- journal_title：American Mineralogist
- Contributor：Ayako Shinozaki ; Hisako Hirai ; Hiroaki Ohfuji ; Taku Okada ; Shin-ichi Machida ; Takehiko Yagi
- Publisher：Mineralogical Society of America
- Date：2013-08-01
- Format：text/html
- Language：en
- Identifier：10.2138/am.2013.4434
- journal_abbrev：American Mineralogist
- issn：0003-004X
- volume：98
- issue：8-9
- firstpage：1604
- section：Articles
High-pressure and high-temperature experiments were carried out in a Mg<sub>2sub>SiO<sub>4sub>-H<sub>2sub> system using laser-heated diamond-anvil cells to understand the influence of H<sub>2sub> fluid on the stability of forsterite. In situ X-ray diffraction experiments and Raman spectroscopic measurements showed the decomposition of forsterite, and formation of periclase (MgO) and stishovite/quartz (SiO<sub>2sub>) in the presence of H<sub>2sub> after being heated in the range between 2.5 GPa, 1400 K and 15.0 GPa, 1500 K. Transmission electron microscopic observation of the samples recovered from 15.0 GPa and 1500 K showed that the granular to columnar periclase grains maintained the original grain shape of forsterite, indicating that the periclase crystals crystallized under high temperature. On the other hand, euhedral columnar stishovite crystals were found at the boundaries between residual forsterite grains and reacted periclase. This implies that the SiO<sub>2sub> component was dissolved in H<sub>2sub> fluid, and that stishovite was considered to have crystallized when the solubility of the SiO<sub>2sub> component became reduced with decreasing temperature. Additional experiment on a SiO<sub>2sub>-H<sub>2sub> system clearly showed the dissolution of quartz in H<sub>2sub> fluid, while those on a MgO-H<sub>2sub> system, periclase was hardly dissolved. These lines of evidence indicate that forsterite was incongruently dissolved in H<sub>2sub> fluid to form periclase crystals in the Mg<sub>2sub>SiO<sub>4sub>-H<sub>2sub> system, which is different from what was observed in the Mg<sub>2sub>SiO<sub>4sub>-H<sub>2sub>O system. The results indicate that the stability of forsterite is strongly affected by the composition of coexisting C-O-H fluid.