THE COMPOSITION OF THE EARTHS CORE - CONSTRAINTS ON S AND SI VS. TEMPERATURE

The Earth's core is an alloy of iron and a light element such as sulph ur, oxygen or silicon. Starting with static equations of state of Fe, Fe3Si, Fe7S, Fe3S 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. Consequen tly, the density of the Earth's con can be explained using much less s ulphur (2.0-8.0 wt%) than that previously estimated. However, the amou nt of sulphur required is still greater than that expected from the ab undance of less volatile elements and suggests that a second light ele ment is needed. The equation of state of Fe3Si shows that the core can accommodate as much 8.7% Si at geophysically reasonable thermal press ures. 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 pres sure of 15 GPa at the core-mantle boundary assuming a -3% density chan ge of melting for Fe. A 15 GPa thermal pressure, in turn, requires a t emperature of 3000 K for a Gruneisen parameter gamma = 1.38. At the in ner 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 nea r 4400 K. Somewhat higher core temperatures would result if the sulphu r content were as low as that recently proposed. It appears that a sul phur-depleted Earth with a chondritic Mg/Si ratio (with excess Si inco rporated into the core) is consistent with the observed density of the core. the calculated equations of state, and the estimated thermal pr essures. Finally, the results obtained here completely remove any need to accommodate oxygen in the core; this is consistent with the previo usly found [14] extreme instability of Fe-FeO alloy phases. (C) 1997 E lsevier Science B.V.