Abstract
Geophysical evidences indicate that the density of the Earth's outer core is lower than that of pure liquid iron. To account for this density deficit between the density of liquid iron and geophysical observation, it has often been suggested that lighter elements are likely to be present in the Earth's core. Although carbon is abundant in the cosmos, its abundance is relatively low in the bulk of the silicate Earth. Carbon is also highly siderophile with iron carbides being observed in meteorites. These arguments are often invoked to favor carbon as a possible light element candidate in the Earth's core. To access whether carbon indeed could explain the geophysical observations, the effect of carbon on the density and bulk sound velocity of liquid iron needs to be constrained. In this study, using first-principles molecular dynamics simulations, we have explored the effect of carbon on the melt structure and equation of state of liquid iron alloy. In particular, we have explored- 4-9 wt.% of dissolved carbon at the pressure and temperature conditions relevant for the Earth's outer core. Our results show that at conditions relevant for the Earth's core-mantle boundary, for each wt. % of dissolved carbon, the density of pure iron liquid is reduced by approximately 0.138 g/cm (super 3) and the bulk sound velocity of pure iron liquid is enhanced by approximately 0.090 km/s. Thus, at pressures and temperatures corresponding to the Earth's core mantle boundary the maximum carbon content for Fe-C alloy is estimated to be approximately 1.4 wt. %. In presence of other light elements such as oxygen, sulfur, and silicon, the carbon content is likely to be lower than the upper limit predicted by this study. Acknowledgement: We acknowledge funds from NSF EAR 1753125 and computing resources from XSEDE.