High-Pressure Equation of State of Molten Anorthite and Diopside

Abstract

New Hugoniot equation of state data for molten diopside (at 1773 K) and molten anorthite (at 1923 K) are reported to 38 and 35 GPa, respectively. The diopside data (initial density, 2.61 Mg/m^3) are described by a straight-line fit to the shock velocity-particle velocity results of U_s = 3.30 + 1.44 U_p km s^(−1), and our preferred fit to the anorthite data (initial density, 2.55 Mg/m^3) is given by U_s = 2.68 + 1.42 U_p km s^(−1). Reduction of the data to a third-order Birch-Murnaghan isentrope assuming the Gruneisen ratio times the density is a constant, and the Mie-Gruneisen equation of state gives K_(0s) = 22.4 GPa and K′_s = 6.9 for diopside. For anorthite we calculate K_(0S) = 17.9 GPa and K′_s = 5.3. The present data for diopside are used to calculate the diopside solidus at high pressures. We expect the solidus to be shallow above ∼ 10 GPa, but the lack of data on the variation of either the Gruneisen parameters of the liquid and crystal or the heat capacity and thermal expansion at elevated pressures makes extrapolation of fusion curves uncertain. Solidus temperatures of 2400–2500 K and 2560–2705 K for diopside are calculated at 10 and 20 GPa, respectively. The new data are combined with those of Rigden et al. [1988] for the Di_(0.64)A_(0.36) eutectic composition to examine the degree to which such liquids mix ideally with respect to volume up to ∼ 25 GPa. For the eutectic composition at 1400°C we calculate the volumes of the An and Di mix nearly ideally to 25 GPa. We find that the ratio of the partial molar volumes of the oxides in silicate melts to that of the crystal oxides at 1673 K and 1 atm is 1.0 ± 0.1 for a wide range of oxide components. For the low-pressure tetrahedrally coordinated oxides (e.g., SiO_2, Al_2O_3, Fe_2O_3) the ratio is >1.3 with respect to oxides such as stishovite, corundum, and hematite in which the cations are octahedrally coordinated by oxygens. If changes in coordination of Al and Si from tetrahedral at low pressures to octahedral at high pressures occur in melts, they do so gradually over an interval of ∼ 40 GPa. Although 1 atm bulk moduli for a wide compositional range of silicate melts are similar, the differences in integrated compression to mixed oxide-like high-pressure configurations are reflected mainly by variations in K^′_T. K^′_T is found to vary inversely with fraction of network forming initially tetrahedrally coordinated cations (e.g., Al^(3+), Si^(4+)). Thus K^′_T, which may be uncertain by ±1.5, is estimated to vary from ≲7 for molten anorthite, enstatite, and diopside to ∼ 8 for molten ferrosilite to ∼ 10 for molten forsterite and to ∼ 11 for molten fayalite.

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