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

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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 high P–T conditions. The mixing properties of Fe and Si for the iron phases were found to be approximated by ideal mixing under high P and T conditions. 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 at P > 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.
Original languageEnglish
JournalPhysics and Chemistry of Minerals
DOIs
Publication statusPublished - 3 Jul 2020

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