EarthRef.org Reference Database (ERR) -- Sherman 1997

The Earth's core is an alloy of iron and a light element such as sulphur, oxygen or silicon. Starting with static equations of state of Fe, Fe (sub 3) Si, Fe (sub 7) S, Fe (sub 3) S and FeS, calculated from density functional theory, we can estimate the core composition as a function of thermal pressure. We can relate the thermal pressure at a given density to temperature using the Mie-Gruneisen formalism. There is a significant (4%) excess volume of mixing along the Fe-FeS binary at core pressures. Consequently, the density of the Earth's core can be explained using much less sulphur (2.0-8.0 wt%) than that previously estimated. However, the amount of sulphur required is still greater than that expected from the abundance of less volatile elements and suggests that a second light element is needed. The equation of state of Fe (sub 3) Si shows that the core can accommodate as much 8.7% Si at geophysically reasonable thermal pressures. A core composition of 7.3% Si and 2.3% S, as proposed by Allegre et al. [10] for a chondritic bulk Earth Si/Mg, implies a thermal pressure of 15 GPa at the core-mantle boundary assuming a -3% density change of melting for Fe. A 15 GPa thermal pressure, in turn, requires a temperature of 3000 K for a Gruneisen parameter gamma = 1.38. At the inner core boundary, a core composition of 7.3% Si and 2.3% S implies a thermal pressure near 35 GPa which, in turn, implies a temperature near 4400 K. Somewhat higher core temperatures would result if the sulphur content were as low as that recently proposed. It appears that a sulphur-depleted Earth with a chondritic Mg/Si ratio (with excess Si incorporated into the core) is consistent with the observed density of the core, the calculated equations of state, and the estimated thermal pressures. Finally, the results obtained here completely remove any need to accommodate oxygen in the core; this is consistent with the previously found [14] extreme instability of Fe-FeO alloy phases.