Conclusion

1
Recycling of atmospheric argon concurrent with
pore water subduction in the Izu-Ogasawara
arc 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)
Im really sorry that I have not been able to
attend this conference. If you have a question
about this poster, please mail me at
shimizu.aya_at_iri-tokyo.jp Aya Shimizu (Now at
Tokyo Metropolitan Industrial Technology Research
Institute)
Introduction 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 Earths reservoirs. Noble
gases are subducted and recycled back to Earths
surface via arc volcanism. Studying this process
is critical for evaluating the evolution history
of the Earths 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.
The 3He/4He and 40Ar/36Ar ratios diagram of
subducting sediments, basalts and gabbros and
volcanic rocks
A box model to calculate the amount of pore water
involved in the genesis of arc magmas
3He/4He and 40Ar/36Ar ratios in each part of the
Earth (modified after Sumino et al., 2005).

• Izu-Ogasawara arc
• 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.

At least 7 of the subducting pore water is
recycled back to the atmosphere through arc
volcanism.
What is a carrier of pore water? Serpentine??
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.
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.
Candidate of atmospheric argon during subduction
processes
Kerrick (2002)
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.
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.
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.

• 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.

3He/4He and 40Ar/36Ar ratios of volcanic gas and
rock samples
The 4He/40Ar vs. 4He 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.
References Allègre C.J., Staudacher T. and Sarda
P. (1986/87) Earth Planet. Sci. Lett., 81
127-150. Barr S.R., Revillon S., Brewer T.S.,
Harvey P.K. and Tarney J. (2002) Geochem.
Geophys. Geosyst., 3(11), 8901,
doi10.1029/2001GC000255. Hirano N., Takahashi
E., Yamamoto J., Abe N., Ingle S.P., Kaneoka I.,
Hirata T., Kimura J-I., Ishii T., Ogawa Y.,
Machida S. and Suyehiro K. (2006) Science, 313
1426-1428. Hiyagon H., Ozima M., Marty B., Zashu
S. and Sakai H. (1992) Geochim. Cosmochim. Acta,
56 1301-1316. Honda M. and Patterson D.B. (1999)
Geochim. Cosmochim. Acta, 63 2863-2874. Jarrard
R.D. (2003) Geochem. Geophys. Geosyst., 4(5),
8905, doi10.1029/2002GC000392. Kerrick D. (2002)
Science, 298 1344-1345. Matsuda J. and Nagao K.
(1986) Geochem. J., 20 71-80. Staudacher T. and
Allègre C.J. (1988) Earth Planet. Sci. Lett., 89
173-183. Staudacher T., Sarda P., Richardson
S.H., Allègre C.J., Sagna I. and Dmitriev L.V.
(1989) Earth Planet. Sci. Lett., 96
119-133. Sumino H., Yamamoto J. and Kumagai H.
(2005) Japanese Mag. Mineral Petrol. Sci., 34
173-185.
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.
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).