Slab-derived halogens and noble gases illuminate closed system processes controlling volatile element transport into the mantle wedge

Masahiro Kobayashi,Hirochika Sumino,Keisuke Nagao,Satoko Ishimaru,Shoji Arai,Masako Yoshikawa,Tatsuhiko Kawamoto,Yoshitaka Kumagai,Tetsuo Kobayashi,Ray Burgess,Chris J Ballentine

doi:10.1016/j.epsl.2016.10.012

Abstract

Halogen and noble gas systematics are powerful tracers of volatile recycling in subduction zones. We present halogen and noble gas compositions of mantle peridotites containing H2O-rich fluid inclusions collected at volcanic fronts from two contrasting subduction zones (the Avacha volcano of Kamchatka arc and the Pinatubo volcano of Luzon arcs) and orogenic peridotites from a peridotite massif (the Horoman massif, Hokkaido, Japan) which represents an exhumed portion of the mantle wedge. The aims are to determine how volatiles are carried into the mantle wedge and how the subducted fluids modify halogen and noble gas compositions in the mantle. The halogen and noble gas signatures in the H2O-rich fluids are similar to those of marine sedimentary pore fluids and forearc and seafloor serpentinites. This suggests that marine pore fluids in deep-sea sediments are carried by serpentine and supplied to the mantle wedge, preserving their original halogen and noble gas compositions. We suggest that the sedimentary pore fluid-derived water is incorporated into serpentine through hydration in a closed system along faults at the outer rise of the oceanic, preserving Cl/H2O and 36Ar/H2O values of sedimentary pore fluids. Dehydration–hydration process within the oceanic lithospheric mantle maintains the closed system until the final stage of serpentine dehydration. The sedimentary pore fluid-like halogen and noble gas signatures in fluids released at the final stage of serpentine dehydration are preserved due to highly channelized flow, whereas the original Cl/H2O and 36Ar/H2O ratios are fractionated by the higher incompatibility of halogens and noble gases in hydrous minerals.

Highlights

Water is one of the most important volatiles in many processes studied in earth sciences
We report halogen and noble gas compositions in mantle xenoliths collected at the volcanic fronts from two different subduction zones, and orogenic peridotites from a peridotite massif, that latter represents a portion of the mantle wedge infiltrated with slab-derived H2O–CO2 rich fluids
The halogen and noble gas signatures of the H2O-rich fluid inclusions in the peridotites sampled along the volcanic fronts from two subduction zones showing contrasting thermal regimes are similar to those of marine sedimentary pore fluids and serpentinites. This observation suggests that serpentine-derived water supplied to the mantle beneath volcanic fronts has preserved its original halogen and noble gas signatures derived from sedimentary pore fluid during the subduction processes

Summary

Introduction

Water is one of the most important volatiles in many processes studied in earth sciences. It is a major component degassed from the Earth’s interior through volcanism at mid-ocean ridges, hot spots, and arcs and is returned via subduction processes. The water is released from the subducting slabs to the mantle wedge through dehydration of those hydrous minerals. Volcanism and seismicity associated with subduction zones is linked to the dehydration processes and release of water from the subducting slabs (Mitsui and Hirahara, 2009; Tatsumi, 1989), the detail of hydrous mineral and water release mass balance remains debated (e.g., Iwamori, 2007; van Keken et al, 2011; Wada et al, 2012)

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