Core Formation , Hse Partitioning and Non - Equilibrium Processes

Introduction: The compositional characteristics of the Earth's core-mantle system are best described by a model that recognizes that the present upper mantle is not in equilibrium with the core and that metal-silicate differentiation was in response to secular changes in equilibrium conditions [1-5 and references therein]. Au) that are ~100-fold depleted relative to chondritic meteorites [6-7]. These HSE characteristics are commonly interpreted as reflecting their late stage (post-core formation) introduction into the silicate Earth, typically invoked as an exogenic contribution (or Late Veneer) [1]. Alternatively, others propose that HSE abundances in the silicate Earth are the result of an equilibrium process [8-10 and references therein] between metal and silicate at P-T-X-fO 2 conditions that satisfy the compositional constraints. Equilibrium models for core-mantle differentiation require metal-silicate partitioning to be similar for all the HSE, with values of 200-800. Experiments documented partitioning in excess of 10 6 for all the HSE at modest temperatures (circa 1500-1900 K) and higher temperature experiments reveal lower partitioning values for some HSE, although the convergence of partitioning at high temperature for all the HSE has not been confirmed. Results of metal-silicate partitioning experiments for osmium and gold and show that, for conditions approximating core-formation, partitioning of osmium likely exceeds 10 7 , and the relative partitioning of osmium to gold is >10 4. In the context of equilibrium core formation models, these results are irreconcilable and require the addition of HSE-bearing material to the mantle following core formation. Methods: Experiments employed samples consisting of synthetic basalt plus a metal mixture, encapsulated in high purity graphite. The added metal mixture contained Au-encapsulated osmium, or in a few cases, Fe powder mixed with 1 to 10 wt% of Au or Au + Os. In the former configuration, preferential wetting of the Os grains by Au texturally isolates the Os from the silicate melt. This configuration inhibits Os nugget formation, which we have found to be a pernicious aspect of experiments containing this element. Experiments were done at temperatures of 2175-2590 K, at a fixed pressure of 2 GPa, using a piston-cylinder apparatus, and employing standard techniques. The amount of Fe dissolved in Au coexisting with FeO-bearing molten silicate was used to calculate the oxygen fugacity, which is expressed here relative to the iron-wustite (IW) buffer. Run products were analyzed for major elements by electron microprobe, and trace elements by laser ablation (LA) ICP-MS. Time-resolved spectra obtained from experiments …