Metal-silicate partitioning of cesium: Implications for core formation

Here we present the first set of metal-silicate partitioning data for Cs, which we use to examine whether the primitive mantle depletion of Cs can be attributed to core segregation. Our experiments independently varied pressure from 5 to 15 GPa, temperature from 1900 to 2400 degrees C, metallic sulfur content from pure Fe to pure FeS, silicate melt polymerization, expressed as a ratio of non-bridging oxygens to tetrahedrally coordinated cations (nbo/t) from 1.26 to 3.1, and fO(2) from two to four log.units below the iron-wustite buffer. The most important controls on the partitioning behavior of alkalis were the metallic sulfur content, expressed as X-S, and the nbo/t of the silicate liquid. Normalization of X-S to 0.5 yielded the following expressions for D-values as a function of nbo/t: logD(Na) = -2.0 + 0.44 x (nbo/t), logD(K) = -2.4 + 0.67 x (nbo/t), and logD(Cs) = -3.2 + 1.17 x (nbo/t). Normalization of nbo/t to 2.7 resulted in the following equations for D-values as a function of S content: logD(Na) = -4.1 + 6.4 x X-S, logD(K) = -7.7 + 13.9 x X-S, and logD(Cs) = -12.1 + 23.3 x X-S. There appears to be a negative pressure effect up to 15 GPa, but it should be noted that this trend was not present before normalization, and is based on only two measurements. There is a positive trend in cesium's metal-silicate partition coefficient with increasing temperature. D-Cs exhibits the largest change and increased by a factor of three over 500 degrees C. The effect of oxygen fugacity has not been precisely determined but in general, loweringfO(2) by two log units resulted in a rise in all D-values of approximately an order of magnitude. In general, the sensitivity of partition coefficients to changing parameters increased with atomic number. The highest D-value for Cs observed in this study is 0.345, which was obtained at nbo/t of 2.7 and a metal phase of pure Fes. This metallic composition has far more S than has been suggested for any credible core-forming metal. We therefore conclude that the depletion of Cs in Earth's mantle is either caused by radically different behavior of Cs at pressures higher than 15 GPa or is not related to core formation. Even so, we have shown that a planet with a sufficient S inventory may incorporate significant amounts of alkali elements into its core. (c) 2007 Elsevier Ltd. All rights reserved.