High-pressure phase relations in the system Fe-Ni-Cu-S up to 14 GPa

Base metal sulfides (Fe-Ni-Cu-S) are ubiquitous phases in mantle and subduction-related lithologies. Depending on their Cu and Ni content, sulfides may melt incongruently, leaving a solid residue called monosulfide solid solution (mss), and a Cu-Ni-rich sulfide melt. However, the persistence of crystalline sulfide phases like mss in the Earth's mantle has long been up for debate, as the presence of both mss and sulfide melt in mantle rocks would lead to the fractionation of chalcophile elements during mantle melting. Recent studies have shown that the average mantle sulfide is fully molten at average mantle potential temperatures (1300-1400 °C) up to 8 GPa (ca. 240 km). However, sulfide inclusions found in diamonds show a broad compositional spectrum, ranging from Ni-poor and Fe-rich (eclogitic), to Ni-rich and Fe-poor sulfides (peridotitic), with their Cu contents being generally low. Results on the melting properties of average mantle sulfide are thus not applicable to the full compositional range shown sulfide compositions sampled by diamonds originating from greater depths. Here we present the results of an experimental study where the melting properties of typical sulfide compositions found in diamond inclusions associated with eclogites and peridotites have been determined. Experiments have been carried out between 0.1 MPa and 14 GPa, and between 850 and 1590 °C. Results show that solid mss in Fe-rich, Ni-poor sulfide inclusions associated with eclogites persist to higher pressures and temperatures compared to their more Ni-rich peridotitic counterparts. Our results have implications for the recycling of chalcophile elements during subduction-related processes.