1. 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. Recycling of atmospheric argon concurrent with pore water subduction in the Izu-Ogasawara arc Aya Shimizu 1, Hirochika Sumino 1, Naoto Hirano 1, Keisuke Nagao 1, Kenji Notsu 1, Shiki Machida 2 and Teruaki Ishii 2 ( 1 Laboratory for Earthquake Chemistry, Graduate School of Science, University of Tokyo, 2 Ocean Research Institute, University of Tokyo) Magmas from the volcanic front region have lower 40 Ar/ 36 Ar 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. Contribution of helium in slab-derived fluid to the mantle wedge is negligible. The difference in 40 Ar/ 36 Ar ratios of volcanic front and back arc regions may reflect the different contributions of argon in slab-derived fluid. The 3 He/ 4 He and 40 Ar/ 36 Ar ratios diagram of subducting sediments, basalts and gabbros and volcanic rocks Candidate of atmospheric argon during subduction processes A box model to calculate the amount of pore water involved in the genesis of arc magmas 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, doi:10.1029/2001GC000255. 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, doi:10.1029/2002GC000392. 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. 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 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. 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. 3 He/ 4 He and 40 Ar/ 36 Ar ratios of gas and rock samples of VF and BA regions. Yellow areas show the 3 He/ 4 He ratio of MORB and blue dotted lines show the 40 Ar/ 36 Ar ratio of Air (296). The 4 He/ 36 Ar and 40 Ar/ 36 Ar 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. The 3 He/ 4 He and 40 Ar/ 36 Ar ratios diagram of subducting sediments and basalts. Triangles show the data of volcanic rock samples and others are the data of input materials. The 4 He/ 40 Ar* vs. 4 He 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. 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. At least 7 % of the subducting pore water is recycled back to the atmosphere through arc volcanism. 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. 3 He/ 4 He and 40 Ar/ 36 Ar ratios in each part of the Earth (modified after Sumino et al., 2005). 3 He/ 4 He and 40 Ar/ 36 Ar ratios of volcanic gas and rock samples 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. 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. What is a carrier of pore water? Serpentine?? Kerrick (2002) 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 shimizu.aya@iri-tokyo.jp Aya Shimizu (Now at Tokyo Metropolitan Industrial Technology Research Institute)