Effect of iron on the elastic behavior of mantle monosulfide solid solution

Monosulfide solid solution (mss), with a metal-to-sulfur ratio of 1:1, is a significant sulfide phase in the upper mantle and commonly hosts elements such as Fe, Ni, Cu, and Co. It forms wide solid solutions and is unstable in all compositions in crustal environments, as evidenced by paragenetic assemblages in mantle rocks, meteorites, and inclusions in diamonds (i.e. pyrrothite, pentlandite, chalcopyrite). However, highpressure and high-temperature (HP/HT) synthesis can stabilize the end-member of this solid solution. In this study, we report on the HP/HT synthesis (multianvil press, 5 GPa, 750°C) of Fe-Cu mss (Fe0.8Cu0.2S) and Ni-Cu mss (Ni0.8Cu0.2S) and their characterization using electron microprobe and single crystal diffraction (laboratories at the Department of Earth Sciences, University of Milano). We then investigate the compressibility of these two synthetic minerals using diamond anvil cell and synchrotron X-ray radiation up to 15 GPa with He as a pressure transmitting medium. Our results indicate that Fe-bearing mss exhibits an elastic anomaly, with an increase in compressibility up to around 6 GPa and significant volume contraction, likely due to high to low spin electronic transition in Fe. Similar anomalies are also detected in natural pyrrothite. In contrast, Ni-bearing mss (Fefree system) does not exhibit such anomaly. These findings demonstrate that sulfides, particularly Fe-bearing mss, exhibit electronic transitions detectable within an accessible P-T range. Further research is needed to determine the exact P-T bracketing of this electronic transition and its implications for the thermodynamic stability and magnetic properties of these materials at planetary conditions.