The Density and Compressibility of BaCO3-SrCO3-CaCO3-K2CO3-Na2CO3-Li2CO3 Liquids: New Measurements and a Systematic Trend with Cation Field Strength

Abstract:

The volumetric properties of multi-component carbonate liquids are required to extend thermodynamic models that describe partial melting of the deep mantle (e.g. pMELTS; Ghiorso et al., 2003) to carbonate-bearing lithologies. Carbonate in the mantle is an important reservoir of carbon, which is released to the atmosphere as CO₂ through volcanism, and thus contributes to the carbon cycle. Although MgCO₃ is the most important carbonate component in the mantle, it is not possible to directly measure the 1-bar density and compressibility of MgCO₃ liquid because, like other alkaline-earth carbonates, it decomposes at a temperature lower than its melting temperature. Despite this challenge, Liu and Lange (2003) and O’Leary et al. (2015) showed that the one bar molar volume, thermal expansion and compressibility of the CaCO₃ liquid component could be obtained by measuring the density and sound speeds of stable liquids in the CaCO₃-Li₂CO₃-Na₂CO₃-K₂CO₃ quaternary system at one bar. In this study, this same strategy is employed on SrCO₃- and BaCO₃-bearing alkali carbonate liquids. The density and sound speed of seven liquids in the SrCO₃-Li₂CO₃-Na₂CO₃-K₂CO₃ quaternary and three liquids in the BaCO₃-Li₂CO₃-Na₂CO₃-K₂CO₃ quaternary were measured from 739-1367K, with SrCO₃ and BaCO₃ concentrations ranging from 10-50 mol%. The density measurements were made using the double-bob Archimedean method and sound speeds were obtained with a frequency-sweep acoustic interferometer. The molar volume and sound speed measurements were used to calculate the isothermal compressibility of each liquid, and the results show the volumetric properties mix ideally with composition. The partial molar volume and compressibility of the SrCO₃ and BaCO₃ components are compared to those obtained for the CaCO₃ component as a function of cation field strength. The results reveal a systematic trend that allows the partial molar volume and compressibility of the MgCO₃ liquid component to be estimated.