Pressure Dependence of Proton Incorporation and Water Solubility in Olivine

摘要

Hydrous forsterite single crystals were synthesized at 8 GPa and 1250 degrees C using a multianvil apparatus under various SiO2 activity conditions. Electron microprobe and transmission Fourier-transform infrared spectroscopy analyses show that the Mg/Si atomic ratio increases with increasing water content. This provides new evidence that protons are mainly stored in Si sites in hydrous forsterite. The contribution of Mg sites for proton incorporation can be significant at <= 2.5 GPa with excess SiO2 but is negligible at higher pressures. Protons in olivine under asthenospheric conditions are therefore mostly maintained in Si sites. A summary of experimental studies suggests that the water solubility of forsterite/olivine equilibrated with excess (Mg,Fe)O is substantially higher than that with excess SiO2; however, this difference decreases with increasing pressure and becomes identical at 12-14 GPa. Owing to the low water solubility of olivine under topmost asthenospheric conditions, the geophysically observed low viscosity and low seismic velocity zones cannot be caused by olivine hydration, and the high conductivity anomaly should not be attributed to free H+-controlled proton conduction in olivine.