The system Mg2SiO4-MgO-H2O at high pressures and temperatures - Possible hydrous magnesian silicates in the mantle transition zone

Details The system Mg2SiO4-MgO-H2O at high pressures and temperatures - Possible hydrous magnesian silicates in the mantle transition zone The system Mg2SiO4-MgO-H2O at high pressures and temperatures - Possible hydrous magnesian silicates in the mantle transition zone

Dec 1980

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980pepi...23..268a&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 23, Issue 4, p. 268-275.

Physics

18

Scientific paper

The system Mg2SiO4-MgO-H2O 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 H2O, 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 Mg2SiO4-MgO-H2O.
PhaseCompositionSpace groupCell parametersDensity (g cm-3)
abcβ
(Å)(Å)(Å)(°)
ChondroditeMg5Si2O10H2P21/c7.9144.75210.350108.713.06
ClinohumiteMg9Si4O18H2P21/c13.6954.74710.284100.643.14
Phase AMg7Si2O14H6P637.8609.5732.96
Phase BMg23Si8O42H6P21/c10.60014.09810.092104.053.32
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 H2O, 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.
Now at Department of Earth Sciences, Kanazawa University, Kanazawa, 920 Japan.

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