Density and sound speed measurements on model basalt (An–Di–Hd) liquids at one bar: New constraints on the partial molar volume and compressibility of the FeO component

Abstract

Density and sound speed measurements were obtained over a wide range of temperature for three model basalt liquids in the An–Di–Hd (CaAl2Si2O8–CaMgSi2O6–CaFeSi2O6) system. High-temperature (1585–1838 K) double-bob density measurements were combined with low-temperature (943–930 K) measurements at the limiting fictive temperature for each sample to provide liquid volume data over a temperature interval of ∼900 K. In addition, relaxed sound speeds were obtained with a frequency sweep acoustic interferometer from 1665–1876 K. An ideal mixing model for molar volume, thermal expansivity, and isothermal compressibility recovers the new data from this study and leads to the following fitted values (±2σ) at 1723 K for V¯FeO (12.86±0.32 cm3/mol), ∂V¯FeO/∂T((3.69±1.16)×10−3 cm3/mol-K) and β¯T,FeO((4.72±0.46)×10−2 GPa−1). These volumetric properties for the FeO component are estimated to reflect Fe2+ in an average coordination of 5.7 (±0.2), based on the relationship between V¯FeO and Fe2+ coordination derived by a comparison to mineral molar volumes (Guo et al., 2013). Application of these volumetric data to a calculation of the pressure dependence of the Fe–Mg exchange reaction between orthopyroxene and basaltic liquid results in a small decrease in Fe–MgKD with pressure. In contrast, partial melting experiments of peridotite show a small increase in Fe–MgKD(opx-liq) with pressure (e.g., Walter, 1998). This difference in the pressure dependence is proposed to reflect the role of alkalis in reducing the average coordination number of Fe2+ toward five compared to the alkali-free model basalt compositions in this study, thus changing the volume and compressibility of the FeO liquid component. The results from this study may be most appropriately applied to lunar basalts, which are impoverished in alkalis.