Geochemical Journal, Vol. 42, pp. 51 to 60, 2008
Geographical distribution of 3He/4He ratios and seismic tomography
in Japan
YUJI SANO1* and JUNICHI NAKAJIMA2
1
Center for Advanced Marine Research, Ocean Research Institute, The University of Tokyo,
Nakano-ku, Tokyo 164-8639, Japan
2
Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University,
Aoba-ku, Sendai 980-8578, Japan
(Received March 12, 2007; Accepted July 13, 2007)
The 3He/ 4He ratios (Ra) of natural gas, volcanic fluid, and groundwater are compiled in the Japanese Islands and their
geographical distributions are discussed in the tectonic frame work of subduction zone together with preciselydetermined seismic velocity structures. In Northeastern (NE) Japan where typical island arc signatures are developed,
there is a clear contrast of 3He/4He ratios perpendicular to the trench axis, low-Ra in the frontal arc and high-Ra in the
volcanic arc. This may reflect the presence or absence of magma with high-Ra in the shallow crust. As a carrier of primordial helium, source melt may be generated in low-V zone of the wedge mantle by dehydration of Pacific slab at about 150
km deep and may flow upward sub-parallel to the slab, which is well constrained by S-wave velocity perturbation. In the
Chugoku and Shikoku districts of Southwestern (SW) Japan, there is a geographical contrast of Ra similar to NE Japan
except for the region at about 100 km from the volcanic front where medium-Ra was found. High-Ra observed in volcanic
arc of the Chugoku district may be attributable to the mantle helium derived from the magma source generated below the
Philippine Sea slab. Medium-Ra in the Shikoku district is explained by dehydration of the young slab with a moderate
aging effect. These features are again consistent with the results of seismic tomography. In the Kinki district of SW Japan,
anomalously high-Ra was observed in the frontal arc region that was called by “Kinki Spot”. Since the high-Ra is located
at much wider region from the volcanic front when compared with NE Japan, the melt generated below the Philippine Sea
slab may penetrate into the fissure of the slab tear and may arrive at the shallow crust by upwelling flow.
Keywords: helium isotopes, seismic tomography, Japanese islands, geotectonics, arc magma
with tectonic settings of island arc as a whole.
The recent development of dense regional highsensitivity seismograph network (up to about 900 stations)
has provided high-quality data and a large amount of
travel-time data is available in Japan. Based on the large
number of earthquakes, the precise travel-time tomography was performed using high-quality P- and S-wave arrival times. Such 3D seismic velocity structure of the
upper mantle beneath the Northeastern (NE) Japan was
well documented by a research group of Tohoku University (Zhao and Hasegawa, 1993; Nakajima et al., 2001;
Hasegawa and Nakajima, 2004). Recently Nakajima and
Hasegawa (2007) have reported seismic tomography beneath the Southwestern (SW) Japan and suggested that a
large low-velocity anomaly beneath the Philippine Sea
slab is the source of arc magma.
In this study we compile helium isotope data of Japan
available in literatures. Based on the geographical distribution of the 3He/ 4He ratio corrected for air contamination and precise 3D seismic velocity structure using highquality travel-time data, we discuss an island arc tectonics and magma genesis in the subduction zone.
INTRODUCTION
After the pioneer work of Sano and Wakita (1985),
many 3He/ 4He ratios of volcanic fluids and natural gases
were obtained throughout the Japanese Islands. These data
have provided new knowledge of geophysics and
geochemistry of the subduction zone. However most of
them were focused on the regional interest such as
Hokkaido island (Sano and Wakita, 1988), Kinki district
(Wakita et al., 1987; Matsumoto et al., 2003; Umeda et
al., 2006), Chugoku district (Sano et al., 2006), and
Shikoku island (Dogan et al., 2006). The other data were
related to single volcanoes such as Oshima (Sano et al.,
1988), Hakone (Sakamoto et al., 1992), Kusatsu–Shirane
(Sano et al., 1984) and Ontake (Sano et al., 1998) in Japan. Since a lot of helium isotopes have been published,
it is timely to compile the data and discuss their relation
*Corresponding author (e-mail: [email protected])
Copyright © 2008 by The Geochemical Society of Japan.
51
HELIUM ISOTOPES AND A RC MAGMA GENESIS
IN NE JAPAN
The Japanese Islands are divided into two major
blocks, NE Japan and SW Japan by Itoigawa–Shizuoka
tectonic line (Fig. 1). NE Japan is considered as a welldefine island arc system with a deep trench, a frontal arc,
a volcanic arc, and a back arc region with a marginal sea
(Kaizuka, 1972). Perpendicular to the trench axis, geophysical data such as terrestrial heat flow, gravity anomaly
and seismic velocity vary significantly across the island
arc (Yoshii, 1977). These geographical and geophysical
signatures were well explained by the subduction of the
Pacific plate beneath the NE Japan. On the other hand,
there are few geochemical data with across the arc variation (Kuno, 1966; Notsu, 1983; Shibata and Nakamura,
1997) and they were limited to within a volcanic arc.
Sano and Wakita (1985) reported the 3He/4He and 4He/
20
Ne ratios of 115 natural gas and volcanic fluids in Japan. Helium in these samples is well explained by the
mixing of three end members: the upper mantle helium
with the 3He/ 4He ratio of about 8Ra (where Ra is the atmospheric 3He/4He ratio of 1.39 × 10–6) and the 4He/ 20Ne
ratio of 1000; the radiogenic helium with the 3He/4He ratio
of 0.02Ra and the 4He/20Ne ratio of 1000; the atmospheric
helium with 1Ra and 4He/20Ne ratio of 0.318. The mixing hypothesis has successfully applied to their data as
well as many other data obtained in Japan (Wakita et al.,
1987; Sano and Wakita, 1988; Dogan et al., 2006). One
can distinguish air helium from the mixture of the mantle
and radiogenic helium based on the 4He/20Ne ratio. Actually it is possible to correct air helium contamination using following equations (Craig et al., 1978).
Rcor = [(3He/ 4He)obs – r]/(1 – r),
(1)
r = (4He/20Ne)air/(4He/20Ne)obs,
(2)
where Rcor and (3He/4He)obs denote the corrected and observed 3He/4He ratios, and (4He/20Ne) air and (4He/ 20Ne)
are the air and observed 4He/20Ne ratio. After correcting
the air helium contribution, we define here the 3He/4He
ratio of samples as follows:
He/ 4He > 4Ra
more than 50% mantle contribution
(3)
medium-Ra 4Ra > 3He/4He > 2Ra
between 50% and 25% mantle contribution
(4)
2Ra > 3He/4He less than 25%
mantle contribution
(5)
high-Ra
low-Ra
3
where high-Ra is principally found in volcanic region with
high heat emission such as hot springs and geothermal
52
Y. Sano and J. Nakajima
Fig. 1. Tectonic configurations of the Japanese Islands with
trench, volcanic front and “Kinki Spot”. “I–S TL” shows the
“Itoigawa–Shizuoka tectonic line”.
activity, while low-Ra is tend to be observed in nonvolcanic region.
We have compiled helium isotopes data in the Tohoku
and Hokkaido districts of NE Japan (Sano and Wakita,
1985, 1988) and plotted in the map based on the definition above after the correction for air contamination (see
Fig. 2a). It is apparent that low-Ra samples is locating
everywhere while medium-Ra and high-Ra fluids distribute limitedly within the volcanic arc region. Figure 2b
shows the relationship between the distance from the quaternary volcanic front and the 3He/4He ratio expressed in
the unit Ra. Sano and Wakita (1985) have emphasized
that these is a geographical difference of Ra between the
frontal arc and volcanic arc; lower in the trench side and
higher in the back arc side in NE Japan. This trend is not
changing at the present case (Fig. 2b). Their interpretation was that the uprising magma is a unique material
which brings primordial helium from the upper mantle to
the shallow crust. The volcanic front was formed at the
location below which subducting slab reaches the depth
where hydrous volatiles supplied from the slab lowers
the solidus temperature of mantle materials and generates a partial melting (Tatsumi et al., 1983; Tatsumi,
1986). Thus geographical contrast of Ra has constrained
the magma genesis immediately below the volcanic front.
Another important feature of the geographical distribution is that medium-Ra and high-Ra situate relatively
narrow region with about 110 km wide from the volcanic
Fig. 2. (a) Geographical distribution of helium isotopes in NE Japan. The seismic tomography is taken along the line A–A ′ . (b)
The 3He/ 4He profile in NE Japan, showing corrected 3He/ 4He ratio versus geographic distance from the site to the volcanic front.
Fig. 3. Vertical cross-section of S-wave perturbations along
line A–A′ in Fig. 1a (from Nakajima et al., 2001). Red and blue
colors represent low and high velocities, respectively, according to the scale at the bottom.
front. There is no high-Ra in the area of more than 110
km far from the front in the back arc side (Fig. 2b). This
trend may provide another constraint on the arc magma
genesis in NE Japan.
Nakajima et al. (2001) reported both P- and S-wave
velocity structures beneath NE Japan by applying a traveltime tomography method. Figure 3 shows a vertical crosssection of S-wave velocity perturbations along line A–A′
in the map of Fig. 2a. Low-velocity zone (both P- and S-
Fig. 4. Schematic diagram of vertical cross-section of the crust
and upper mantle beneath NE Japan along line A–A′ in Fig. 1a
(from Hasegawa and Nakajima, 2004 with some modification).
waves) were extensively distributed along the volcanic
front in the uppermost mantle and are expanding downward to the back arc side in the mantle wedge across the
arc. They suggested that the partial melting zones existed
Helium isotopes and seismic tomography
53
Fig. 5. (a) Geographical distribution of helium isotopes in Chugoku and Shikoku, SW Japan. The seismic tomography is taken
along the line B–B ′ . (b) The 3He/ 4He profile in SW Japan, showing corrected 3He/ 4He ratio versus geographic distance from the
site to the volcanic front.
in the mid-crust right beneath active volcanoes. Taking
into account of these tomography studies, Hasegawa and
Nakajima (2004) have presented a schematic diagram of
vertical cross-section of the crust and upper mantle beneath NE Japan where genesis of arc magma was well
explained.
Figure 4 shows a schematic diagram of NE Japan
which describes geographical distribution of helium isotopes. This is principally the same as the Plate 5 of
Hasegawa and Nakajima (2004) with some modifications
to accommodate with helium isotopes in the back-arc region. Aqueous fluids with low-Ra are supplied from
subducting Pacific Plate at several depths. The cause of
the low-Ra in the oceanic crust will be given latter. More
precisely at about 80–130 km deep of NE Japan the fluids are derived from a metamorphic reaction of jadeite
lawsonite blueschist to lawsonite amphibole eclogite. At
about 100–150 km deep another dehydration of lawsonite
amphibole eclogite to eclogite may occur and the fluids
are again supplied (Hacker et al., 2003). These fluids may
react with overlying mantle materials and form secondary hydrous minerals such as serpentine and chlorite in
the mantle wedge (Iwamori, 1998), which could make
helium isotopes medium-Ra.
As following the subducting slab, fluids hosted in
hydrous minerals may have been dragged down to depths
of 150–200 km where breakdown of these secondary minerals may occur (Iwamori, 1998). The breakdown may
release a large amount of aqueous fluids which may move
upward and probably initiates partial melting in the inclined low-V zones with high-Ra. The melt is transported
54
Y. Sano and J. Nakajima
sub-parallel to the subducting slab by the upwelling flow
and finally meets the Moho beneath the volcanic front.
Then the melt separates from the flow and distributes
below the Moho along the volcanic front. This is the principle source of high-Ra magma in NE Japan. A part of
the flow may arrive at the Moho of the back-arc side and
the separated melt may lead to the source magma of
volcanism in the back-arc region (see Fig. 3). There is a
constraint that the partial flow can travel about 100 km
from the volcanic front in horizontal direction based on
the high-Ra distribution (see Fig. 2b). Anyway the
upwelling flow with low velocity signature is the principle carrier of the mantle helium with high-Ra.
HELIUM ISOTOPES IN C HUGOKU AND SHIKOKU
OF SW JAPAN
In SW Japan a well-defined island arc system has not
developed. The volcanic front is not as clear as in NE
Japan except for Kyushu and heat flow values are relatively high in the trench region (Yamano et al., 1984).
These features are generally attributable to the subduction of the young and warm lithosphere of the Philippine
Sea plate beneath the Eurasian plate (Shiono and Sugi,
1985). In addition the Pacific plate descends from the east
beneath the Philippine Sea and Eurasian plates. This has
resulted in a complicated surface geology (Taira et al.,
1989) and lateral heterogeneity in the upper mantle structure of SW Japan (Nakajima and Hasegawa, 2007).
Helium isotope data were significantly lacking in the
Chugoku and Shikoku districts until year 2005 (Sano and
Wakita, 1985; Wakita et al., 1987). Recently Dogan et al.
(2006) have reported 38 3He/ 4He and 4He/20Ne ratios of
gas and water samples from the Shikoku district. At the
same time Sano et al. (2006) have published 34 helium
isotope data in the Chugoku district. We have compiled
these data and plotted them in the map as following above
definition of helium isotopes (Fig. 5a). It is apparent that
high-Ra is located only in the volcanic arc while low-Ra
fluids occur everywhere. The tendency is significantly
similar to that of NE Japan. On the other hand several
medium-Ra are found in the frontal arc region of Shikoku
district, which is discrepant from NE Japan. Similar
medium-Ra was reported in the frontal arc region of the
North Island, New Zealand (Sano et al., 1987).
In order to explain the medium-Ra in Shikoku, Dogan
et al. (2006) have suggested two mechanisms. First a
major active fault in Japan called “the Median Tectonic
Line” can provide an efficient path for the mantle helium
which was librated from the Philippine Sea plate, which
is similar to the explanation of medium-Ra in New Zealand by Giggenbach et al. (1993). Second hydrous fluids
again derived from the descending slab may cause fracturing within the crust, thereby easing the transfer of the
medium-Ra to the surface. This means that the uprising
fluids themselves made the path to the surface which is
supported by the evidence that non-volcanic deep tremors occur, about 30 km deep in the region (Obara, 2002).
If the latter is the case, medium-Ra should be found in
the other region where the long-period tremors occur in
SW Japan such as southwestern Tokai area. However
Byakko and Yuya springs of the Tokai area showed lowRa (0.79Ra and 1.24Ra, respectively from Sano and
Wakita, 1985). Thus the mechanism of medium-Ra is not
so simple as described by Dogan et al. (2006).
Figure 5b indicates the relationship between the distance from the helium volcanic front and the 3He/4He ratio expressed in the unit Ra. This front was defined by
the geographical distribution of helium isotopes in
Chugoku district (Sano et al., 2006). It is noted that
medium-Ra found in Shikoku is located relatively narrow region at about 100 km from trench side of the front.
This suggests that the medium-Ra is somehow related to
the depth of the upper surface of the Philippine Sea plate.
The helium may be derived from the aqueous fluids expelled from the subducting slab. It is necessary to calculate the magma aging effect which may lower the helium
isotopes significantly from the original MORB-type helium.
Sano et al. (2006) have reported high-Ra in the volcanic arc region of Chugoku district and have described
that it is impossible to explain the high-Ra by the slab
melting of the descending Philippine Sea plate based on
the magma aging effect. The other source with pristine
mantle material was required. They further suggested that
Fig. 6. Magma aging effect on the 3 He/ 4 He ratio of
holocrystaline tholeiite, glassy tholeiite and bulk oceanic crust
due to radiogenic production of helium. The 3He/4He ratio of
holocrystaline tholeiite starts to decrease after 1000 year while
that of glassy basalt is constant until 107 year.
the magma source is related to the Pacific plate at the
great deep when taking into account of the highest and
average helium isotopes as well as strontium isotopes of
volcanic rocks.
MAGMA AGING EFFECT ON HELIUM I SOTOPES
Above discussions on medium-Ra in Shikoku and
high-Ra in Chugoku require the evaluation of the magma
aging effect. Therefore we have re-calculated the decrease
of helium isotopes (Ra) by addition of radiogenic helium
with time. The method is principally the same as the
magma aging reported by Torgersen and Jenkins (1982).
The initial condition of tholeiite magma such as U, Th
and 3He contents are referred from Tatsumoto (1966) and
Dymond and Hogan (1974). We follow the radiogenic
production of 4He from the equation by Craig and Lupton
(1976). We assume that production ratio of 3He/4He based
on the nuclear reaction: 6Li(n,α)3H(β)3He is 0.03Ra (or
4.17 × 10–8 ) as described by Mamyrin and Tolstikhin
(1984). Then we calculate helium isotope variation with
time. It is noted that the variation of nucleogenic production ratio of 3He/4He does not affect the result significantly.
Figure 6 shows the magma aging effect on helium isotopes of holocrstaline and glassy tholeiite as defined by
Torgersen and Jenkins (1982) where initial 3He contents
are 7 × 10–15 and 7 × 10–11 ccSTP/g, respectively. The
helium isotopes of holocrstaline tholeiite start to decrease
Helium isotopes and seismic tomography
55
Fig. 7. Vertical cross-section of S-wave perturbations along
line B–B′ in Fig. 4a (from Nakajima and Hasegawa, 2007). Red
and blue colors represent low and high velocities, respectively,
according to the scale at the bottom. Symbol (䉲) shows a trench
axis.
even after 1000 year, while the ratio of glassy tholeiite
does not change until 10 7 year. We should assume the initial content of 3He in bulk oceanic crust. It is unrealistic
to take the initial contents of holocrstaline or glassy
tholeiite. Probably the actual value exists between those
values of 7 × 10–15 and 7 × 10–11 ccSTP/g. Kumagai et al.
(2003) have reported helium contents of gabbros and
peridotites recovered from oceanic core complex on the
Southwest Indian Ridge. They admitted that total helium
abundances of these samples are lower than MORB
glasses (glassy tholeiites) by two to three orders of magnitude. We calculate the average of 3He contents in their
gabbros as 7 × 10 –13 ccSTP/g except for the sample with
very low 3He/4He ratio. The standard deviation of the
contents indicates the variation of factor three at the maximum. Taking into account of these data, we assume that
the initial 3He content of bulk oceanic crust is 7 × 10–13
ccSTP/g with uncertainty of factor three, which means
that the bulk oceanic crust consists of the mixture of 99%
holocrstaline tholeiite and 1% glassy tholeiite. The possible magma aging effect on the hypothetical oceanic crust
is shown by a hatched zone with a factor of three uncertainty of the initial 3He content in Fig. 6.
In NE Japan descending Pacific plate is old and cold.
Formation age of the oceanic crust at the trench is estimated about 130 Ma by the magnetic anomalies
(Nakanishi et al., 1992). Magma aging calculation leads
to the 3He/4He of values between 0.10Ra and 0.65Ra, sig56
Y. Sano and J. Nakajima
Fig. 8. Schematic diagram of vertical cross-section of the crust
and upper mantle beneath Chugoku and Shikoku, SW Japan
along line B–B′ in Fig. 4a (from Nakajima and Hasegawa, 2007
with some modification). Symbol (夹) shows earthquake foci.
nificantly lower than the low-Ra with 25% mantle contribution. Therefore the low-Ra is dominant component
in the frontal arc region of NE Japan, even though aqueous fluids derived from the Pacific plate may arrive at
the shallow crust.
In SW Japan subducting Philippine Sea plate is young
and warm. The age of the slab at the trench is estimated
15–25 Ma (Shih, 1980) and about 20 Ma by the magnetic
anomalies (Okino et al., 1994). The calculated 3He/4He
ratios vary from 0.40Ra to 3.4Ra, suggesting that
medium-Ra is available in aqueous fluids derived from
the descending slab in some case. On the other hand, it is
difficult to maintain the high-Ra in the oceanic crust.
Figure 7 shows a vertical cross-section of S-wave velocity perturbations along line B–B′ in the map of Fig. 5a
(Nakajima and Hasegawa, 2007). This indicates the existence of a large low-velocity anomaly below the Philippine Sea slab in the Chugoku and Shikoku districts at
least down to depths of 300 km. Nakajima and Hasegawa
(2007) have inferred that the low-velocity zone is an
upwelling flow from great deep. There is a shallow lowvelocity zone toward the Quaternary volcanoes in
Chugoku which is connecting with the large anomaly
below the Philippine Sea slab as going around the tip of
the plate.
Figure 8 indicates a schematic diagram of Chugoku
and Shikoku of SW Japan which explains geographical
distribution of helium isotopes. This is basically the same
as figure 10A of Nakajima and Hasegawa (2007) with
Fig. 9. (a) Geographical distribution of helium isotopes in Kinki, SW Japan. The seismic tomography is taken along the line C–
C′ . (b) The 3He/4He profile in SW Japan, showing corrected 3He/ 4He ratio versus geographic distance from the site to the volcanic front. Note that high-Ra is locating almost everywhere.
some modifications to accommodate with helium isotopes
in Shikoku. Based on the magma aging calculation, it is
possible to maintain medium-Ra in the subducting Philippine Sea plate at present. Aqueous fluids supplied from
the oceanic crust at the depth between 30–50 km by metamorphic dehydration reactions (Hacker et al., 2003;
Yamasaki and Seno, 2003) may be the principle source
of medium-Ra in Shikoku. In the case of NE Japan, expelled fluids produce secondary minerals which may have
been dragged down to deep in the mantle. In the SW Japan, the fluids may have hardly chance to form secondary minerals at such shallow depth and may move upward by their buoyancy. This aqueous uprise may cause
the non-volcanic deep tremors in the region (Obara, 2002).
Medium-Ra is located narrow region at about 100 km
trench side of the front (see Fig. 5b), which is well explained by the hydration hypothesis since the depth of
the Philippine Sea plate may arrive at 30–50 km in the
area. Magma aging effect suggests that high-Ra in the
volcanic arc of Chugoku is not attributable to the re-melting of the oceanic crust and pristine mantle material was
required (Sano et al., 2006). An upwelling flow from great
deep, probably related to the Pacific plate, could be the
carrier of such high-Ra in the shallow crust of the region.
HELIUM ISOTOPES AND KINKI SPOT IN SW JAPAN
4
Sano and Wakita (1985) found anomalously high 3He/
He ratio in the frontal arc region of the Kinki district,
SW Japan. Since the high-Ra was locating in roughly circular area, they called this “Kinki Spot”. Wakita et al.
(1987) reported additional helium data as well as microseismic activity continuing over a long period at shallow
depths. Taking into account of high terrestrial heat flow,
hypocentral distribution of micro-earthquakes, and the
presence of molten materials inferred from the characteristic phases of SxP and SxS reflected waves in microseismograms, they suggested the existence of a shallow
magma body beneath the area. Matsumoto et al. (2003)
have confirmed high-Ra in the “Kinki Spot” and inferred
that the mantle helium was derived from the dehydration
of young and warm Philippine Sea Plate. They further
suggested that the occurrences of non-volcanic deep tremors in the Kii Peninsula were caused by the movement of
fluids derived from the subducting slab. Recently Umeda
et al. (2006) have reported again high-Ra emanation and
conductive anomaly in the region. They have attributed
these features into the aqueous fluids expelled from the
Philippine Sea Plate. However the magma aging effect
indicates that it is difficult to maintain the high-Ra in the
Philippine Sea Plate as discussed above. It is not likely
that the high-Ra was derived from the dehydration suggested by Matsumoto et al. (2003) and Umeda et al.
(2006). In addition the slab melting can not account for
the high-Ra. The other source with pristine mantle material is definitely needed in the region.
We have compiled helium isotopes data in the Kinki
district of SW Japan (Sano and Wakita, 1985; Wakita et
Helium isotopes and seismic tomography
57
Fig. 11. Schematic diagram of vertical cross-section of the crust
and upper mantle beneath Kinki, SW Japan.
Fig. 10. Vertical cross-section of S-wave perturbations along
line C–C ′ in Fig. 8a (from Nakajima and Hasegawa, 2007).
Red and blue colors represent low and high velocities, respectively, according to the scale at the bottom.
al., 1987; Matsumoto et al., 2003; Umeda et al., 2006)
and plotted in the map based on the above definition (Fig.
9a). The volcanic front is not really defined in northern
Kinki region. We tentatively extend the volcanic front
from the Chugoku district to the northern Kinki. It is apparent that medium-Ra and high-Ra are locating even in
frontal arc region, which is completely different from
geographical distributions of helium isotopes in NE Japan (Fig. 2a), and Chugoku and Shikoku, SW Japan (Fig.
5a). Figure 9b shows the relationship between the distance from the hypothetical volcanic front and the 3He/
4
He ratio expressed in the unit Ra. This helium profile is
apparently anomalous compared with those of NE Japan,
and Chugoku and Shikoku, SW Japan. In order to describe
the cause of high-Ra in the frontal arc of SW Japan, Sano
and Wakita (1985) proposed newly developing volcanism
in the region by moving of the volcanic front. If this is
the case, it is necessary to explain the observation that
high-Ra is located at about 250 km wide region across
the island arc system. This is significantly wider than
those in NE Japan, about 100 km (see Fig. 2b) and in the
Chugoku district, less than 100 km (see Fig. 5b). Breakdown of secondary minerals may not produce aqueous
fluids from the descending slab with such a long distance
and/or variable depth (Iwamori, 1998). Thus the magma
genesis model of NE Japan (Fig. 4) may not be easily
58
Y. Sano and J. Nakajima
applicable to the Kinki district.
Figure 10 indicates a vertical cross-section of S-wave
velocity perturbations along line C–C′ in the map of Fig.
9a (from Nakajima and Hasegawa, 2007). This indicates
the existence of a large low-velocity anomaly below the
Philippine Sea slab, very similar to those of the Chugoku
and Shikoku districts. Taking into account of the absence
of a high-velocity anomaly corresponding to the
subducted Philippine Sea slab beneath the Kii Peninsula,
Nakajima and Hasegawa (2007) have suggested that the
shallow low-velocity zone is connecting with a large lowvelocity anomaly below the Philippine Sea plate somewhat passing through the slab. Figure 11 shows a schematic diagram of the Kinki district of SW Japan which
explains geographical distribution of helium isotopes.
There may be a large fissure of the Philippine Sea slab
tear induced by the different dip angle of the subducting
ocean plate. Upwelling mantle materials with high-Ra
may penetrate through the fissure and may arrive at the
shallow crust of the region. The fissure might have occurred along the fossil spreading ridge of the Shikoku
Basin that is now subducting beneath the Kii Channel.
Even though this model is highly schematic and admittedly oversimplified, this can explain the observation of
significantly wide distribution of high-Ra across the island arc. In order to constrain the model more precisely,
extensive sampling and analysis of helium isotopes in
northern Kinki region are highly desirable together with
more tomographic researches.
CONCLUSIONS
This paper describes a geographical distribution of
helium isotopes in Japan complied from literatures and
presents new interpretation on the magma genesis based
on the precise seismic tomography data. In NE Japan, a
carrier of the primordial helium is well visualized by lowvelocity upwelling flow in the upper mantle generated in
dehydration of Pacific slab. The source magma may be
separated from the flow in the lower crust immediately
below the volcanic front. Radiogenic helium in frontal
arc is simply attributable to the absence of the magma. In
volcanic arc of Chugoku, SW Japan, the mantle helium
may be derived from the upwelling flow originated beneath the Philippine Sea plate, which looks passing
through the tip of the plate by a contour of low velocity
anomaly. In Shikoku of SW Japan medium values of helium isotopes are situating at the area about 100 km from
the volcanic front. The helium characterized by significant mantle component may be attributed to the dehydration of the young slab with a moderate aging effect. We
try to explain the cause of “Kinki Spot” by fissure type
tear of the subducting Philippine Sea slab. However it is
not fully resolved in this work. Extensive sampling and
analysis of helium isotopes in the region are needed to
constrain the formation mechanism of high-Ra in the frontal arc in addition to tomographic analysis in future.
Acknowledgments— We would like to thank A. Hasegawa, S.
Ueki, J. Yamamoto, H. Kumagai and T. Shibata for the fruitful
discussion. Constructive comments by B. Marty and anonymous
reviewer were useful to revise the paper. This work was partially supported by a grant (No. 17101001) from the Ministry
of Education, Culture, Sports, Science and Technology of Japan.
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