Platinum Group Metals in Silicate Melts

The platinum group metals, rhenium, and gold, are collectively known as the highly siderophile elements (HSEs) due to their strong preference for iron metal. Because of this property, they are expected to be almost completely sequestered by Earth’s Fe-rich core when metal and silicate melts segregate during planetary accretion. HSE abundances estimated for Earth’s mantle, however, are orders of magnitude greater than expected on the basis of these elements’ chemical behaviour.

Several explanations for this discrepancy have been proposed. One possibility is that high pressure and temperature makes the HSEs less siderophile (less iron-loving). If the HSEs become less siderophile, they will not be partitioned so strongly into the metal phase during core formation. Alternatively, it has been suggested that a small amount of material is accreted to Earth subsequent to core-formation, elevating HSE abundances to their observed level. To determine which of these explanations is correct, we need experiments that show how siderophile the HSEs are at high pressure and temperature. Performing these experiments, however, is made difficult by the formation of HSE nanonuggets in silicate melt.

These figures show the signal for platinum measured by a mass spectrometer as a function of time for sample with (top) and without (bottom) nuggets

The presence of dispersed HSE nuggets in the silicate portion of experimental run-products prevents the dissolved HSE content of the silicate melt being accurately determined. To overcome this problem, we developed experimental methods that prevent transient high HSE solubility conditions during the early stages of an experiment. This prevents the over-saturation of silicate melt later in the experiment that leads to precipitation of metallic HSE nuggets. With these methods, we were able to measure robust HSE concentrations in silicate melt and demonstrate that HSEs remain extremely siderophile at high pressure and temperature. This suggests that the elevated HSE abundances of Earth’s mantle arise due to the accretion of material to Earth subsequent to core-formation.

Associated Papers
Bennett, N. R. & Brenan, J. M. 2013. Controls on the solubility of rhenium in silicate melt: Implications for the osmium isotopic composition of Earth’s mantle, Earth and Planetary Science Letters, 361, 320-332.

Bennett, N. R., Brenan, J. M., Koga, K. T. 2014. The solubility of platinum in silicate melt under reducing conditions: Results from experiments without metal inclusions, Geochimica et Cosmochimica Acta, 133, 422-442.

Bennett. N. R., Brenan, J. M., Fei, Y. 2015. Metal-silicate partitioning experiments at high pressure and temperature: Experimental methods and a procedure for the highly siderophile elements, Journal of Visualized Experiments, 100.

Brenan, J. M., Bennett, N. R., Zacjacz, Z. 2016. Experimental Results on Fractionation of the Highly Siderophile Elements (HSE) at Variable Pressures and Temperatures during Planetary and Magmatic Differentiation, Reviews in Mineralogy and Geochemistry, 81.

Bennett, N. R., Brenan, J. M., Fei, Y. 2016. Magma ocean thermometry: Controls on the metal-silicate partitioning of gold, Geochimica et Cosmochimica Acta, 184, 173-192.