Partitioning of K, U, and Th between Sulfide and Silicate Liquids: Implications for Radioactive Heating of Planetary Cores

The possibility of heating of planetary cores by K radioactivity has been extensively discussed, as well as the possibility that K partitioning into the terrestrial core is the reason for the difference between the terrestrial and chondritic K/U. We had previously suggested that U and Th partitioning into FeFeS liquids was more important than Ko Laboratory FeFeS liquid, silicate liquid partition coefficient measurements (D) for K, U, and Th were made to test this suggestion. For a basaltic liquid at 1450øC and 1.5 GPa, D U is 0.013 and D K is 0.0026; thus U partitioning into FeFeS liquids is 5 times greater than K partitioning under these conditions. There are problems with 1-atm experiments in that they do not appear to equilibrate or reverse. However, measurable U and Th partitioning into sulfide was nearly always observed, but K partitioning was normally not observed (D K • 10-4). A typical value for D U from a granitic silicate liquid at one atmosphere, 1150øC, and low fO_ is about 0.02; DTh is similar. At low fO 2 and higher temperature, experiments with basaltic liquids produce strong Ca and U partitioning into the sulfide liquid with D U > 1. DTh is less strongly affected. Because of the consistently low DK/Du, pressure effects near the core-mantle boundary would need to increase D K by factors of ~103 with much smaller increases in D U in order to have the terrestrial K and U abundances at chondritic levels. In addition, if radioactive heating is important for planetary cores, U and Th will be more important than K unless the lower mantle has K/U greater than 10 times chondritic or large changes in partition coefficients with conditions reverse the relative importance of K versus U and Th from our measurements.