Iron-rich eutectic liquid composition in Fe-Si system at core pressures: Ex-situ textural and chemical characterization of DAC samples

Abstract:

Melting phase diagram of iron–light-element system is helpful to identify the light component in the core. If the outer core contains a single light element and the relevant binary phase diagram has a eutectic point, the core composition must lie on the iron-rich side of the eutectic. The eutectic liquid compositions in such binary systems have been examined at high pressure in a diamond-anvil cell (DAC), based on the disappearance of one of solid phases from in-situ X-ray diffraction patterns (e.g., Seagle et al., 2008 EPSL for Fe–FeO; Morard et al., 2008 EPSL for Fe–Fe₃S). However, the loss of a given solid phase from the X-ray diffraction pattern does not necessarily mean its disappearance from a sample but is sometimes caused by extensive grain growth. And, it is difficult to eliminate a solid phase when axial temperature gradient is strong in a laser-heated sample. Alternatively, ex-situ textual and chemical characterization of a DAC sample could provide more solid information on the compositions of quenched liquid and coexisting solid. Here we estimated the eutectic liquid composition and the difference in silicon abundance in coexisting liquid and solid in an iron-rich portion of the Fe-FeSi binary system at high pressures to 134 GPa. Partially melted samples (Fe with 2, 4, 6.5, and 9 wt.% Si) were recovered from a DAC, and then examined with dual beam scanning microprobe (FIB + FE-SEM) (Versa 3D^TM, FEI) or field-emission-type electron probe microanalyzer (FE-EPMA) (JXA-8530F, JEOL). The X-ray map of Si demonstrated the differentiation of silicon between coexisting liquid and solid phases quantitatively. The results demonstrate that the Si content in eutectic liquid decreases rapidly with increasing pressure. If the outer core includes ~12 wt.% Si as a single light element, it crystallizes the CsCl-type (B2) phase that is more enriched in silicon than coexisting liquid. Therefore, silicon cannot be the sole light component in the core, although its presence is strongly supported by Si isotopic composition as well as Mg/Si ratio of the mantle.