HIGH PRESSURE REDOX GEOCHEMISTRY OF TUNGSTEN IN METAL-SILICATE SYSTEMS: IMPLICATIONS FOR CORE FORMATION IN THE EARTH
Shofner, Gregory A.
Campbell, Andrew J
Walker, Richard J
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Geochemical models of core formation are commonly based on core and mantle abundances of siderophile elements. Because of the affinity of these elements for metallic phases, they are thought to be highly concentrated in Earth's core. Tungsten is a moderately siderophile element that may provide constraints on the pressure, temperature, composition, and oxygen fugacity conditions, and on the timing of core formation in the Earth. Previous experimental studies suggest that pressure exerts little influence over tungsten metal/silicate partitioning up to 20 gigapascals (GPa). But, core formation models, based on W, predict metal-silicate equilibration pressures outside the available experimental pressure range, thus, requiring extrapolation. Therefore, higher pressure experimental data on tungsten were needed to constrain this important parameter. High pressure melting experiments were conducted to 50 GPa and 4400 K using a diamond anvil cell, and to 26 GPa and 2500 K using a multi-anvil press. Diamond anvil cell samples were sectioned using a focused ion beam instrument. The W-WO2 oxygen fugacity buffer was characterized to high pressure, also using diamond anvil cells and a multi-anvil press, combined with synchrotron x-ray diffraction. Combining the high pressure W-WO2 oxygen fugacity buffer and the database of metal/silicate partitioning data, a new approach was taken to model the Fe-W exchange reaction. Compared to the common linear method of parameterizing metal-silicate partitioning data, this approach captures the non-linear pressure dependence on oxygen fugacity, and allows for modeling of the excess Gibbs energy of mixing based on the activity ratios of Fe, FeO, W, and WO2. Applying this non-linear parameterization to the problem of core formation in the Earth, a pressure-temperature solution of 38 GPa and 3100 K in a peridotite silicate composition for a single-stage, magma ocean core formation model was determined that constrains equilibrium core formation conditions in the Earth. This solution was further constrained by Ni, Co, and Mo parameterizations from the literature. The pressure and temperature conditions of this solution represents a combination of the averaged effects of a deepening magma ocean through time and the "Moon-forming" impact.