Ultrasonic Sound Velocity of Carbonate Liquids at High Pressure and Temperature

The knowledge of the physical properties (e.g., density, compressibility, viscosity) of carbonate liquids at high pressures can help understand their stability and migration in the mantle, which in turn are important in understanding a wide range of geological problems pertaining to the deep carbon cycle. Examples of these problems include the thermodynamic modeling of carbonatitic magmas at depth, the recycling of carbonates to deep Earth through subduction (Hammouda and Keshav, 2015), the mechanism for the fast ascent of kimberlite at depth (Russell et al., 2012), and possible velocity/electrical conductivity anomalies in the mantle caused by carbonatite melts (e.g., Presnall and Gudfinnsson, 2005; Gaillard et al., 2008). However, the density and elastic properties of carbonate liquids at high pressures remain poorly constrained, due to the fact that commonly used techniques for liquid density measurements (e.g., sink-float, X-ray absorption, shock compression) are not suitable to carbonate liquids. In this study, we determined the sound velocity of dolomite (CaMg(CO₃)₂) liquid under high pressure and temperature conditions from 1 to 6 GPa and 1573 to 1873 K in a multi-anvil apparatus, using the ultrasonic technique combined with synchrotron X-ray diffraction and imaging. These data can help constrain the equation of state (EOS) of dolomite liquid, as sound velocities measured at high pressures can provide direct information on the bulk modulus (K) and its pressure derivative (K'). Our results show that the sound velocity of dolomite liquid increases with pressure, decreases with temperature, and is much slower than that of a typical silicate liquid (e.g., diopside liquid) at similar conditions. This suggests that carbonatite melts could reduce the seismic wave velocity more efficiently than silicate melts if a small amount were present in the mantle. The results of this study can be used to derive density and velocity profile for carbonate liquids at high pressures. Combined with previous viscosity measurements on carbonate liquids (Kono et al., 2014), these results will help us better model the mobility and ascending rate of carbonatite melts at depth.