Chemical compositions of the outer core examined by first principles calculations

Abstract

We have examined the density and bulk sound velocity of liquid iron alloys, (Fe, Ni)X(H, Si, O, S, C)1-X, at Earth's outer core pressure and temperature conditions based on first-principles molecular dynamics simulations. The nonideal mixing effects on volume and velocity were found to be negligible for all combinations of different liquid alloys examined. By comparing the results with seismological observations, we searched for possible chemical compositions for the outer core. Hydrogen is found to be a primary light element when the inner-core boundary temperature TICB is 4,800 K to 5,400 K. If this is the case, it is suggested that a large amount of water was delivered to the Earth during its accretionary stage and that the present-day core temperature is relatively low. On the other hand, oxygen is the most important light element if TICB = 6,000 K, consistent with the previous calculations by Badro et al. (2014) at TICB = 6,300 K. To further constrain the chemical composition of the outer core, it is necessary to take into account other constraints besides its density and bulk sound velocity; melting temperature, simultaneous solubilities of multiple of light elements, and so forth.