CSEDI: Collaborative Research: Experimental Partitioning of Highly Siderophile Elements at Ultratrace Level for Understanding the Conditions of Core Formation

Washington University, Saint Louis MO

Investigators

Abstract

Gold, platinum, osmium, irridium, ruthenium, rhodium, palladium, and rhenium (known collectively as highly siderophile elements) are among the rarest elements available to mankind. Their widespread use in technology and the arts results in a high cost. It also justifies extensive mining, which has a high environmental and human health impact. The reason for their scarcity is that they were scavenged into the core when Earth separated into a silicate outer layer (mantle and crust) and a metallic core. Even if these elements are highly depleted in the mantle, previous experimental work indicates that they are overabundant relative to expectation for scavenging by the core. Available experiments suggest that the mantle should be completely barren of these elements, which is not what is seen. A likely explanation for this discrepancy between experiments and observations is that the highly siderophile elements were delivered into the Earth's mantle by the late impact addition of meteoritic material after the core had formed. There are differences however between the composition of the mantle and meteorites, notably for ruthenium, and an important question is whether previous experiments reliably predict the scavenging of highly siderophile elements in the core. These experiments were limited in the pressure-temperature conditions that they could achieve and relies on large extrapolations to make inferences about the scavenging efficiency of the core. A new experimental approach relying on the laser ionization of selected highly siderophile elements coupled with experiments done in diamond anvil cells that can routinely reach pressures of 700,000 atmospheres and temperatures of 4500 K, will allow the partitioning of highly siderophile elements to be measured under core formation conditions without relying on large extrapolations. This work will apply a relatively new technique (Resonant Ionization Mass Spectrometry - RIMS) to in situ ultratrace analyses, which can find applications in a variety of fields outside of Earth sciences, including material sciences/development and nuclear forensics. The project is a multidisciplinary collaboration between geochemists, physicists and instrument developers, an experimental petrologist, and a high-pressure mineral physicist. The project will support two graduate as well as undergraduate students, who will be trained on a multidisciplinary research project. The PIs will also be involved in outreach at the K-12 level through the French-American Science Festival. The reason for the depletions in highly siderophile elements (HSEs) in the mantle is their removal into Earth’s core, and their subsequent replenishment by late accretion of extraterrestrial material representing ~0.5 % of Earth’s mass. To first order, this model of late delivery of chondritic material to the Earth can account for the abundance of HSEs in the mantle but it fails to explain the elevated Ru/Pt and Pd/Pt ratios in the mantle relative to chondrites and other HSEs. One explanation for these high ratios is that Ru and Pd may be less siderophile or chalcophile compared to other HSEs, resulting in their partial retention in mantle when the core formed. Testing this hypothesis is however difficult because the relevant metal/silicate partitioning experiments have been done at P-T conditions that are quite remote from those that are thought to have prevailed during core formation. The investigators will study the origin of HSEs in Earth’s mantle by applying a novel ultra-trace element quantification technique known as RIMS to measure the concentrations of selected HSEs in metal-silicate experiments done using piston cylinders and diamond anvil cells (DACs). Through this collaboration between geochemists, physicists and instrument developers, an experimental petrologist, and a high-pressure mineral physicist, the research group will study the effect of nano/micro metal nuggets on metal/silicate partition data, and will measure the partition coefficients of Ru, Pd, and Pt at 0–70 GPa and 2100–4500 K, which spans conditions relevant to core formation. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.