Download INVITED REVIEW Petit-spot volcanism: A new type of volcanic zone

Geochemical Journal, Vol. 45, pp. 157 to 167, 2011
INVITED REVIEW
Petit-spot volcanism: A new type of volcanic zone discovered near a trench
NAOTO HIRANO*
Center for Northeast Asian Studies, Tohoku University, Kawauchi 41, Aoba-ku, Sendai 980-8576, Japan
(Received March 23, 2010; Accepted December 8, 2010)
One extremely young volcano (0.05–1 Ma) and other young volcanoes (1.8, 4.2, 6.0, and 8.5 Ma) composed of strongly
alkaline magma were recently discovered on the abyssal plain of the Early Cretaceous (135 Ma) Pacific Plate. These
volcanoes were dubbed “petit-spots”. The petit-spot volcanic province represents more than 8 Myr of activity over a large
area (~600 km along the direction of plate motion), but with a relatively small volume of magma production, thus indicating a small supply of heat inconsistent with a hotspot. The low-flux petit-spot volcanoes may be related to the occurrence
of a tensional field of lithosphere caused by plate flexure, with the ascending melt derived from a mantle source susceptible to partial melting.
Rock samples from the young volcanoes are highly vesicular (up to 60%) despite high hydrostatic pressures at 6000 m
water depth, indicating volatile-rich magmas. The depleted heavy rare earth elements and high radiometric isotopic ratios
of noble gases indicate the magma was derived from upper mantle. Nevertheless, the low 143Nd/ 144Nd, high 87Sr/ 86Sr, low
206
Pb/ 204Pb, and low 207Pb/204 Pb ratios are similar to enriched or fertile compositions such as oceanic island basalts. These
apparently conflicting data are explained by the extremely small degree of partial melting of recycled materials in the
degassing mantle of the asthenosphere, probably with carbonate in the source. The petit-spot volcanoes, therefore, provide a unique window into the nature of the oceanic plate and underlying asthenosphere prior to subduction.
Keywords: petit-spot, volcano, Pacific Plate, volatiles, basalt
et al., 2001, 2006) has revealed a new type of volcanism,
different from the three types mentioned above. Monogenetic petit-spot volcanoes located near the Japan Trench
(Site A in Fig. 1) are less than 2 km in diameter and yield
ages of 1.8, 4.2, 6.0, and 8.5 Ma, indicating the episodic
eruption of magma over a period involving 600 km of
plate motion, without any systematic spatial trend in age
such that seen along oceanic island/seamount chains moving over a hotspot (Hirano et al., 2001, 2006, 2008).
Moreover, these volcanoes represent 8 million years of
activity over a large eruption area but with low volumes
of magma production, indicating that this geological setting is distinct from that of hotspot volcanoes on the
present-day Pacific Plate.
Hirano et al. (2006) suggested that petit-spot magma
originates within the asthenosphere, without the development of a hotspot, based on geological/geochronological grounds and geochemical data including noble gas
isotope data and the abundance of heavy rare earth elements (HREEs). Hofmann and Hart (2007) noted that this
type of small-volume, within-plate volcanism is not completely unknown, because previous studies have described
alkaline volcanism that is unrelated to mantle plumes or
hotspots (e.g., off-ridge seamounts: Batiza and Vanko,
1984; Cameroon Line: Fitton and Dunlop, 1985). How-
INTRODUCTION
Chemical and isotopic variability in the mantle is quantified by analyzing lavas from multiple sources collected
at oceanic island volcanoes and submarine volcanoes.
However, volcanoes occur only locally within the ocean,
with three types recognized based on their magmatic system and tectonic setting: (1) those at divergent plate
boundaries (mid-ocean ridges, MOR), (2) those at convergent plate boundaries (e.g., island arcs), and (3) those
within plates (e.g., hotspots).
MOR lavas are the only type to provide a window into
the geochemistry of the shallow mantle (i.e.,
asthenosphere); the geochemistry of island arc and hotspot
lavas reflects the input of components from the subducted
slab and mantle plume, respectively. However, MOR lavas
and their source mantle might show a compositional bias
toward the asthenosphere, because the distribution of
MOR is restricted to below newly producted lithosphere
on Earth’s surface.
The recent discovery of petit-spot volcanoes (Hirano
*E-mail address: [email protected]
Copyright © 2011 by The Geochemical Society of Japan.
157
Kuril
Trench
Pacific Plate
~10 cm/yr.
Site A
outer rise
Japan
Site B
Japan
Trench
Site C
ts
n un
a
o
b
Jo eam
S
J
Se apa
am ne
Izuou se
nts
Ogasawara
Trench
–10000
–8000
–6000
Shtsky Rise
–4000
Depth (m)
–2000
0
Fig. 1. Bathymetric map of the NW Pacific Plate off NE Japan, based on data from Amante and Eakins (2009). The three petitspot volcanic fields are known as Sites A, B, and C. Black stars show the sites of petit-spot volcanoes from which samples have
been retrieved. The circle indicates an area containing more than 80 potential petit-spots at Site C. The thick arrow represents the
present-day constant plate motion (approximately 10 cm/yr; Gripp and Gordon, 2002). The thin arrow shows the possible eruption range of 0–8.5 Ma volcanoes.
ever, the formation of such young volcanoes upon old,
cold oceanic crust is uncommon, and none of these volcanoes described in the previous studies formed at sites
located far from spreading centers, hotspots, or, more
generally, areas of thermal upwelling (Hirano and
Koppers, 2007). The petit-spot magmas, therefore, could
represent the first discovery of melting product transported directly to the surface from the asthenosphere below an old plate prior to subduction.
Early in the theory of plate tectonics, conditions at
the boundary between the lithosphere and underlying
asthenosphere were thought to be suitable for melting, as
158 N. Hirano
indicated by observations of a seismic low-velocity zone,
a layer with high electrical conductivity, and the experimentally determined peridotite solidus in the presence of
H2O and CO2 (e.g., Wyllie, 1988). However, the possibility of melting in the asthenosphere has remained a topic
of controversy since Karato (1990) and Karato and Jung
(1998) proposed that the presence of small amounts of
water in olivine could explain the above geophysical observations and may control the rheology of the mantle;
consequently, they argued that the presence of a partial
melt is unnecessary. On the other hand, laboratory measurements of Vs (shear-wave velocity) and Qs (shear-wave
Site A
Site B
understandings of this newly discovered magmatism at
the bottom of the ocean. Thus, the final section of this
article will discuss possible future direction of the research
associated with the petit-spot volcanoes.
outer-rise
Japan
Trench
Fig. 2. Model of the formation of petit-spot volcanoes, modified after Hirano et al. (2006, 2008).
attenuation) in mantle material have recently been performed to investigate the existence of partial melt in the
asthenosphere as a function of temperature and pressure
(e.g., Faul and Jackson, 2005). Moreover, Mierdel et al.
(2007) concluded that melting must occur in the
asthenosphere because the minimum capacity of water in
orthopyroxene coincides with the depth of the lowvelocity zone in the asthenosphere. The results of experimental studies have provided support for several incompatible processes proposed to explain the high electrical
conductivity of the asthenosphere (Karato, 1990; Wang
et al., 2006; Yoshino et al., 2006, 2009). Therefore, the
presence of melt in the asthenosphere remains a topic of
debate.
The newly discovered petit-spot volcanoes may provide evidence relevant to the question of whether melting occurs in the asthenosphere, as the magma that feeds
such volcanoes may escape to the surface from the
asthenosphere along fractures caused by bending of the
lithosphere (Fig. 2). If this interpretation is correct, then
it is noteworthy that such volcanoes are ubiquitous on
flexed and fractured ocean floor, and that acoustic reflective data indicate the possible presence of petit-spot volcanoes at another site off the southern Japan Trench (i.e.,
possibly more than 80 volcanoes are located close to Site
C in Fig. 1), as well as on the oceanward slope of the
Tonga Trench (Hirano et al., 2008). Although it is clear
that the surface morphology and distribution of petit-spot
volcanoes on the NW Pacific Plate are influenced by
cracks in the lithospheric that reach the surface, it remains
uncertain whether petit-spot volcanoes form wherever
oceanic plate is flexed and fractured.
An important objective of this contribution is to review current state of our understanding on the origin and
mechanisim of formation of the newly recognized type
of magmatism based mainly on our multidisciplinary studies of petit-spots and the subducting Pacific Plate. It will
be shown that there are a lot of missing pieces for full
THE DISCOVERY OF PETIT-S POT VOLCANOES
The discovery of a sample of young alkali basalt (ca.
6 Ma; Hirano et al., 2001), collected during dive 10K#56
by ROV KAIKO (of the Japan Agency for Marine-Earth
Science and Technology, JAMSTEC) in the Japan Trench,
was unexpected because nobody had anticipated such
volcanism on the Cretaceous Pacific Plate. The dive revealed continuous outcrops and collapsed angular blocks
of basalt for 50 m along a steep cliff on the oceanward
slope of the trench (Kawamura, 2007).
To explore these young volcanoes, extensive surveys
were carried out from 2003 to 2005 using the research
ships R/V Kairei and Yokosuka, and the submersible
Shinkai6500 of JAMSTEC. High-resolution acoustic
multibeam surveys of the ocean floor are required to detect these volcanoes because they are only 1–2 km in diameter and only several hundred meters in height (Hirano
et al., 2006). It has been shown that the acoustic reflectivity of a petit-spot is more than three times as high as
that of the surrounding abyssal plain on the old plate
(Hirano et al., 2008). In the present case, areas of high
acoustic reflectivity are probably hard ocean floor, covered with only a thin layer of soft pelagic sediment, much
thinner than the surrounding pelagic sediment layer on
the old Pacific Plate. Following sections will explore the
geological and lithological overviews about the petit-spot
volcanoes and rock samples recovered from them.
Geology and lithology
The volume of each petit-spot knoll is less than 1 km3
(Hirano et al., 2008), and the many knolls at Site A are
aligned, defining three lines of WNW to ESE. The alignment of monogenetic volcanoes has generally been explained in terms of the maximum horizontal compression
(Koyama and Umino, 1991) or upward bending of the
plate (Natland, 1980; Ozawa et al., 2005).
Rock samples were recovered by dredges and submersible dives from several “spotty” areas at Sites A and B
that show high acoustic reflectivity (Figs. 3A and B)
(Hirano et al., 2006). Most of the rocks are highly vesicular, and specimens with quench features are associated with lava lobes and breccias within the pelagic
sediments. Volcanic cones and ridges at Site A can be distinguished from the surrounding seafloor based on
bathymetric and acoustic reflectivity data, where the slope
is characterized by trench-parallel normal faults (horsts
and graben) resulting from extensional bending of the
subducting Pacific Plate, and where the cliffs generally
Petit-spot volcanism 159
indicate 100–500 m of vertical displacement upon the
faults (Ogawa et al., 1997). An important difference between the area with trench-parallel normal faults and other
parts of the subducting plate is the presence of hummocky
structures within the former area (Site A in Fig. 3A)
(Hirano et al., 2001). The volcanic features consist of
moderately or highly vesicular lavas (~10–60 vol% vesicles). Elsewhere, lavas recovered from along the fault
outcrops below the ridges are generally less vesicular (0–
20 vol%). Erosion of sediments by the strong bottom currents in the trench (Owens and Warren, 2001), together
with localized normal faulting, has exposed the bases of
the volcanoes, which consist of non-vesicular sheet-flow
lavas.
The sequences and lithologies at Site B represent only
the exposures at the top of volcanic cones, and typically
represent just one eruptive phase. These rocks include
highly vesicular pillow lava, water-chilled bombs,
hyaloclastite, peperite, and contact-metamorphosed mud.
Petrology and geochemistry
Notable and obvious feature of the specimens recovered from Sites A and B are significant freshness of the
samples (even the interior surfaces of vesicles and olivine
crystals are unaltered). This feature is in clear contrast
with lavas obtained from Cretaceous seamounts (Koppers
et al., 2000; Awaji et al., 2004). This is because most of
the Cretaceous volcanic rocks have undergone a long period of submarine alteration. Large olivine crystals within
the analyzed samples have been interpreted as xenocrysts
from the mantle, from as deep as 14 km (Hirano et al.,
2004). In fact, the petit-spot lavas contain several kinds
of xenoliths and xenocrysts 2–100 mm in diameter, representing material brought up from the oceanic crust and
the upper mantle, including dolerite, olivine gabbro, and
peridotite (Hirano et al., 2010; Yamamoto et al., 2009).
XRF analyses of major elements in whole-rock petitspot lavas, together with electron microprobe analyses of
quenched glass, show that the rocks can be classified as
potassium-rich shoshonite, trachybasalt, basanite, and
sodium-rich basanite (SiO 2 = 45–54 wt%, total alkalis =
5–9 wt%). MgO contents range from 4 to 14 wt%, with
the Mg# (100 × Mg/[Mg + Fe]) in the range 47–70 (Fig.
4). The rare earth element (REE) contents of quenched
glasses (measured using ICP-MS with the ArF Excimer
laser ablation system) and bulk compositions (measured
using ICP-MS) show a depletion in HREEs, as represented
by high [La/Yb] CN values (La/Yb normalized by
chondrite) of 19–40 (Fig. 5).
AGE DETERMINATIONS
Determining the age of petit-spot eruptions is important in gaining an understanding of the tectonic setting of
160 N. Hirano
the eruption site and temporal variations in the magmatic
process. During submersible dives over petit-spots at Site
B, water-chilled bombs, covered with only a thin layer of
pelagic sediment, were observed scattered on the seafloor
around the volcanic cones. This finding suggests a recent
eruption (Hirano et al., 2006), as also indicated by the
thickness of palagonite in quenched glass rims (1–4 mm;
Hirano et al., 2006), which is estimated to have formed
via alteration in the period from 0.05 to 1 Ma (0.003–
0.02 µm/yr: Moore et al., 1985).
At Site A, non-quenched, crystalline rocks from the
interior of the volcano were exposed by strong bottom
currents and were easily sampled in areas that contained
good outcrops of basement lava along the horst and graben
fault structures in the trench (Hirano et al., 2006). In contrast, the three petit-spot volcanoes at Site B are only exposed along the top, quenched parts of the volcano, showing eruptive products such as hyaloclastite, peperite, volcanic bombs, volcaniclastic rocks, and pillow lavas, which
are all composed mainly of glasses and the alteration product palagonite (Hirano et al., 2006; Fujiwara et al., 2007),
with intersertal texture. Such materials are unsuitable for
Ar–Ar age dating because in amorphous material, when
39
Ar is created from 39K by a fast neutron in the nuclear
reactor during irradiation of the sample, the 39Ar may
move from one phase to another in a process called “recoil”, creating a disturbed age spectrum (McDougall and
Harrison, 1988). Accordingly, Ar–Ar methods for dating
petit-spot volcanics were only successful for the
holocrystalline rocks at Site A. This approach was also
supported by Iwata and Kaneoka (2000), who concluded
that fresh, holocrystalline rocks should be analyzed for
precise Ar–Ar dating.
Base on above notion, four holocrystalline rock samples collected from petit-spot volcanics at Site A have
been subjected to age determination by the Ar–Ar method.
As shown in Fig. 6, the Ar–Ar method yield ages ranging
from 1.8 to 8.5 Ma. The data for sample 6K#880-R3B
(Fig. 6) indicate an age of 1.76 ± 0.58 Ma, based on the
inverse isochron. This age is clearly younger than the plateau age, reflecting excess Ar of 340 ± 21 for the initial
40
Ar/ 36Ar ratio for the inverse isochron, which is higher
than atmospheric values (approximately 296). The three
other samples show atmospheric values in the initial 40Ar/
36
Ar ratio for the inverse isochron, along with successFig. 4. Major element compositions of petit-spot lavas, modified after Valentine and Hirano (2010). The ages in the SiO2
vs. Na2O + K 2O diagram are based on Ar–Ar data from representative samples of each volcano (color-coded). The different
trends obtained for each volcano in the SiO2 vs. Mg# diagram
show the variable degrees of fractionation of each petit-spot
magma (i.e., fractionation occurred independently at each volcano).
A)
JapanTrench axis
–7500
–7000
–6500
–6000
–5500
depth (m)
465.387
–5000
0
5
1563.920
5419.260
10 km
B)
140
–6000 –5800 –5600 –5400 –5200 –5000 –4800 –4600
depth (m)
0
10
300
350
400
450
500
600
700
800
1000 8700
20 km
Fig. 3. Maps showing the bathymetry (left-hand panels) and acoustic reflectivity (right-hand panels) obtained from on-board
multibeam surveys at Sites A and B (see Fig. 1). Yellow lines in upper panels A) of Site A and arrows in lower panels B) of Site B
indicate sites sampled by dredges and dives during 2003–2005 (Hirano et al., 2006).
Site A
Site B
KR03-07 D1, 10K#056
6K#878
KR03-07 D2
KR04-08 D07 & 08
KR04-08 D02
6K#880 R1,R2,R5
6K#880 R3
REE normalized to concentration in chondrite
1000
Na2O + K2O (wt%)
9
8
4.2 Ma
7
0~1 Ma
1.8 Ma
6
8.5 Ma
6.0 Ma
5
4
70
10
1
La Ce Pr Nd
65
Mg #
100
60
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 5. Chondrite-normalized rare earth element (REE) patterns. The data are from Hirano et al. (2006). REE abundances
are normalized to C1 chondrite values (Sun and McDonough,
1989). Color legends are the same as in Fig. 4.
55
50
45
45
46
47
48
49
50
51
52
53
54
SiO2 (wt%)
Fig. 4.
Petit-spot volcanism 161
10K#056 R002
Site
A;
Japan
Trench (fault wall)
0.004
0.004
no isochron
0.003
0.003
36Ar/40Ar
36Ar/40Ar
0.002
0.002
Isochron Age: 5.69 ± 0.43 Ma
N = 8 (all fractions)
(40Ar/36Ar)t = 0: 304.9 ± 9.4
MSWD: 1.1
0.001
0
0
30
0.439Ar/40Ar 0.6
0.2
0.001
0
0
30
0.8
20
Age
(Ma)
5.95 ± 0.31 Ma
Plateau Age:
Released 39Ar : 94% (n = 4)
10
0
0.004
0.439Ar/40Ar 0.6
0.8
50
% 39Ar cumulative
100
6K#880-R3B
Site A; Japan Trench (fault wall)
0
0.004
0.003
0.003
36Ar/40Ar
0.002
0
1.76 ±1000
0.58
Ma
C)
N = 7 (650
(40Ar/36Ar)t = 0: 340 ± 21
0 MSWD: 0.15
0
0.4
30
0.839Ar/40Ar 1.2
20
0
0
30
20
Age
(Ma)
0
50
% 39Ar cumulative
0.2
0.439Ar/40Ar 0.6
0.8
8.53 ± 0.18 Ma
Age
(Ma)
Plateau Age: 2.54 ± 0.76 Ma
Released 39Ar : 83% (n = 8)
10
100
Isochron Age: 8.3 ± 1.2 Ma
N = 5 (860 1200 C)
(40Ar/36Ar)t = 0: 302 ± 24
MSWD: 10.7
0.001
1.6
50
% 39Ar cumulative
KR04-08 D02-009
Site A; Japan Trench (fault wall)
0.002
Isochron Age:
0.001
4.23 ± 0.19 Ma
Plateau Age:
Released 39Ar : 89% (n = 3)
10
36Ar/40Ar
0
0.2
20
Age
(Ma)
0
KR03-07 D2-211
Site A; Japan Trench (knoll)
Plateau Age:
Released 39Ar : 91% (n = 6)
10
100
0
0
50
% 39Ar cumulative
100
Fig. 6. Results of Ar–Ar dating of petit-spot samples. All the samples, namely 10K#056 R002 (Hirano et al., 2001), KR03-07 D2211, KR04-08 D02-009 (Hirano et al., 2006), and 6K#880-R3B (Hirano et al., 2008), are from Site A. Analytical methods were
adopted from Saito et al. (1991) and Ebisawa et al. (2004).
fully obtained plateau ages (Fig. 6).
Radiometric dating and geochronological analysis of
several monogenetic petit-spot volcanoes at Sites A and
B reveal that the volcanoes erupted independently from
each other, from different sources, possibly as a result of
squeezing out of magma during the period 0–8.5 Ma
(Hirano et al., 2001, 2006, 2008). As mentioned above,
these dates are geologically important in recording petitspot activity during a period involving a total plate motion of 600 km, which is inconsistent with the occurrence
162 N. Hirano
of a single hotspot on the Pacific Plate. If the
asthenosphere contains a zone of partial melt, then the
process of petit-spot formation may potentially occur
anywhere on the ocean floor, triggered by tectonic events
that enable the melt to ascend from the mantle. Thus, as
will be further discussed in the next section, it is anticipated that the occurrence of petit-spot volcanoes might
be controlled by the condition of the lithospheric part of
the ocean directly above the zone of possible melt production.
hinge of
outer rise
trench
oceanward
slope
subdu
upper lithosphere's
stress field
in concave-warping
petit-spots
ction
pelagic sediments
oceanic crust
lithospheric
mantle
asthenospheric
mantle
not to scale
lower lithosphere's
stress field
in concave-warping
Fig. 7. Model of ascending dikes in concavely warped
lithosphere (modified after Valentine and Hirano, 2010; Hirano
et al., 2006, 2008). White arrows and white dotted lines show
the tensional field at the base of lithosphere, under the petitspot. The magma accumulates and ascends due to tension in
the lower lithosphere and subjacent asthenosphere that reflects
concave warping of the plate; otherwise, the dike ascends
through the upper lithosphere, entraining xenoliths, and experiences a 90° rotation in σ 3 when passing through the middle
lithosphere (black ellipses). Consequently, the petit-spot volcanoes are aligned perpendicular to the hinge line of the flexure
in the outer rise.
ROLE OF LITHOSPHERIC FLEXURE
Old, cold lithosphere behaves elastically, and it may
be deformed or flexed as a result of ocean-island or
seamount loading, or flexed as the plate enters a subduction zone (McAdoo and Martin, 1984; Watts and Zhong,
2000). The NW Pacific Plate, close to the Japan Trench,
rises 300–800 m higher than predicted in models of plate
subsidence (Parsons and Sclater, 1977; Stein and Stein,
1992; Carlson and Johnson, 1994). This area is called the
outer rise, representing a convex flexing that occurs during the initiation of plate subduction and that yields a
positive gravity anomaly (Sandwell and Smith, 1997). The
petit-spot magmas are likely to rise to the surface along
fractures oriented perpendicular to the direction of the
maximum horizontal compression, controlled by the stress
field in the down-warping Pacific Plate at the eastern edge
of the outer rise. Of note, the WNW-ESE alignment of
volcanic cones at Site A is essentially perpendicular to
the hinge line of the bending plate at the outer rise (Hirano
et al., 2006).
Valentine and Hirano (2010) suggested that the tensional field in the mantle results in the accumulation and
ascent of melt, producing low eruption-flux volcanoes
within intra-plate volcanic provinces, because regional
tectonic deformation controls the way in which partial
melts accumulate. Regarding the NW Pacific petit-spots,
flexuring of the plate creates tension in the lower
lithosphere and subjacent asthenosphere as a result of
concave warping of the plate (Hirano et al., 2006, 2008),
whereas regional tectonics results in the formation of
dikes that ascend under the eruption sites and experience
a 90° rotation of σ 3 in the middle lithosphere (Fig. 7).
The base of the lithosphere, therefore, is extended so that
σ3 is perpendicular to the flexure axis, while in the upper
lithosphere, the least compressive horizontal stress is parallel to the flexure axis. This stress rotation may cause
many of the ascending dikes to stall in the lithosphere
(Valentine and Hirano, 2010) (Fig. 7), one result being
the observed variable degree of fractionation in the volcanic products (Fig. 4). Dikes that propagate upward from
these temporary reservoirs are oriented perpendicular to
the least horizontal compressive stress in the upper
lithosphere, perpendicular to the flexure axis (Hirano et
al., 2006; Valentine and Hirano, 2010).
Petit-spot volcanoes may be ubiquitous on the ocean
floor, with magma being squeezed upward by tectonic
forces associated with plate flexure if the source mantle
is susceptible to partial melting. For example, potential
petit-spot volcanoes have been identified on the
oceanward slope of the Tonga Trench, where horst and
graben structures are developed in association with subduction of the southern Pacific Plate (Hirano et al., 2008).
Ranero et al. (2005) produced bathymetric maps near the
Chile Trench, showing tiny knoll-like petit-spots on the
subducting plate (the authors anticipate hotspot-related
volcanoes from bathymetry). The above features may represent good examples of fracture-related petit-spot
volcanism. Pending more detailed rock sampling on the
oceanward slopes of trenches and further age dating of
lavas, it is suggested that petit-spot volcanic activity may
prove to be a ubiquitous phenomenon on the flexed parts
of all subducting tectonic plates. Fissures that form within
outer rises appear to provide the mechanism for
asthenospheric melts to escape to the surface, forming
petit-spot volcanoes.
A MANTLE SOURCE FOR PETIT-SPOT MAGMA
AND AN ISOTOPIC P ARADOX
As noted earier, the depleted HREEs in petit-spot lavas
indicates that the magma was derived from garnetbearing mantle, suggesting a source deeper than 90 km
(Arth, 1976), possibly corresponding to the low-velocity
zone (asthenosphere) below the base of the lithosphere,
as detected by seismography. Previous studies have interpreted the presence of interstitial magma in the
asthenosphere (Lambert and Wyllie, 1968; Mierdel et al.,
2007); therefore, the asthenosphere is a potential source
of petit-spot magmas (Hirano et al., 2001, 2006).
Petit-spot volcanism 163
Isotopic ratios in petit-spot lavas provide further information on the source region in the mantle. Hirano et
al. (2006) examined the isotopes of noble gases because
they reveal whether the magma originated from the deep
mantle or shallow mantle. MORB and oceanic island basalt (OIB) are generally regarded to be derived from depleted mantle and primordial mantle, respectively (e.g.,
Kaneoka, 2000; Ozima and Podosek, 2002). Secondary
nuclides are expected to differentiate most clearly the
nature of the mantle source. Thus, 21Ne/ 22Ne and 40Ar/
36
Ar ratios in MORBs are comparatively higher than those
in OIBs, due to the addition of radiogenic 21Ne and 40Ar
from the radioactive decay in a source depleted in nonradiogenic isotopes ( 22 Ne and 36 Ar). The isotope
geochemistry of petit-spot lavas examined in the present
study was determined after extracting the gases from
quenched glass by applying the vacuum crushing method
to glass rind in samples collected from Site B (Hirano et
al., 2006). This approach was taken because the magmatic
gas would have been trapped in the glass when the magma
was quenched during eruption. The low 21Ne/22Ne values
and high 40Ar/36Ar values obtained from the samples suggest that the petit-spot magma was derived from a MORBtype mantle source (Hirano et al., 2006).
Isotopic compositions of Nd, Pb and Sr of samples
from Sites A and B are investigated by Machida et al.
(2009). It was reported that petit-spot samples from Sites
A and B yielded relatively low 143Nd/ 144Nd (0.5125–
0.5127) and 206Pb/204Pb (16.87–17.93) ratios with relatively elevated 87Sr/86Sr (0.7042–0.7047) ratios. Note that
such a feature is similar to those found in ocean island
basalts derived from enriched or fertile compositions, as
in Pitcairn, Tristan Da Cunha, and Walvis Ridge (e.g., an
EM-1 mantle component: Zindler and Hart, 1986; the
Dupal isotopic signature: Dupré and Allègre, 1983). The
results reported by Machida et al. appear to be at odds
with the isotopic compositions of noble gases, which indicate the magma was derived from depleted mantle.
However, this apparent discrepancy is resolved by considering the extremely small degrees of partial melting
involved and the higher mobility of noble gases in the
mantle due to their more volatile behavior when compared with other solid elements (Machida et al., 2009).
Compared with a depleted peridotite source, the solidus
of an enriched component (e.g., recycled former oceanic
crust) would be more than 100°C lower at pressures of 3
GPa in the asthenosphere (Takahashi et al., 1998;
Hirschmann, 2000). Therefore, the effect of enriched
materials may prevail, even though they are minor constituents in the depleted mantle, if the small degrees of
partial melting occur in the asthenosphere.
Other key geochemical component of a mantle source
is carbon dioxide. Petit-spot lavas are highly vesicular
(up to 60%) despite the high hydrostatic pressures en-
164 N. Hirano
countered at 6000 m water depth (Hirano et al., 2006).
This observation suggests that the vesicularity is caused
by CO2, as the solubility of CO2 is very low in alkaline
magmas (Dixon, 1997) compared with the high solubility of H2O (ca. 300 ppm versus 0.5–1.0 wt%, respectively). Because a few percent of melt might be present if
small amounts of H 2 O or CO 2 are present in the
asthenosphere (Wyllie, 1995), it is anticipated that petitspot magmas originate in the asthenosphere as incipient
partial melts that form as a result of the presence of CO2.
More recently, carbonatite melt has been proposed as a
key material in explaining the electrical conductivity of
oceanic asthenosphere (Gaillard et al., 2008; Yoshino et
al., 2010). The preliminary observation of high CO2 contents in petit-spot lavas, based on their vesicularity, raises
the possibility that CO2 affects the source components
and their melting. However, it is necessary to carefully
estimate the fluctuations in CO2 content that occur in ascending magma due to degassing (Gonnerman and
Mukhopadhyay, 2007) in order to investigate the significance of CO2 in an asthenospheric source of petit-spot.
Furthermore, the presence of entrained carbonate rocks
(e.g., submarine limestone) might increase the CO2 content of the magma (Allard et al., 1991).
As stated above, there is a lack of direct evidence for
melting in the asthenosphere with which to complement
experimental and geophysical observations (Karato, 1990;
Karato and Jung, 1998; Faul and Jackson, 2005; Wang et
al., 2006; Yoshino et al., 2006, 2009, 2010; Mierdel et
al., 2007; Gaillard et al., 2008). In addition, geophysical
data are ambiguous regarding the specific nature of the
asthenosphere below the NW Pacific Plate (Shimamura
et al., 1983; Shinohara et al., 2008). In this context, petitspots on the Pacific Plate provide a potential window into
the occurrence of partial melting of the asthenosphere.
FUTURE D IRECTIONS
The discovery of petit-spot volcanoes upon old
subducting plates requires the revision of some established
ideas in geosciences. If we could determine that melt is
present in the source mantle, petit-spots would be recognized as a ubiquitous phenomena on Earth; otherwise, the
widespread occurrence of petit-spots could be indicated
by the discovery of petit-spots at other sites (e.g., off the
Tonga and Chile trenches). In such a case, the distribution and alignment of monogenetic petit-spot volcanoes
from various areas may reveal that fractures in the
lithosphere are important controls on petit-spot formation.
The article of “perspectives” by McNutt (2006) on the
paper by Hirano et al. (2006) (in the same issue of Science) noted that the model of petit-spot formation may
also apply to hotspot volcanoes, because some workers
consider that the magma at hotspots ascends along a crack
in the plate from the asthenosphere to the surface, rather
than being caused by plume activity (e.g., McNutt et al.,
1997). The contrasting views regarding the ascent of
hotspot magma constitute the “plume debate”. As mentioned above, the sizes and volumes of hotspot volcanoes
are several orders of magnitude greater than those of petitspot volcanoes, with the latter being less than 1 km3 in
volume (Hirano et al., 2008) and being underlain by small
sills (Fujiwara et al., 2007). Therefore, in order to accept
an alternative to the hotspot plume model, an additional
explanation should be given how such a large volume of
melt could be produced solely by melting of the
asthenosphere. Otherwise, knowledge about formation of
petit-spots would not directly be applicable to the plume
debate (Hirano and Koppers, 2007; Hofmann and Hart,
2007).
Although the formation of fissures upon an outer rise
seems to be the logical mechanism for enabling
asthenospheric melt to escape to the surface, Valentine
and Hirano (2010) suggested that the presence of
fracture-controlled pathways is not necessary because ascending dikes can propagate their own fractures. The distribution of petit-spot volcanoes may depend on the presence of melt in the source. Therefore, if the accumulation
of partial melts due to tension (Valentine and Hirano,
2010) occurs in the mantle, the distribution of petit-spots
may be sufficiently limited to prevent them forming linear arrays along tectonic fissures at the surface. On the
other hand, if horizontal melt-rich layers are embedded
in otherwise meltless mantle in the asthenosphere
(Kawakatsu et al., 2009), petit-spot volcanoes could be
formed ubiquitously wherever the plate flexes under the
influence of plate tectonics.
A multidisciplinary research project is currently being undertaken on the NW Pacific Plate, including oceanbottom seismic observations, measurements of electrical
conductivity, and a broad survey using multi-channel seismic equipment. The detailed results of this project promise to increase our understanding of the asthenosphere
and the structures in the lithosphere beneath petit-spot
volcanoes. Further work on the concentrations of isotopes
and volatiles in petit-spot lavas could provide important
information on the chemical composition of the
asthenosphere. Moreover, a petrological, rheological and
geochemical study of xenoliths entraining petit-spot lavas,
together with information from the 21st Century SuperMohole Project (Hirano et al., 2010), provides us with
the opportunity to sample and understand the entire oceanic lithosphere. The fourth type of volcanic zone recognized on Earth, the petit-spot, provides a unique window
into the nature of old plates, in areas where bathymetry
was largely ignored prior to the discovery of petit-spots.
Acknowledgments—I would like to thank H. Kagi and the
Executive Editor Y. Sano for encouraging me to write this paper. The present work on petit-spot volcanoes was fully supported by the author’s former supervisors: K. Saito, Y. Ogawa,
T. Ishii, E. Takahashi, I. Kaneoka, H. Staudigel, A. A. P.
Koppers, H. Kagi, and T. Morishita. I gratefully acknowledge
the assistance of I. Kaneoka, K. Nagao, K. Saito, Y. Takigami,
K. Fukunaga, Y. Miura, T. Hanyu, N. Iwata, K. Sato, H. Tamura,
H. Sumino, J. Yamamoto, and N. Ebisawa in performing the
Ar–Ar and K–Ar analyses, as well as the Radioisotope Center
of the University of Tokyo, the International Research Center
for Nuclear Materials Science, and the Institute for Materials
Research, Tohoku University. I also owe my thanks to T.
Yoshida, S. Arai, T. Nakano, T. Hirata, J. Kimura, S. Takizawa,
M. Kurosawa, H. Sato, S. Haraguchi, S. P. Ingle, A. Tamura, S.
Takeuchi, S. Machida, S. Okumura, K. Sato, and S. Yoshimura
for their help and discussions on geochemical matters. I also
thank Captains S. Ishida, Y. Imai, O. Yukawa, and H. Tanaka,
the crews of R/V Kairei, R/V Yokosuka, and the submersible
Shinkai6500, K. Tamaki, G. Valentine, M. Nakanishi, T.
Fujiwara, S. Umino, K. Okino, J. Smith Jr., T. Kanamatsu, N.
Abe, M. Ichiki, B. Eakins, K. Baba, S. Haraguchi, T. Sasaki, B.
Miyahara, T. Mizukami, K. Kawamura, S. Machida, J. Konter,
Y. Harigane, T. Ayu, and N. Takehara for their help during the
YK05-06, KR04-08, and KR03-07 cruises and related offshore
studies. This study has been financially supported in part by
the Grants-in-aid for Scientific Research in Japan (No.
22740350). The author greatly appreciates the constructive comments of reviewers, Y. Orihashi, H. Shimoda, J. Kevin, and the
Associate Editor, T. Matsumoto.
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