Colloquium: Richard J. Walker

Richard J. Walker, Professor
Department of Geology, University of Maryland

Siderophile (iron-loving) elements are important geochemical tools for constraining terrestrial accretionary processes, as well as tracking global geodynamic processes through time. The highly siderophile elements (HSE) constitute a suite of elements characterized by low-pressure metal-silicate distribution coefficients of >10⁴. The moderately siderophile elements (MSE) are characterized by low-pressure metal-silicate distribution coefficients that are typically >10 but <10⁴. Assuming generally chondritic abundances of HSE and MSE in the bulk Earth, mass balance calculations suggest that ~98% of the Earth’s HSE and ~90% of its MSE (this fraction varies much more per element for the MSE than the HSE) reside in its metallic core. In addition to core-mantle segregation, the relative abundances of these elements present in the mantle today were likely strongly affected by both mantle and crustal processes (including crustal recycling into the mantle), especially those processes involving sulfides. The heavily biased distribution of HSE and MSE between the core and the silicate Earth make these element suites potentially useful for number of important applications including establishing the timing and conditions of terrestrial core formation, constraining proportions and genetics of late accreted materials, and monitoring long-term mantle mixing rates. These and other applications of the HSE and MSE will be discussed.