The system Mg 2SiO 4MgOH 2O at high pressures and temperatures — Possible hydrous magnesian silicates in the mantle transition zone

Abstract

The system Mg 2SiO 4MgOH 2O was investigated at pressures between 85 and 160 kbar and at temperatures between 750 and 1200°C. In runs for a gel with Mg/Si ratio of 3 and with 4.0 wt.-percent H 2O, a dense hydrous magnesian silicate, denoted phase B by Ringwood and Major, was found at pressures from about 100 kbar to at least 160 kbar in the whole temperature range studied. In the following table the crystallographic parameters and chemical formula of phase B, determined in this study, are compared with those of the other dense hydrous silicates in Mg 2SiO 4MgOH 2O. Phase Composition Space group Cell parameters Density (g cm −3) a b c β (Å) (Å) (Å) (°) Chondrodite Mg 5Si 2O 10H 2 P2 1/c 7.914 4.752 10.350 108.71 3.06 Clinohumite Mg 9Si 4O 18H 2 P2 1/c 13.695 4.747 10.284 100.64 3.14 Phase A Mg 7Si 2O 14H 6 P6 3 7.860 9.573 2.96 Phase B Mg 23Si 8O 42H 6 P2 1/c 10.600 14.098 10.092 104.05 3.32 Below about 100 kbar, phase B decomposed into the assemblage of phase A + forsterite + periclase at relatively low temperatures and into the assemblage of chondrodite + periclase + forsterite at relatively high temperatures. In runs for a gel with an Mg/Si ratio of 2 and with 2.4 wt.-percent H 2O, phase A was found to coexist with forsterite (or modified spinel) and clinoenstatite up to 135 kbar at 750°C. In the synthesis field of phase B, however, no indication of the reaction, forsterite + water = phase B + enstatite, was detected. While this study indicates the stability of phase B for bulk compositions which are richer in MgO than accepted mantle rocks, phase B is regarded, in addition to phase A, as a potential dense hydrous silicate in the depth range from about 300 km to at least 460 km in the mantle, particularly in the subduction zone.