Thermodynamics of the system Fe-Si-O under high pressure and temperature and its implications for Earth's core

摘要

The thermodynamics of the system Fe-Si-O under high pressure (P) and temperature (T) was examined, starting with modelling the phase transition between a face-centred cubic (fcc) and hexagonal close-packed (hcp) structure in Fe-Si alloy which was previously examined by experiment under highP-Tconditions. The mixing properties of Fe and Si for the iron phases were found to be approximated by ideal mixing under highPandTconditions. The entropy changes upon melting of the end-members of the system are fairly large, and therefore the melting temperature of the Si-bearing fcc or hcp phases needs to be insensitive to the Si content, to account for the reported close compositions of coexisting liquid and solid (< 1 wt%Si atP > 50 GPa). The solidus and liquidus temperatures of Fe-Si iron alloy would therefore, not significantly be changed by the presence of Si at the inner core-outer core boundary, which enables us to evaluate the melting curve of Fe-Si fcc and hcp phases. From thus-constrained melting curve, I assessed a thermal equation of state of Si-bearing iron liquid. I then estimated a seismologically consistent outer core composition as a function of Si and O contents using the EoS for liquids constructed in this study and the literature. The best-fit composition is Fe-5.8(0.6) wt%Si-0.8(0.6) wt%O, which however does not precipitate a solid iron phase that is consistent with the inner core density. Therefore, Earth's core cannot be fully represented by the system Fe-Si-O and it should include another light element.