Density and Compressibility of FeO-bearing Silicate Melt: Relevance to Magma Behavior in the Earth.

Abstract

Developing an equation of state (P-V-T relation) for magmatic liquids that includes iron-bearing liquids is necessary for thermodynamic calculations of crystal-melt equilibrium and oxygen fugacity as a function of pressure. Currently, there is a scarcity of high-quality density and compressibility data on Fe2+-bearing silicate liquids. Moreover, there is abundant spectroscopic evidence in the literature that the average coordination number of Fe2+ in silicate liquids varies as a function of composition, which is anticipated to affect the partial molar volumetric properties of the FeO component. This dissertation systematically explores the density and compressibility of FeO-bearing melts as a function of composition using the double-bob Archimedean method and a frequency-sweep acoustic interferometer. For CaO-FeO-SiO2 (CFS) liquids, both the partial molar volume and isothermal compressibility of the FeO component increase systematically with CaO concentration, consistent with a composition-induced decrease in the average Fe2+ coordination number. A similar behavior is seen in Na2O-FeO-SiO2 (NFS) liquids, except that the compressibility of the FeO component varies inversely with its molar volum, which is unexpected. It is hypothesized that not just Fe2+ coordination changes, but also topological changes to the silicate melt network that affects the compressibility of the NFS liquids. When density and compressibility measurements are extended to melt compositions in the FeO-CaO-MgO-Al2O3-SiO2 system (model basalts), the resulting partial molar volume of FeO (by comparison to mineral volumes) suggests an average Fe2+ coordination closer to 6-fold than 5-fold. However, when alkalis (Na2O and K2O) are added to the model basalt system, the partial molar volume of FeO is larger than it is in alkali-free melts, consistent with spectroscopic evidence in the literature that shows increasing Na/Ca ratios lower the average Fe2+ coordination number. Evaluating the effect of alkalis on the compressibility of FeO in model basalts is complicated by the complex compressibility behavior of the Al2O3 component, which itself varies as a function of melt composition. In this dissertation, revised model equations for both the density and compressibility of magmatic liquids are presented, which account for the complex behavior related to Fe2+ coordination change, previously unconsidered.