Influence of H₂ fluid on the stability and dissolution of Mg₂SiO₄ forsterite under high pressure and high temperature

High-pressure and high-temperature experiments were carried out in a Mg₂SiO₄-H₂ system using laser-heated diamond-anvil cells to understand the influence of H₂ 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 (SiO₂) in the presence of H₂ 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 SiO₂ component was dissolved in H₂ fluid, and that stishovite was considered to have crystallized when the solubility of the SiO₂ component became reduced with decreasing temperature. Additional experiment on a SiO₂-H₂ system clearly showed the dissolution of quartz in H₂ fluid, while those on a MgO-H₂ system, periclase was hardly dissolved. These lines of evidence indicate that forsterite was incongruently dissolved in H₂ fluid to form periclase crystals in the Mg₂SiO₄-H₂ system, which is different from what was observed in the Mg₂SiO₄-H ₂O system. The results indicate that the stability of forsterite is strongly affected by the composition of coexisting C-O-H fluid.