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Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab

Tatsuhiko Kawamoto, Masako Yoshikawa, Yoshitaka Kumagai, Ma. Hannah T. Mirabueno, Mitsuru Okuno and Tetsuo Kobayashi
Proceedings of the National Academy of Sciences of the United States of America
Vol. 110, No. 24 (June 11, 2013), pp. 9663-9668
Stable URL: http://www.jstor.org/stable/42706062
Page Count: 6
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Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab
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Abstract

Slab-derived fluids play an important role in heat and material transfer in subduction zones. Dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow. Nevertheless, direct observations of the fluid chemistry and pressuretemperature conditions of fluids are few. This report describes CO₂-bearing saline fluid inclusions in spinel-harzburgite xenoliths collected from the 1991 Pinatubo pumice deposits. The fluid inclusions are filled with saline solutions with 5.1 ± 1.0% (wt) NaCl-equivalent magnesite crystals, CO₂-bearing vapor bubbles, and a talc and/or chrysotile layer on the walls. The xenoliths contain tremolite amphibole, which is stable in temperatures lower than 830 °C at the uppermost mantle. The Pinatubo volcano is located at the volcanic front of the Luzon arc associated with subduction of warm oceanic plate. The present observation suggests hydration of forearc mantle and the uppermost mantle by slab-derived CO₂-bearing saline fluids. Dehydration and decarbonation take place, and seawater-like saline fluids migrate from the subducting slab to the mantle wedge. The presence of saline fluids is important because they can dissolve more metals than pure H₂O and affect the chemical evolution of the mantle wedge.

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