Preferential dissolution of SiO2 from enstatite to H2 fluid under high pressure and temperature
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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.
KeywordsH2 fluid Pyroxene Laser-heated diamond-anvil cell SEM TEM
The authors thank Mr. Ohhashi (Ehime University) for his great help with high-pressure and high-temperature experiments. We thank Mr. Moroyama and Mr. Yoshida (The University of Tokyo), respectively, for supporting our use of CP and SEM, and Dr. Moriwaki (Japan Synchrotron Radiation Research Institute; JSARI) for supporting FTIR measurements at BL43IR, SPring-8. We also thank Ms. Fujimoto (The University of Tokyo) for her assistance with IR measurements. The authors are grateful to Prof. Kogure (The University of Tokyo) for helpful suggestions. This study was supported by the G-COE program Deep Earth Mineralogy. This study was financially supported by JSPS KAKENHI Grant Number 26246039. This study was conducted under the Visiting Researcher’s Program of the Institute for Solid State Physics, The University of Tokyo. The synchrotron radiation experiments were performed at BL18-C of KEK with the approval of the Photon Factory Program Advisory Committee (Proposal No. 2012753) and at BL43IR of SPring-8 with the approval of JASRI (Proposal No. 2014A1038). A part of this work was supported by “Nanotechnology Platform” (Project No. 12024046) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We are grateful to Dr. Catherine McCammon for handling this manuscript and two anonymous reviewers for their comments, which improved the manuscript.
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