Melting and Phase Relations of Fe-Ni-Si Determined by a Multi-Technique Approach

High-pressure studies on the melting curves and solid phase relations of iron alloys provide a valuable experimental approach for constraining temperature profiles and thermal evolution models of terrestrial planets, whose cores are expected to consist of iron, nickel, and some light elements. Precise constraints on core temperatures are essential for understanding major processes like inner core crystallization, magnetic field generation, core-mantle boundary heat flow, and the complex structures and phase relations of Earths lowermost mantle (Dobrosavljevic et al. 2019, Minerals). Silicon has been commonly suggested as a plausible candidate for a light element in the cores of Earth and Mercury. However, the combined effects of nickel and silicon on the high-pressure phase diagram of iron are sparsely studied and poorly understood. At the same time, the melting curve of pure iron remains controversial due to discrepancies across experimental techniques and melt-detection diagnostics. We present a multi-technique approach for measuring the melting and solid phase relations of iron alloys using laser-heated diamond anvil cell (DAC) experiments that we apply to Fe0.8Ni0.1Si0.1, a composition compatible with recent estimates for the cores of Earth and Mercury. At beamline 3-ID-B of the Advanced Photon Source (APS) of Argonne National Laboratory, we use synchrotron Mössbauer spectroscopy to probe the atomic dynamics of the iron nucleus across the solid-liquid phase boundary (Jackson et al. 2013, EPSL; Zhang et al. 2016, EPSL). At beamline 13-ID-D of the APS, using samples from the same bulk material under the same DAC loading conditions, we conduct a series of X-ray diffraction experiments to detect the onset of liquid diffuse scattering, constrain the hcp-fcc transition boundary, and measure in-situ thermal pressure evolution. We report the effect of 10-mol% silicon on the phase diagram of Fe0.9Ni0.1 and demonstrate excellent reproducibility and strong agreement in melting temperatures determined separately by the two techniques. This approach demonstrates the advantages of combining complementary experimental techniques in investigations of melting at extreme conditions and can be applied to other iron-bearing materials relevant to planetary interiors.