THE EFFECTS OF CORE FORMATION on VOLATILE ELEMENT ABUNDANCES IN SILICATE EARTH

The currently accepted model of planetary formation is that a large number of moon to Mars-sized planetary embryos with different metal/silicate/volatile ratios grew from the dust and gas surrounding the young sun in ~1 M.yr. As Earth accreted from such embryos through a succession of impacts, the metallic core segregated and some volatile elements may have been lost. The result is that silicate Earth has a complex pattern of element depletion relative to undifferentiated chondritic meteorites which reflects these different processes. The aim of this work is to use metal-silicate partitioning experiments, in conjunction with the chemical and isotopic composition of silicate Earth to deduce the conditions and timing of core formation, volatile addition and volatile loss from the growing Earth. Simultaneous consideration of the depletion factors and metal-silicate partition coefficients of a large number of elements in silicate Earth lead to the following general conclusions: (1) The average pressure of core segregation on Earth was high >30 GPa , implying depths of >800 km. (2) Earth began as a small, strongly reduced body and became more oxidised as it grew. (3) Volatile elements such as Pb, In, Tl were added during most of accretion and their relative abundances have been affected by core formation. They have not been significantly affected by late loss during the moon-forming giant impact. (4) The differences between accretionary timescales given by ¹⁸²Hf-¹⁸²W, ^235,238U - ^207,206Pb and ²⁰⁵Pb-²⁰⁵Tl isotopic systems can be explained by addition of a small amount of core (~10%) during the giant impact. (5) The depletion of silicate Earth in Si relative to CI chondrites is predominantly due to Si dissolution in the core (~6% by mass) at high pressures and temperatures.