, pilot deep-sea borehole seismic experiments have not
considered whether the core phases are observable. In order to fill the observational gap,
seismologists thirst for observations in the deep oceanic areas. Here we discuss the
outlook of the seismic study on the Earth’s core with seismic observation below sea floor.
Seismic phases called PKP are transmitted in the mantle and core as P-waves,
which are suitable for the studies of the inner core and the bottom of the outer core.
Although we can observe them in a wide frequency ranges, short period components
around 1 seconds are particularly important in order to discriminate complicated branches.
Seismic anisotropy has been a robust feature in the inner core [Morelli and Dziewonski,
1987]. Recent studies are focusing lateral and radial variations of the anisotropy [Sun and
Song, 2008]. To date, the hemispherical structure of inner core anisotropy observed by
Tanaka and Hamaguchi [1997] is confirmed by recent many observations, and Aubert et
al [2008] proposed an interesting interpretation for the core dynamics. However, finer
structure is not clear because of the insufficient sampling. Especially, we have still a small
number of data sampling the arctic area where is important for understanding the tangent
cylinder in the outer core. Also, the discovery of inner core super rotation [Song and
Richards, 1996] is standing on long-term observations at a very small number of stations
in the polar regions. The innermost inner core proposed by Ishii and Dziewonski [2003] is
examined by antipodal PKP phases that are newly observed in China and Venezuela [Niu
and Chen, 2008]. On the other hand, SmKS phases are transmitted in the mantle as
S-wave, in the outer core as P-wave, and returned to mantle as S-waves after (m-1) time
reflection under the core-mantle boundary, which are suitable for the studies of the
outermost core. We can observe them in the period of several tens seconds. So far,
available SmKS data suggests a low velocity region in the outermost core compared to
that of PREM [Tanaka, 2007; Alexandrakis and Eaton, 2007]. However, the covered area
is less than half of the total surface.
Sampling areas depend on the location of seismic stations as well as
earthquakes. Since observable epicentral distance ranges for PKP and SmKS phases are
narrow and distribution of earthquakes is limited, it is worthwhile to discuss the possible
sampling area due to assumed seismic stations. The International Ocean Network (ION)
proposed 27 sites for future drilling sites to install broadband seismograph. Some of the
ION proposed sites are certainly appropriate for sampling unexplored regions such as
arctic by stations in North Atlantic as well as the other interesting area where can not be
obtained by land observations. We should understand that the noise level at a deep-sea
borehole station is suitable for our purpose. Here we look at the seismic noise spectra
observed at pilot deep-sea borehole broadband stations, e.g., WP-1 in the Philippine Sea,
WP-2 in the northwestern Pacific, and OSN in the central Pacific. At all of the stations,
the sensors are deployed in the basement below a sediment layer. The amplitude of the
noise spectra are much lower than that of a sea floor observation, especially for the
vertical component around the period of 1 seconds, and the horizontal components in the
period of several tens seconds. Comparing to the Low Noise Model (LNM) and High
Noise Model (HLM) that are defined from typical noise characteristics on lands, the noise
spectra of a deep-sea borehole stations do not exceed the HNM for the periods that we
concern. This indicates that a deep-sea borehole broadband seismic observation has
enough potential for the study of the Earth’s core. Actually, the broadband seismograms
at WP-1 detect possible signals of PKP and SmKS phases from South American deep
earthquakes but we do not find good signals in the seismograms at NOT1, a pop-up ocean
bottom seismograph deployed near WP-1.
As concluding remarks, I would like to briefly mention my hope. To achieve a
significant scientific result under modern conditions, it would be insufficient that an
isolated single station is deployed in the middle of an ocean as the ION proposed. A
multiple borehole stations in a close area that forms a seismic array or collaboration with
land stations would be effective in searching for the unexplored regions and highly
reliable observations.
Paleomagnetic problems to be solved beyond 2013 by IODP
Toshitsugu Yamazaki (Geological Survey of Japan, AIST)
In the present Initial Science Plan (ISP), the importance of the paleomagnetism is
recognized as the statement that “a more complete understanding of the variability of Earth’s
magnetic field through time, in both magnitude and direction, is an important component of
drilling studies of the Earth system”, although a paleomagnetic theme is not included in the
eight initiatives. The two specific problems, source of marine magnetic anomalies and a
possible relationship between the frequency of change in the polarity of Earth’s magnetic field
and major geodynamic events including that of the Cretaceous normal superchron and
superplume, are given in the ISP. These problems have not yet been settled at present, and will
continue to be the issues to be solved beyond 2013.
The most significant progress in the geomagnetism and paleomagnetism during the last
about 10 years would be numerical simulations of the geodynamo. The first result that
succeeded to make a geomagnetic polarity reversal was published in 1995 by Glatzmaier and
Roberts. At that time, however, parameters used for the simulation were still far from the
conditions in the core of the real Earth. Since then, simulations have become closer to the Earth
in accordance with the development of super-computers like the Earth Simulator. In 2005,
Takahashi et al. attained a simulation in a quasi-Taylor state, the Earth-like dynamo. When
considering new and revised strategies for paleomagnetism beyond 2013, it is necessary to try
to merge observations and simulations; drilling targets should be focused to obtain the data that
can give strong constraints to simulations. The present ISP lacks this viewpoint. From the
modern geodynamo simulations (see the abstract of Takahashi), the following observations will
be important for further progresses: construction of a time-averaged field (TAF) model
including non-dipole components (during the last ca. 5 m.y. for the target period at first),
relationship between the reversal frequency and the strength of the field, in particular the
paleointensity during the Cretaceous normal superchron, and differences of geomagnetic field
variations inside and outside the tangent cylinder (a virtual cylinder aligned with the rotation
axis in touch with the inner core at the equator). The proposals of Yamamoto, Mochizuki, and
Kanamatsu, respectively, target these problems. For constructing TAF models, the global data
coverage is essential, and thus paleomagnetic data from the southern oceans are desired. From a
geochemical point of view, Kawamura proposes an implementing strategy for selecting
sediment cores suitable for paleomagnetic studies.
Arguments for the fixity of hotspots and the True Polar Wander (TPW) are a fundamental
issue of the geodynamics, and paleomagnetism can provide essential data for settling the
problem (see the proposal of Harada). Implementation of the IODP drilling proposal of the
Louisville seamount chain, which is currently in the SAS of IODP, will be the first step. It is
necessary to drill more hotspot tracks to solve the problem: at least one in the Atlantic,
preferably the Tristan hotspot track. This may not be completed before 2013, and be carried
forward to the next phase of IODP.
Understanding geomagnetic field variations within the whole Earth system is an important
viewpoint because the energy sources of the geodynamo may not be restricted within the core
but comes from outside the core. An IODP proposal for studying geomagnetic field variations
both intensity and direction during the last ca. 10 m.y., and examining the possibility of orbital
modulation of the geomagnetic field was submitted in 2002 (Proposal 612-Full3), and already
forwarded to SPC. However, drilling may not be completed before 2013 and carried over to the
next phase of IODP, partly because this proposal requires to occupy globally distributed sites,
which will be implemented as a piggy-back style.
Improvement of drilling technology is an important aspect of paleomagnetism (see the
proposal submitted to the Technology Development WS). Drilling induced remanent
magnetization has often been annoyed paleomagnetists. Coring with APC sometimes produces
artificial remanent magnetization that cannot be removed by alternating-field (AF)
demagnetization (e.g., ODP Leg 154). Such cores are unfortunately useless for paleomagnetism.
The artificial remanent magnetization is probably acquired by deformation of sediments in a
strong magnetic field of the drilling instruments: core-barrel, cutting shoe, and so on. Coring
hard rocks with RCB also often induces secondary remanent magnetization. This is usually
erased by AF demagnetization, but for understanding sources of marine magnetic anomalies,
recovering in situ magnetization before demagnetization is essential. Improvement of drilling
technology for avoiding drilling induced magnetization should be seriously considered in the
next phase of IODP for maximizing scientific output. A demand of fully oriented cores is not
only for paleomagnetism but also for other fields including structural geology. Orientation of
APC cores using the tensor tool available at present is far from satisfactory; it can be used only
for judging the polarities, normal or reversed, but not for secular variations of declination.
Introduction of up-to-date technology will enable accurate orientation of cores, including RCB.