Shock-wave equation of state of molten and solid fayalite

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Scientific paper

Shock-wave equations of state (EOS) of initially solid (300K) and molten (1573K) fayalite (Fe2SiO4, Fa) are measured over the pressure ranges of 23-212 and 5-47GPa, respectively. The 300K data indicates that Fa undergoes a phase change from the low-pressure olivine structure (Lpp) over the 35-55GPa range. In agreement with earlier analyses of Rockport fayalite shock data and diamond cell recovery experiments, the high pressure phase (Hpp) data are consistent with an oxide (2FeO+SiO2, stishovite) mixture. A fit to the Hpp Hugoniot data in the shock velocity (uS)-particle velocity (uP) plane yields: uS=4.07(0.22)km/s + 1.43 (0.06)uP. Here, the initial density, ρ0 is 4.375 (0.027)Mg/m3. The 1573K data yields: uS=2.63(0.02)km/s + 1.59 (0.01)uP. Initial density calculated from temperature data is 3.750 (0.018)Mg/m3 and K0S=25.9+/-0.4GPa, and K0S'=5.36+/-0.04GPa. The bulk modulus, K0S, compares favorably with Agee's result [Geophys. Res. Lett. 19 (1992a) 1169], 24.4GPa, but the pressure derivative is much less than the K0T'=10.1GPa previously reported. Molten Fa compression data >40GPa are closely fit with an ideal mixture of oxides, SiO2 (stishovite) + 2FeO (Lpp), in support of the hypothesis of Rigden et al. [J. Geophys. Res. 94 (1989) 9508]. A model molten basalt incorporating previous molten anorthite (An)-diopside (Di) eutectic and the present molten Fa (EOS) data implies that a zone of basic silicate liquid, could be neutrally buoyant at a depth of ~250-400km, upon partial melting of a peridotite mantle as discussed by Rigden et al. [Science 226 (1984) 1071]. This conclusion is based on comparison of the density of the model basalt: (An0.36Di0.64)0.85Fa0.15, with Dziewonski and Anderson's Preliminary Reference Earth model [Phys. Earth Planet. Inter. 25 (1981) 297].

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