HR: 0800h
AN: V41A-1428    [Abstracts]
TI: Slab-Derived Noble Gases Preserved in Wedge Mantle Peridotite From the Sanbagawa Belt, Shikoku, Japan
AU: * Sumino, H
EM: sumino@eqchem.s.u-tokyo.ac.jp
AF: Laboratory for Earthquake Chemistry, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Mizukami, T
EM: miz@nagoya-u.jp
AF: Division of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Chikusa-ku, Nagoya, 464-8602 Japan
AU: Wallis, S R
EM: swallis@eps.nagoya-u.ac.jp
AF: Division of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Chikusa-ku, Nagoya, 464-8602 Japan
AB: Subduction volcanism is generally considered to form a 'subduction barrier' that efficiently recycles volatile components contained in subducted slabs back to the Earth's surface. Water-rich fluids released from subducting slabs play an important role in this volatile circulation. However, noble gases in the fluids are not well investigated. The Higashi-akaishi peridotite body in the Sanbagawa metamorphic belt, southwest Japan, is a possibly unique example of a km-scale sliver of a former mantle wedge exhumed from depths of at least 100 km. The body mainly consists of dunite with minor amounts of garnet-bearing rocks. Penetrative porphyroclastic microstructure in the dunite is associated with the development of olivine LPO patterns implying hydrous and high-stress conditions. Serpentine dominated micro-inclusions in the porphyroclastic olivine grains are regarded as relics of former water-rich inclusions developed in the wedge mantle above a subducting slab. Thus, it is expected that these micro-inclusions should preserve noble gas characteristics of slab-derived fluids and measurements of their compositions could provide important constraints on geochemical models of subduction zones. To extract noble gases from the micro-inclusions we first step heated the samples up to 800°C, above the breakdown temperature of serpentine minerals, before crushing them in vacuo. This treatment ensures both platy antigorite matrix grains and cross-cutting lizardite veins that formed after the micro-inclusions broke down during the heating and their contained noble gases were almost entirely released at this stage. In contrast, micro-inclusions are enclosed in thermally stable "capsules" of olivine and their noble gases should be dominantly released during the subsequent crushing. The determined isotopic characteristics of noble gases in the micro-inclusions are as follows. (1) {}³He/{}⁴He ratios of about 2 Ra representing a mixture of mantle and radiogenic He. (2) {}⁴⁰Ar/{}³⁶Ar ratios up to 440 that are close to atmospheric ratios with a small contribution of mantle and/or radiogenic Ar. The mantle He and atmospheric Ar should be an intrinsic feature of the micro-inclusions. This implies that Ar in the wedge mantle was heavily contaminated by an atmospheric component derived from the subducting slab. Ar analyses of neutron-irradiated samples revealed insignificant contribution of in situ radiogenic {}⁴⁰Ar accumulated after the formation of micro-inclusions that predates cooling ages of the Sanbagawa metamorphism (c. 76-94 Ma). However, it remains uncertain whether slab-derived He (possibly crust-like, enriched in radiogenic He) is contained in the micro-inclusions, due to relatively poor constraints on the amount of in situ radiogenic He production. The noble gas isotopic features of the micro-inclusions are similar to those of arc volcanic gases and rocks. This implies that the Higashi-akaishi peridotite body has frozen in and preserved an inferred but previously unseen part of the recycling process whereby noble gases (and probably other volatiles) are injected into the wedge mantle just above the subducting slab before being recycled back to the atmosphere via arc volcanism.
DE: 1000 GEOCHEMISTRY
DE: 1030 Geochemical cycles (0330)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005