Deciphering the behaviour of chalcophile and siderophile elements during planetary differentiation

The highly siderophile, or iron-loving, elements (HSE) include the more familiar precious metals (platinum, palladium, rhodium and gold) as well as exotic varieties (rhenium, osmium, ruthenium and iridium). They are all useful to society, owing to exceptional thermal, surface and electronic properties. The HSE are also of value to geologists, as they are unique in their tendency to avoid oxygen, and bond to iron, sulfur, as well as other “sulfur-like” elements, such as arsenic. The HSE are therefore sensitive to how planets separate into a metallic core, mantle and crust. My research program has involved laboratory simulation of these processes, with a specific focus on determining how the HSE get concentrated above normal background levels; be it by iron metal, other minerals, or unusual iron-sulfur (sulfide) melts. The planned research will continue work to understand the efficiency of HSE extraction from magma by sulfide melt, and also investigate the role of arsenic, an element normally scarce in the earth, but concentrated when magmas digest sedimentary rock. Research will also move into a poorly known area, and that is the role of highly pressurized water, containing dissolved solvents, at transporting the HSE, as well as the related sulfur-loving (or chalcophile) elements. Knowledge of the scavenging capacity of sulfide melts makes it easier to inventory the HSE content of other planets from measurements of their surface lavas, whose sulfide has remained at depth. This is particularly important for understanding the origin of the moon, for which interior samples are unavailable, as paradoxically, the HSE levels in lunar lavas seems to be too low if it experienced the same formation history as the earth. In some ore deposits, arsenic minerals containing high concentrations of the more valuable HSE (Pt, Rh) are found, so determining how and when they form will help to evaluate arsenic as a “primary collector” of the precious metals. Information on the role of pressurized water in dissolving the HSE and “sulfur-like” elements is used to determine the extent to which fluids produced when the ocean floor is downthrust beneath adjoining tectonic plates carry these elements upwards, into the zone of melting that gives rise to volcanoes and mineral deposits. The composition of the residual, dewatered material can then be predicted as well, which provides insight into the longterm compositional changes of the deep earth, as well as the origin of unusual element concentrations brought back to the surface when this material is melted at depth. In sum, this work seeks to provide the essential information for using the HSE and related elements to understand how planets form and evolve. Information gained from this research is also of value to the exploration and mining industry, as it provides insight into how the mechanisms by which these elements may be concentrated to economic levels.