Temperature profile of the outer core based on X-ray diffraction of Fe-Fe3S and (Fe,Ni)-(Fe,Ni)3S system

The Earth’s core is considered to be composed of Fe-Ni alloy with light element(s). Sulfur in particular has been considered as one of the most plausible light elements based on the solar abundances and high pressure partitioning experiments. Therefore, it is important to investigate the Fe-FeS system up to the core conditions to discuss its chemical and physical properties. Nevertheless, there are not previous works under the core conditions. Therefore, we have investigated the melting relationships in the Fe-Fe3S system up to 190 GPa, and those in the (Fe,Ni)-(Fe,Ni)3S system up to 100 GPa using a double-sided laser-heated diamond anvil cell (LHDAC) combined with the synchrotron X-ray. A foil of Fe86.8S13.2 or Fe75.0Ni10.0S15.0 was sandwiched between NaCl pellets. The sample was heated using fiber lasers. The shape of a fiver laser beam was changed to a flat-top using a beam shaping system at the BL10XU, SPring-8. The melting detection was based on disappearance of the X-ray diffraction (XRD) peaks derived from Fe3S or (Fe,Ni)3S. Disappearance of XRD from Fe3S was observed at 64.2, 78.0, 113.7, 136.3, and 181.6 GPa. That of XRD from (Fe,Ni)3S was observed at 74.5 and 104.6 GPa. The melting temperatures of the present results in the Fe-Fe3S system are consistent with previous works up to around 70 GPa (e.g., Campbell et al., 2007; Morard et al., 2008). However, the slope of the melting temperature in the Fe-Fe3S system is slightly gentle above 100 GPa compared to the extrapolated melting curves of previous studies. On the other hand, the melting temperatures in the (Fe,Ni)-(Fe,Ni)3S system are similar to those in the Fe-Fe3S system. This suggests that 10 at. % Ni does not affect on melting significantly. The present results were fitted to the Kraut-Kennedy law in order to extrapolate the melting temperature of the Fe-Fe3S system up to the ICB condition. The eutectic temperature of the Fe-Fe3S system at the pressures relevant to the CMB and ICB conditions are 3010(80) and 4440(140) K, respectively. If S is the only light element in the core, the eutectic temperature of 4440 K at ICB condition suggests the lower bound of TICB and the upper bound of TICB which corresponds to the temperature of the total melting temperature of the Fe-Fe3S system because the Fe-Fe3S system may show the eutectic system at the core conditions. The present ICB temperature is about 800 K below the temperature estimated by Ma et al. (2004). Campbell et al. (2007) estimated the temperature at ICB conditions to be at least 4500 K by assuming that the depression of melting temperature due to sulfur was 800 K. The present result is consistent with the lower bound of ICB temperature estimated by Campbell et al. (2007). If the adiabatic gradient through the whole outer core can be assumed, the temperature at the top of the outer core is calculated to be 3400 K assuming the temperature at ICB of 4440 K and γth of 1.3 (Stacey, 1995). The estimated temperature at the CMB condition based on the adiabatic gradient is 390 K higher than the eutectic temperature at CMB conditions.