High-Pressure Spin Transition and Compressional Behavior of Fe- and Ni-Bearing Monosulfide Solid Solutions

Monosulfide solid solutions (mss) with a metal-to-sulfur ratio of 1:1 represent major sulfide phases in the Earth’s upper mantle and are commonly found as inclusions in diamonds and mantle-derived rocks (Stachel & Harris, 2008; Beyer et al., 2022). Their compositions commonly involve Fe, Ni, Cu, and Co, forming extensive solid solutions. The presence of transition metals in these phases may trigger pressure-induced electronic transitions (Lavina et al., 2010), associated with anomalous elastic and structural behavior under deep Earth conditions In this study, Fe-Cu and Ni-Cu mss end-members ((Fe0.90Cu0.08)S and (Ni0.92Cu0.08)S respectively) were synthesized at 5.2 GPa and 1000 °C using a multianvil apparatus. Their high-pressure behavior was investigated up to apx. 9 GPa by synchrotron single-crystal X-ray diffraction in diamond anvil cells. Synchrotron Mössbauer source spectroscopy was also performed on the Fe-bearing sample. Fe-bearing mss exhibits a pronounced elastic anomaly from 3-5 to 6 GPa, characterized by enhanced compressibility and a volume decrease associated with a pressure-induced high-spin to low-spin transition of Fe. Specifically, the room-pressure high-spin configuration is characterized by a 6% volume excess compared to the volume expected extrapolating the low-spin equation of state. Mössbauer spectroscopy independently confirmed the occurrence of a continuous electronic transition. In contrast, the Fe-free Ni-Cu mss displays smooth compression without detectable anomalies, demonstrating that the observed behavior is intrinsically linked to the electronic state of iron. Comparison with natural troilite (FeS) and pyrrhotite (Fe7S8) highlights the strong influence of spin transitions on the compressional properties of Fe-bearing sulfides, consistent with previous observations on FeS systems (Kobayashi et al., 1996). These findings demonstrate that electronic spin transitions significantly affect the density and elastic behavior of mantle sulfides and must be explicitly considered when estimating trapping pressures of sulfide inclusions in diamonds (Gilder et al., 2011) and modeling the physical properties of sulfide-bearing assemblages in planetary interiors. References Beyer C. et al. (2022) - High-pressure phase relations in the system Fe-Ni-Cu-S up to 14 GPa: Implications for the stability of sulfides in the earth’s upper mantle. Contributions to Mineralogy and Petrology, 177(10), 99, https://doi.org/10.1007/s00410-022-01966-x Gilder S. A. et al. (2011) - Anatomy of a pressure‐induced, ferromagnetic‐to‐paramagnetic transition in pyrrhotite: Implications for the formation pressure of diamonds. Journal of Geophysical Research: Solid Earth, 116(B10), https://doi.org/10.1029/2011JB008292 Lavina B. et al. (2010) - Effect of dilution on the spin pairing transition in rhombohedral carbonates. High Pressure Research, 30(2), 224-229, https://doi.org/10.1080/08957959.2010.485391 Stachel T. & Harris J. W. (2008) - The origin of cratonic diamonds-constraints from mineral inclusions. Ore Geology Reviews, 34(1-2), 5-32, https://doi.org/10.1016/j.oregeorev.2007.05.002