Sound Velocities of Fe-C and Fe-Si alloying liquids at high pressures

Geophysical and geochemical observations suggest light elements such as S, Si, C, O, H, etc., are likely present in the Earth's outer core and the molten cores of other terrestrial planets and moons including Mercury, Mars, Earth's Moon, and Ganymede. In order to constrain the abundances of light elements in planetary cores, it is crucial to determine the density and sound velocity of Fe-light element alloying liquids under core conditions. In this study, sound velocities of Fe-rich liquids were determined by combining the ultrasonic measurements with synchrotron X-ray radiography and diffraction techniques under high-pressure and temperature conditions from 1 to 6 GPa and 1573 to 1973 K. An Fe-C composition (Fe-5wt%C) and four Fe-Si compositions (Fe-10wt%Si, Fe-17wt%Si, Fe-25wt%Si, and FeSi) were studied. Compared to our previous results on the velocity of Fe and Fe-S liquids at high pressures (Jing et al., 2014, Earth Planet. Sci. Lett. 396, 78-87), the presence of both C and Si increases the velocity of liquid Fe, in contrast to the effect of S. The measured velocities of Fe-C and Fe-Si liquids increase with compression and decrease slightly with increasing temperature. Combined with 1-atm density data in the literature, the high-pressure velocity data provide tight constraints on the equations of state and thermodynamic properties such as the adiabatic temperature gradient for Fe-C and Fe-Si liquids. We will discuss these results with implications to planetary cores.