The system Mg2SiO4MgOH2O at high pressures and temperatures — Possible hydrous magnesian silicates in the mantle transition zone

摘要: Abstract The system Mg2SiO4MgOH2O was investigated at pressures between 85 and 160 kbar temperatures 750 1200°C. In runs for a gel with Mg/Si ratio of 3 4.0 wt.-percent H2O, dense hydrous magnesian silicate, denoted phase B by Ringwood Major, found from about 100 to least in the whole temperature range studied. following table crystallographic parameters chemical formula B, determined this study, are compared those other silicates Mg2SiO4MgOH2O. Phase Composition Space group Cell Density (g cm−3) b c β (A) (°) Chondrodite Mg5Si2O10H2 P2 1 /c 7.914 4.752 10.350 108.71 3.06 Clinohumite Mg9Si4O18H2 13.695 4.747 10.284 100.64 3.14 A Mg7Si2O14H6 P6 7.860 9.573 2.96 Mg23Si8O42H6 10.600 14.098 10.092 104.05 3.32 Below kbar, decomposed into assemblage + forsterite periclase relatively low chondrodite high temperatures. an 2 2.4 coexist (or modified spinel) clinoenstatite up 135 750°C. synthesis field however, no indication reaction, water = enstatite, detected. While study indicates stability bulk compositions which richer MgO than accepted mantle rocks, is regarded, addition A, as potential silicate depth 300 km 460 mantle, particularly subduction zone.