PLANETARY DIFFERENTIATION IN THE LABORATORY: METHODS FOR METAL-SILICATE PARTITIONING EXPERIMENTS IN THE DIAMOND ANVIL CELL AND THEIR APPLICATION TO TUNGSTEN

Siderophile element abundances in the mantle provide an important constraint on the conditions of core formation. Many studies have sought to understand the distribution of siderophile elements between the core and mantle using high temperature experiments (e.g. Drake et al., 1989; Hillgren et al. 1994; Wade & Wood, 2005; Siebert et al. 2011). This work, however, typically relies upon extrapolating results to more extreme conditions for application to Earth’s accretion. Recently, it has become possible to both perform and recover metal-silicate equilibration experiments at conditions directly relevant to the terrestrial magma ocean (Bouhifd & Jephcoat, 2011; Siebert et al. 2013).

We have used laser heated diamond anvil cells to equilibrate metal and silicate melts at 20-50 GPa, and >3000 K. A synthetic chondrite mixture was used as the starting material for experiments, which was prepared by mixing high purity metal and oxide powders. To promote sample homogeneity at the small scales required for laser-heating, Fe, W and Si metals were first ball-milled individually, before being combined with the oxide powders and undergoing a final step of ball-milling. Either polished single crystals or pressed foils of MgO were used as pressure media. To better assess equilibrium, several cells were loaded with two samples, with W present as 1) WO₃ 2) metallic W. This allows for comparison between experiments that approach equilibrium through either reduction or oxidation of W respectively.

Samples were brought to temperature by double-sided laser heating at the Advanced Photon Source, beamline 13-ID-D (GSECARS). X-ray diffraction patterns were collected before, during and after heating, to monitor sample pressure and the onset of melting. After heating, samples were laser cut from the Re gaskets and mounted on faceted aluminium blocks for sectioning by focused ion beam. Once oblique sections through the heated spots are exposed, samples are rotated to horizontal to facilitate compositional analysis by field emission electron-probe microanalysis. This method permits the recovery of multiple heating spots from each cell.

Preliminary results suggest that at high pressures and temperatures, tungsten becomes less siderophile, in agreement with predictions made on the basis of experiments done using large volume techniques.