Transcript PowerPoint プレゼンテーション - MARGINS

Recycling of atmospheric argon concurrent with pore water subduction in the Izu-Ogasawara arc
I’m really sorry that I have not been able to attend this conference.
If you have a question about this poster, please mail me at [email protected]
Aya Shimizu (Now at Tokyo Metropolitan Industrial Technology Research Institute)
Aya Shimizu1, Hirochika Sumino1, Naoto Hirano1, Keisuke Nagao1, Kenji Notsu1, Shiki Machida2 and Teruaki Ishii2
(1Laboratory for Earthquake Chemistry, Graduate School of Science, University of Tokyo, 2Ocean Research Institute, University of Tokyo)
Air
Introduction
3He/4He = 1R （1.4 10-6)
A
40Ar/36Ar = 296.0
Noble gases are considered to be ideal
geochemical tracers of volatile behavior during
subduction processes because of their chemical
inertness and the large isotopic variation found in
Earth’s reservoirs.
Noble gases are subducted and recycled back to
Earth’s surface via arc volcanism. Studying this
process is critical for evaluating the evolution
history of the Earth’s interior.
The aim of this study is to investigate recycling
of volatile materials associated with the subduction
process, based on the behavior of the different
noble gas species.
OIB
5~50RA
<8000
Subduction zone
<8  1RA
<3000
The 3He/4He and 40Ar/36Ar ratios diagram of subducting
sediments, basalts and gabbros and volcanic rocks
(RA)
Plume
Plume source
(less degassed mantle)
50RA
>8000
and 40Ar/36Ar ratios in each part of the Earth (modified after Sumino et al., 2005).
(1800C heating)
Gabbro
10
1
Sea water
Less-altered oceanic crust
Radiolarian chert
Mantle wedge materials
Iyuzan
Input materials
Basalts and gabbros
from Petit spot
Kozushima
(reported by Hirano et al., 2006 )
Altered oceanic crust
0.01
200
400
600
800 1000 1200 1400
40Ar/36Ar
Magmas from the volcanic front region have lower 40Ar/36Ar ratios
than those of any subducting materials, strongly suggesting that argon in
the volcanic products is affected by not only argon trapped in the
subducting materials but also the atmospheric argon possibly dissolved in
subducting fluid.
What is a carrier of pore water? Serpentine??
F(m) = (mX / 36Ar)sample / (mX / 36Ar)air
mX = 4 He, 20Ne, 84Kr and 132 Xe
2
MORB
1
Air
Miyakejima
n
Volca
Sediments and basalts
from the ODP Site 1149
Serpentinite
Mikurajima
Candidate of atmospheric argon during subduction processes
ic f ro
Kerrick (2002)
0
1
Aogashima
Sampling sites of volcanic gases and rocks from the northern part of
the Izu-Ogasawara arc. Green and red characters show sampling points of
volcanic front (VF) and back arc (BA) regions, respectively.
Map of the Izu-Bonin-Mariana arc (modified after Barr et al., 2002)
and 40Ar/36Ar ratios of volcanic gas and rock samples
10000
1000
The 4He/40Ar*
Bubble formation process
4He/40Ar*
100
10
1
0.1
△:VF
△:BA
0.01
0.001
0.1
1
10
4He
100
(10-10)
2
3
4
F(84)
1000
10000
Subducting Pacific plate (7.0 km)
Basalts from
the ODP Site 801
Deep sea water
0
nt
Serpentinites from the
Hahajima seamount
Hachijojima
3He/4He
At least 7 % of the subducting pore water is recycled back to the atmosphere through
arc volcanism.
The 3He/4He and 40Ar/36Ar ratios diagram of subducting sediments and basalts. Triangles show the
data of volcanic rock samples and others are the data of input materials.
Izu-Oshima
Irozaki
Niijima
Shikinejima
MORB
Addition of radiogenic
4 He and 40Ar
0.1
The Izu-Ogasawara arc is located at an intra-oceanic convergent margin between the Pacific and Philippine Sea
plates. This arc is suitable to investigate the recycling of volatile elements concurrent with subduction process, because
contribution of continental crustal noble gases can be negligible.
We have measured noble gas isotopic composition of :
Volcanic gases (hot spring gases and fumaroles) and volcanic rocks (olivine phenocrysts in the volcanic rocks) from
the northern part of the Izu-Ogasawara arc as output materials.
Serpentinites from the Hahajima seamount in the Izu-Ogasawara forearc as a mantle wedge materials.
Sediments (pelagic clay and radiolarian chert) and basalts (altered oceanic crust) drilled at ODP Site 801 and 1149,
and xenoliths of gabbros, basalts (less-altered oceanic crust) from Petit spot as input materials.
Volcanic gases
and rocks from
the Northern part
of the IzuOgasawara arc
Pelagic clay
100
F(20)
Continental crust
<0.02RA
<70000
MORB source
(degassed mantle)
8  1RA
>40000
Izu-Ogasawara arc
Output materials
A box model to calculate the amount of pore water involved in the genesis
of arc magmas
Addition of extraterrestrial 3He
3He/4He
3He/4He
Mid ocean ridge
Trench
Seawater
Pelagic clay (180 m)
Chert (230 m)
Altered oceanic crust
(1.4 km)
Less-altered
oceanic crust (1.6 km)
Gabbro (4.0 km)
4He
vs.
concentration of rock
samples of VF and BA
regions. Blue dotted arrows
show the bubble formation
process during shallow
level atmospheric
contamination (Honda and
Patterson, 1999). This
graph shows that no
shallow level atmospheric
contamination was
observed.
The 4He/36Ar and 40Ar/36Ar ratios diagram of the volcanic rock samples (triangles) together with average
value of the subducting sediments and crust (stars), using the thickness of the sediments, crust and gabbros.
Since noble gases in subducting sediments and crust are not
atmospheric, sea water (with dissolved atmospheric argon) is considered
to be the best candidate for transporting argon to sub-arc depths. This
means that pore water in the subducting materials may play critical role in
the transportation of noble gases in subduction zones.
F(84) vs. F(20) plots of the rock samples. Red and
green symbols show the data of rock samples from
volcanic products and brown symbols show the data of
subducting materials. The blue stars indicated the
compositions of atmosphere and deep seawater (Allègre
et al., 1986/87). The range of MORB glasses (Staudacher
et al., 1989; Hiyagon et al., 1992) and old oceanic crust
and oceanic sediments (Matsuda and Nagao, 1986;
Staudacher and Allègre, 1988) are shown for comparison.
Although serpentinite from Hahajima
seamount may not the carrier of pore water,
serpentinite formed during faulting at the
outer rise (Kerrick, 2002) may be a possible
candidate for carrying pore water into the
mantle.
Conclusion
Based on our results, it is difficult to evaluate whether heavy noble
gases in subducting materials are completely degassed beneath the back
arc region. However, we show that subducting atmospheric argon is
effectively introduced into the mantle wedge associated with the
subducting slab -at least beneath the volcanic arc. We speculate that pore
water containing dissolved atmospheric noble gases may be transporting
gases into the mantle during the subduction process, possibly to depths
beyond the zone of arc magma generation.
References
3He/4He
and 40Ar/36Ar ratios of gas and rock samples of VF and
BA regions. Yellow areas show the 3He/4He ratio of MORB and blue
dotted lines show the 40Ar/36Ar ratio of Air (296).
Contribution of helium in slab-derived fluid to
the mantle wedge is negligible. The difference in
40Ar/36Ar ratios of volcanic front and back arc
regions may reflect the different contributions of
argon in slab-derived fluid.
Acknowledgement
This research used samples provided by the Ocean Drilling Program, which is sponsored by the U.S. National
Science Foundation and participating countries under management of Joint Oceanographic Institutions, Inc. We
greatly appreciate the thoughtful reviews and comments of Masahiko Honda (Australian National University),
Hikaru Iwamori (University of Tokyo) and Alison Shaw (Woods Hole Oceanographic Institution). A. Shimizu was
partly supported by the Sasagawa Scientific Research Grant from the Japan Science Society and COE program of
the University of Tokyo.
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