Preferential dissolution of SiO2 from enstatite to H2 fluid under high pressure and temperature |
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| Authors: | Ayako Shinozaki Hiroyuki Kagi Hisako Hirai Hiroaki Ohfuji Taku Okada Satoshi Nakano Takehiko Yagi |
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| Affiliation: | 1.Geochemical Research Center, Graduate School of Science,The University of Tokyo,Hongo,Japan;2.Department of Earth and Planetary Sciences, Graduate School of Environmental Studies,Nagoya University,Nagoya,Japan;3.Geodynamics Research Center,Ehime University,Bunkyo-cho, Matsuyama,Japan;4.Institute for Solid State Physics,The University of Tokyo,Kashiwanoha, Kashiwa,Japan;5.National Institute for Materials Science,Namiki, Tsukuba,Japan |
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| Abstract: | Stability and phase relations of coexisting enstatite and H2 fluid were investigated in the pressure and temperature regions of 3.1–13.9 GPa and 1500–2000 K using laser-heated diamond-anvil cells. XRD measurements showed decomposition of enstatite upon heating to form forsterite, periclase, and coesite/stishovite. In the recovered samples, SiO2 grains were found at the margin of the heating hot spot, suggesting that the SiO2 component dissolved in the H2 fluid during heating, then precipitated when its solubility decreased with decreasing temperature. Raman and infrared spectra of the coexisting fluid phase revealed that SiH4 and H2O molecules formed through the reaction between dissolved SiO2 and H2. In contrast, forsterite and periclase crystals were found within the hot spot, which were assumed to have replaced the initial orthoenstatite crystals without dissolution. Preferential dissolution of SiO2 components of enstatite in H2 fluid, as well as that observed in the forsterite H2 system and the quartz H2 system, implies that H2-rich fluid enhances Mg/Si fractionation between the fluid and solid phases of mantle minerals. |
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