Shock-wave equation of state of molten and solid fayalite

Abstract

Shock-wave equations of state (EOS) of initially solid (300 K) and molten (1573 K) fayalite (Fe_2SiO_4, Fa) are measured over the pressure ranges of 23–212 and 5–47 GPa, respectively. The 300 K data indicates that Fa undergoes a phase change from the low-pressure olivine structure (Lpp) over the 35–55 GPa 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+SiO_2, stishovite) mixture. A fit to the Hpp Hugoniot data in the shock velocity (u_S)–particle velocity (u_P) plane yields: u_S = 4.07 (0.22)km/s + 1.43 (0.06)u_P. Here, the initial density, ρ_0 is 4.375 (0.027) Mg/m^3. The 1573 K data yields: u_S = 2.63(0.02) km/s + 1.59 (0.01)u_P. Initial density calculated from temperature data is 3.750 (0.018) Mg/m^3 and K_(0S)=25.9±0.4 GPa, and K_(0S)′=5.36±0.04 GPa. The bulk modulus, K_(0S), compares favorably with Agee’s result [Geophys. Res. Lett. 19 (1992a) 1169], 24.4 GPa, but the pressure derivative is much less than the K_(0T)′=10.1 GPa previously reported. Molten Fa compression data >40 GPa are closely fit with an ideal mixture of oxides, SiO_2 (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 ∼250–400 km, 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: (An_(0.36)Di_(0.64))_(0.85)Fa_(0.15), with Dziewonski and Anderson’s Preliminary Reference Earth model [Phys. Earth Planet. Inter. 25 (1981) 297].