Metal-Silicate Partitioning of Uranium at High Pressures and Temperatures

Abstract:

Uranium, along with the other long-lived radioactive elements Th and K, are important heat sources in the Earth, contributing as much as 50% of the total heat flux at the surface.[1] However, it is unknown whether these elements are also an important heat source in the core. Previous metal-silicate partitioning experiments of U have noted an increase in partition coefficient (D(U)) with decreasing oxygen fugacity and increasing S content. Regardless, they conclude that the partition coefficients are too low, even under the most extreme conditions, for U to be an important heat source in the Earth’s core. However, all of those experiments were conducted at relatively low pressures (<29 GPa) and moderate temperatures (<2700 K). We present metal-silicate partitioning experiments conducted in the diamond anvil cell at conditions more relevant to a basal magma ocean, 40-55 GPa in pressure and 3700-4900 K in temperature. Under relatively reducing conditions (ΔIW= -1.5 to -2.5, with 15-30 wt% Si in the metal) and with no sulfur present, we measure partition coefficients for U in the range of 0.01-0.1, significantly higher than has been previously reported. From these results, we are unable to rule out the possibility that U is a significant heat source in the core.

[1] Lay, et al. 2008. Nature Geo. 1, 25-32.

[2] Bouhifd, et al. 2013. GCA. 114, 13-28.

[3] Malavergne, et al. 2007. GCA. 71, 2637-2655.

[4] Wheeler, et al. 2006. GCA. 70, 1537-1547.