Chemical and physical properties of magmatic sulfides and sulfide liquids at conditions of Earth’s deep upper mantle

• Applicants: Dr. Christopher Beyer; Professor Dr. Raúl Fonseca
• Subject Area: Mineralogy, Petrology and Geochemistry ; Palaeontology
• Term: from 2020 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 451014777

Base metal sulfides (Fe-Ni-Cu-S +/- Co, and Zn) and sulfide melts are ubiquitous accessory phases in basalts, peridotites, and mantle xenoliths, and are the most frequent inclusions in diamonds. Due to their nature, they are a sink for highly siderophile elements (HSE) and economically important chalcophile elements like platin-group elements, Cu, Zn, Ag, and Pb. The abundances of HSE in planetary materials are commonly used to track the evolution of planetary reservoirs and core formation, which in conjunction with information of the Re-Os radioactive decay systems, provides us with insights into the chronology of planetary-scale processes. Fe-Ni-Cu-S inclusions are overabundant in diamonds and suggests that a genetic link between sulfides and diamonds exists. We plan to investigate compositions that resemble natural inclusions recovered from lithospheric and sublithospheric diamonds more closely, i.e. compositions with a distinct eclogitic and peridotitic metal to S and Fe to Ni ratio. To complement existing experimental data, carried out at shallow mantle conditions, we will conduct high-pressure/high-temperature experiments at deep mantle conditions (i.e. below 200 km depths to the base of the upper mantle) in equilibrium with different silicates/oxides (olivine, wadsleyite, ringwoodite, majorite, and stishovite) to unravel the conditions of sulfide saturation and their effect on element partitioning between sulfides, sulfide melts, and silicates.Sulfide melts have distinctly different physical properties (e.g. density, viscosity, electrical conductivity) compared to silicates and silicate melts. If sulfide melts develop an interconnected network, which depends on the solid-solid and solid-melt interfacial energies between sulfide melts and coexisting silicates, as well as oxygen and sulfur fugacity, they will significantly contribute to the transport of siderophile and chalcophile elements, as well as the likelihood of efficient metal segregation during planetary core formation. Moreover, sulfide melts can dissolve significant amounts of C and play a key role in the genesis of large diamonds. We will determine the dihedral angle of sulfide melts coexisting with high-pressure mantle phases to identify the conditions under which sulfide melts are mobile in the deep mantle. Moreover, we will address the solubility of C and the precipitation of diamonds from sulfide melts at pressures that represent Earth’s transition zone.

• DFG Programme: Research Grants
• Co-Investigators: Dr. René Hoffmann; Dr. Niels Jöns