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FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1
A dynamic model of hot fingers in the mantle wedge in
northeast Japan
Yoshihiko Tamura
Research Program for Geochemical Evolution, Institute for Frontier Research on Earth Evolution (IFREE)
Introduction
Total alkalis, Al2O3 and K2O
Tamura et al. (2001; 2002) showed that Quaternary volcanoes in the Northeast Japan arc can be grouped into 10 elongate volcanic Groups striking transverse to the arc; each of
these has an average width of 50km and linear separations of
30-75km (Fig. 1). This grouping of volcanic centers is closely
correlated with linear topographic highs, low-velocity regions
in the mantle wedge, and local negative Bouguer gravity
anomalies along behind the volcanic arc (Tamura et al., 2001;
2002). Mantle melting and the production of magmas in the
NE Japan arc may be controlled by locally developed hot
regions within the mantle wedge that have the form of
inclined, 50km wide fingers (Tamura et al., 2001; 2002) (Fig.
1). Although these hot fingers have been captured by seismic
tomography (e.g., Zhao et al., 1992), the role they play in arc
magma genesis is not fully understood. Do these hot mantle
fingers melt to produce arc magmas, or they play a role mainly
as heat sources? Are arc magmas generated by mantle diapirs,
which form in the lower part of the mantle wedge and rise
through the overlying hot fingers in the mantle wedge (Tamura, 1994)? Across-arc variations of isotope and trace element
compositions in volcanoes of NE Japan (e.g., Shibata and
Nakamura, 1997) do not support the concept of melting of a
uniform mantle source. Kersting et al. (1996), on the other
hand, identified along-strike isotopic variations and suggested
that crustal contamination contributed to arc magmatism in
NE Japan. The chief aim of this paper is to explore further the
two-dimensional geochemical variations among volcanic clusters in NE Japan and to clarify the role that hot fingers might
play in the production of arc magmas.
87
Sr/86Sr data from forty-four volcanoes are reviewed to
identify systematic variation of magma sources in the mantle
wedge. These data include those from basalt, andesite, dacite
and rhyolite. In addition, we present across- and along-arc
variations of major element compositions from Quaternary
basalts in NE Japan.
Transverse geochemical variations across volcanic arcs
have been reported from many areas (Kuno, 1960; 1966; for
summary, see Gill, 1981, p.209). Chemical analyses of basalt
from 20 Quaternary volcanoes in NE Japan arc are reviewed
to evaluate across- and along-arc variations. A plot of total
alkalis (Na2O+K2O) against SiO2 of basalts in NE Japan (Fig.
2a) serves to identify the three roughly parallel zones: (1) low
alkali tholeiite (LAT) with low alkalis lying along the volcanic
front, (2) high-alumina basalt (HAB) with intermediate alkalis
lying west of the front. (3) alkali (olivine) basalt (AOB) with
higher alkalis lying further to the west. AOB and HAB in
Kampu and Megata volcanoes, respectively, (Figs. 1 and 2)
show that HAB and AOB have some degree of overlap.
Importantly, however, all basalts from volcanic front volcanoes are LAT (Figs. 1 and 2).
Kuno (1960) defined high-alumina basalt (HAB) in a study
that only involved aphyric rocks. Practically, however, the
term “high-alumina basalt” is confusing when it is applied to
both aphyric and porphyritic lavas. Uto (1986) studied Al2O3
variation in late Cenozoic undifferentiated basalts in Japan and
concluded there is no significant variation in Al2O3 content
among the three parental basalt magma types (LAT, HAB and
AOB). Data presented in this study confirm Uto’s findings for
the basalt bearing volcanoes of NE Japan (Fig. 2b).
A striking characteristic of orogenic andesites and associated rocks within volcanic arcs of modest width is the consistent
increase of their incompatible element concentrations, most
notably K 2O, away from the plate boundary (Gill, 1981).
Apparently, K2O is a more sensitive variable than total alkalis
(Fig. 2c); basalts along the NE Japan volcanic front contain
significantly less K2O than those from the rear of the arc.
Interestingly, however, along-arc K2O values in NE Japan are
not rigorously related to local slab depth. For example, the subducting slab lies 150km beneath the low-K Group 10 volcanoes
Toya, Kuttara and Shikotsu, whereas it is only 100km deep
beneath the volcanic front volcanoes in Groups 1-4 to the south
(Fig. 1). Basalts from the Group 10 volcanoes, however, are the
low-K variety (LAT), despite the greater local depth to the
downgoing slab. To further complicate the situation, medium-K
basalts (HAB) further west of the volcanic front in Groups 2, 5,
and 6, also lie 150km above the subducting slab (Fig. 1).
Variations in basalt composition across and along
the NE Japan arc
Present-day volcanism along the NE Japan arc is dominated
by andesites (Aramaki and Ui, 1978), but in many Quaternary
volcanoes there are no analyses to indicate that basalts were
erupted (Committee for Catalogue of Quaternary Volcanoes in
Japan (1999) and references therein). Basalt is, however, conspicuous in some volcanoes. Basalts are defined here simply
as volcanic rocks with SiO2 <53wt.% calculated on an anhydrous basis. All discussions in this paper refer to analyses that
have been normalized to 100% on a volatile-free basis with
total iron calculated as FeO.
Across- and along-arc 87Sr/86Sr variations
Forty-four Quaternary volcanoes are reviewed to evaluate
two-dimensional strontium isotopic variations in NE Japan. It
can be seen that isotopic variability from the same volcano
having more than ten analyses of 87Sr/ 86Sr, ranges from
<0.0003 (Hachimantai and Toya volcanoes) to >0.002 (Akagi
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FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1
AOB or from low- to high-K in the NE Japan arc mainly
reflect the decrease in partial melting, which could be controlled by thermal structure in the mantle wedge.
Volcanoes in the volcanic front have similar 87Sr/86Sr values
of 0.7040-0.7045 (Fig. 3), despite the increase of slab depth of
<100km in the south to >150km in the north. Thus, the configuration of the thermal structure and the magma source materials at depth appear to be independent of slab depth, and we
suggest this is the result of processes acting at the tips of the
hot mantle fingers.
and Nasu volcanoes). To investigate 87Sr/86Sr variability from
the same volcano, Tamura and Nakamura (1996) reviewed
isotopic data from 38 arc volcanoes worldwide. The differences of the ranges are <0.0003 in the 12 of the 38 volcanoes
that are situated on thin continental crust. Those volcanoes
located on thicker crust exhibit much larger, but systematic
differences of 87Sr/86Sr ranges; rocks having intermediate compositions (andesite and dacite) yield the lowest and the highest
87
Sr/86Sr; basalts and rhyolites are intermediate in terms of
87
Sr/86Sr values (Tamura and Nakamura 1996). Such features
preclude simple mantle-source heterogeneity. If the mantle
diapir model of Tamura (1994) is applicable to other arc volcanoes, isotopic variability within an arc volcano could be
ascribed to crustal contamination (Tamura and Nakamura,
1996). Thus the lowest 87Sr/86Sr values of individual volcanoes
would best represent source mantle values.
Two-dimensional variations of lowest 87Sr/86Sr values of
individual volcanoes and 87Sr/86Sr contours of the source mantle are shown together in Fig. 3. Although there is a general
decrease of 87Sr/86Sr from volcanic front to the back-arc, the
slope of the 87Sr/86Sr surface is irregular. For example, the
87
Sr/86Sr surface slopes more steeply to the west in Groups 4
and 5 than it does in Groups 8 and 9 (Fig. 3).
As noted by Notsu (1983), volcanic front volcanoes from
Group 4 to Group 10 have almost constant 87Sr/86Sr values of
0.7040-0.7045, Because the depth of the subducting slab
beneath these volcanic front volcanoes increases from <100km
in Group 4 to >150km in Group 10, slab depth seems to have a
questionable relationship to the constant 87Sr/ 86Sr values
observed.
A dynamic model
Kushiro (1990) discussed evolution of the crust in the
northeast Japan arc based on melting experiments of a peridotite under hydrous conditions and crustal compositions estimated from deep crustal xenoliths and seismic velocity data.
He concluded that the mass of the present mantle wedge is not
sufficient to produce the present crust and cumulates in the NE
Japan arc, and he suggested that the mantle wedge may have
been replenished several times by the addition of fertile mantle
materials (Kushiro, 1990).
Shibata and Nakamura (1997) suggested that the volume of
subducted material (oceanic sediments and altered MORB)
added to the mantle wedge decreases with increasing distance
from the volcanic front. They suggested that the melting of
these wedge additions, having been metasomatized by rising
fluids, would produce arc magmas showing the isotopic variations of Pb, Sr and Nd observed in Quaternary volcanoes. On
the other hand, Tamaki et al. (1992) noted that major opening
associated with vigorous basin volcanism of Japan Sea
occurred about 28 to 18Ma. Interestingly, basaltic rocks from
the sea of Japan exhibit a broad range in isotopic compositions, and the isotopic characteristics of these backarc lavas
are similar to those of Quaternary arc volcanics from NE
Japan (Cousens and Allan, 1992; Tamaki et al., 1992). This
indicates that a large range in isotopic compositions already
existed within the mantle wedge of NE Japan since the backarc basin opened. Alternatively, we suggest that a MORB-like
mantle source (87Sr/86Sr~0.703) in the mantle wedge is replenished by fertile mantle materials (87Sr/86Sr~0.705) through
convection induced by the subducting lithosphere. Such convection, coupled with mantle diapir model of Tamura (1994),
could explain the two-dimensional variations in degrees of
melting and 87Sr/86Sr values in the volcanoes of NE Japan.
According to the model of Tamura (1994), mantle diapirs
consisting of hydrous peridotite form in the lower part of the
mantle wedge and rise through anhydrous peridotite. Fig. 4a
shows the dynamic convection within the mantle wedge and
genesis of arc magmas by mantle diapirs. As suggested by previous workers (e.g., Furukawa, 1993; Kincaid and Sacks, 1997),
subduction of a rigid oceanic slab into the viscous mantle
induces convection in the mantle wedge. Tamura et al. (2001;
2002) refined this concept and suggested that in NE Japan this
convection takes the form of hot, finger-like regions in the mantle wedge that move toward the volcanic front. Conveyor-like
return flow is interpreted to carry the remnants of these fingers
to depth along the top of the subducting slab. In the act of
returning, it is likely that the fingers lose their identity and are
smeared out to take the form of a thin, continuous sheet at the
Discussion
Slab depth vs. thermal structure in the mantle
wedge (hot fingers)
Cross-arc geochemical variations have been thought to be a
function of slab depth (e.g., Ryan et al., 1995; Shibata and
Nakamura, 1997). Such a relationship, however, is not valid
along the total length in the NE Japan arc (Figs. 1 and 3).
Along the axis of any of the 10 volcano Groups, K2O increases
from east to west (Figs. 1 and 2). Group 10 volcanoes, offset
slightly to the NW, contain low-K basalts or LAT, which are
similar to lavas erupting at the volcanic front to the south, but
are different from the medium-K basalts or HAB erupting at
volcanoes lying at the same distance above the subducting slab
(Fig. 1). Sakuyama and Nesbitt (1986) presented major and
trace element data for a series of lavas from 17 volcanic centres in the NE Japan arc. They suggested that the major control
of melt chemistry (except for Pb, Rb and Th) is the degree of
partial melting of a common source. Kushiro (1994) showed
the variations of SiO2 and total alkalis (Na2O+K2O) of partial
melts formed from peridotite (PHN-1611) to be functions of
pressure and degree of melting. The partial melt composition
changes from higher alkalis to lower alkalis with increasing
degree of melting at each pressure. At 10 and 15kbar, lowalkali tholeiite is produced by relatively high degrees of partial
melting (>20wt.%), but alkali basalt is produced by less than
10wt.% melting at more than 15kbar (Kushiro, 1994). Thus
across-arc variation of basalt types from LAT through HAB to
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FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1
Proc. ODP, Sci. Res., 127/128, 1333-1348, 1992.
Tamura, Y., Genesis of island arc magmas by mantle-derived bimodal
magmatism: evidence from the Shirahama Group, Japan, J.
Petrol., 35, 619-645, 1994.
Tamura, Y., and E. Nakamura, The arc lavas of the Shirahama Group,
Japan: Sr and Nd isotopic data indicate mantle-derived bimodal
magmatism, J. Petrol., 37, 1307-1319, 1996.
Tamura, Y., Y. Tatsumi, D. Zhao, Y. Kido, and H. Shukuno, Distribution of Quaternary volcanoes in the Northeast Japan arc: geologic
and geophysical evidence of hot fingers in the mantle wedge,
Proc. Japan Acad., 77, 135-139, 2001.
Tamura, Y., Y. Tatsumi, D. Zhao, Y. Kido, and H. Shukuno, Hot fingers in the mantle wedge: new insights into magma genesis in
subduction zones, Earth Planet. Sci. Lett., 197, 105-116, 2002.
Uto, K., Variation of Al2O3 content in late Cenozoic Japanese basalts:
a re-examination of Kuno’s high-alumina basalt, J. Volc. Geotherm.
Res., 29, 397-411, 1986.
Zhao, D., A. Hasegawa, and S. Horiuchi, Tomographic imaging of P
and S wave velocity structure beneath Northeastern Japan, J. Geophys. Res., 97, 19909-19928, 1992.
base of the wedge. Given that mantle diapirs are formed in the
lower part of the mantle wedge (Tamura, 1994), greater
amounts fertile material would be involved in diapirs beneath
the volcanic front, and lesser amounts would be involved in
areas behind the front (Fig. 4a). As a result, the magmas formed
beneath the volcanic front would have higher 87Sr/86Sr, and
those formed further “back” would have lower 87Sr/86Sr.
Hot fingers within the mantle wedge extend from deep
mantle (>150km) below the back-arc region towards the shallower mantle (~50km) beneath the volcanic front (Tamura et
al., 2001; 2002). Across-arc variation of basalt type (Figs. 1
and 2) may be controlled by the configuration of hot fingers in
the mantle wedge (Fig. 4a). When mantle diapirs cease their
ascent, those below the volcanic front are still enclosed within
the hot fingers and, therefore, still heated, but those beneath
the back-arc region will have risen above the hot fingers and
will cool before magma segregation. Thus, primary basaltic
magmas are formed by higher degree of partial melting near
the volcanic front, whereas they are formed by smaller degree
of partial melting toward the west.
Acknowledgements. I. Kushiro, Y. Tatsumi and T. Hanyu are
thanked for discussion. R. S. Fiske helped with manuscript revision.
References
Aramaki, S., and T. Ui, Major element frequency distribution of the
Japanese Quaternary volcanic rocks, Bull. Volc., 41, 390-407, 1978.
Committee for Catalogue of Quaternary Volcanoes in Japan, Catalogue of Quaternary Volcanoes in Japan, Volcanological Society
of Japan, Tokyo, 1999.
Cousens, B. L., and J. F. Allan, A Pb, Sr, and Nd isotopic study of
basaltic rocks from the Sea of Japan, Legs 127/128, Proc. ODP,
Sci. Res., 127/128, 805-817, 1992.
Furukawa, Y., Magmatic processes under arc and formation of the
volcanic front, J. Geophys. Res., 98, 8309-8319, 1993.
Gill, J., Orogenic andesites and slab tectonics, Berlin: Springer-Verlag, p.390, 1981.
Kersting, A. B., R. J. Arculus, and D. A. Gust, Lithospheric contributions to arc magmatism: isotope variations along strike in volcanoes of Honshu, Japan, Science, 272, 1464-1468, 1996.
Kincaid, C., and I. S. Sacks, Thermal and dynamical evolution of the
upper mantle in subduction zones, J. Geophys. Res., 102, 1229512315, 1997.
Kuno, H., High-alumina basalt, J. Petrol., 1, 121-145, 1960.
Kuno, H., Lateral variation of basalt magma type across continental
margins and island arcs, Bull. Volc., 29, 195-222, 1966.
Kushiro, I., Partial melting of mantle wedge and evolution of island
arc crust, J. Geophys. Res., 95, 15929-15939, 1990.
Kushiro, I., Recent experimental studies on partial melting of mantle
peridotites at high pressures using diamond aggregates, J. Geol.
Soc. Jpn, 100, 103-110, 1994.
Notsu, K., Strontium isotope composition in volcanic rocks from the
Northeast Japan arc, J. Volc. Geotherm. Res., 18, 531-548, 1983.
Ryan, J. G., J. Morris, F. Tera, W. P. Leeman, and A. Tsvetkov,
Cross-arc geochemical variations in the Kurile arc as a function of
slab depth, Science, 270, 625-627, 1995.
Sakuyama, M., and R. W. Nesbitt, Geochemistry of the Quaternary
volcanic rocks of the Northeast Japan arc, J. Volc. Geotherm.
Res., 29, 413-450, 1986.
Shibata, T., and E. Nakamura, Across-arc variations of isotope and
trace element compositions from Quaternary basaltic volcanic
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between subducted oceanic slab and mantle wedge, J. Geophys.
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Tamaki, K., K. Suyehiro, J. Allan, J. C. Jr. Ingle, and A. Pisciotto,
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Figure 1. Ten elongate volcanic groups striking transverse to the
Northeast Japan arc, and distribution of basalt-bearing volcanoes and
those consisting exclusively of andesite and dacite. Basalt is absent in
the arc-front volcanoes Groups 5, 8 and 9. Dashed lines show depth
contours to the surface of the dipping seismic zones.
Figure 2. Plots of total alkalis, Al2O3 and K2O against SiO2 for NE
Japan basalts. (a) Total alkalis vs SiO2. The two lines (after Kuno,
1966) mark the boundaries of low-alkali tholeiite (LAT), high-alumina basalt (HAB) and alkali olivine basalt (AOB). (b) Al2O3 vs SiO2,
showing HAB, LAT and AOB have overlapping Al2O3 contents, but
note that not all HAB is rich in Al2O3. (c) K2O vs SiO2. The boundary
lines, originally defined by Gill (1981) to subdivide the andesite field
(>53wt.% SiO2), are here extrapolated into the basalt field.
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FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1
87
86
Figure 3. Sr/ Sr map of source mantle for selected volcanoes in the NE Japan arc. The 87Sr/86Sr values shown are
abbreviated as follows (lowest 87Sr/ 86Sr –0.7)×10 5. Lines
show 87Sr/86Sr contours of the source mantle based on the
lowest 87Sr/86Sr.
Figure 4. Schematic cross sections collectively showing three-dimensional convection within the mantle wedge of subduction zones. Solid triangles denote NE Japan
volcanoes. Insert map shows cross-front sections A-A’, B-B’ and a front-parallel
section (C-C’). Numbers enclosed in squares denote volcano Groups. (a) Section
through a hot finger. A MORB-like mantle in the ambient mantle wedge
(87Sr/86Sr~0.703) is replenished by fertile mantle materials (87Sr/86Sr~0.705) by conveyor-like convection. See text for explanation. (b) Section through an area lying
between two hot fingers. The upper part of the conveyor-like system is absent, but
its lower (downgoing) part is represented by a spreading sheet-like mass. (c) Frontparallel section; tips of hot fingers (+) are moving toward the viewer; sheet-like
return flow (–) is moving away.
91