Partitioning of Au up to 23 GPa: Implications for core formation of the Earth

Details Partitioning of Au up to 23 GPa: Implications for core formation of the Earth Partitioning of Au up to 23 GPa: Implications for core formation of the Earth

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.v42c0375d&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #V42C-0375

Physics

1015 Composition Of The Core, 1025 Composition Of The Mantle, 3630 Experimental Mineralogy And Petrology, 3672 Planetary Mineralogy And Petrology (5410), 8125 Evolution Of The Earth

Scientific paper

The core formation process (the segregation of metal and silicates) left an excess of siderophile elements in the mantle compared to siderophile element abundances predicted from metal-silicate partitioning experiments conducted at 1 bar (Ringwood, 1979). One hypothesis that explains the mantle abundances of siderophiles is that metal-silicate equilibration occurred at greatly elevated temperatures and pressures, where siderophilic behavior may be decreased. Because there is a paucity of data for highly siderophile element partitioning at mantle relevant conditions (P, T, X), we are investigating Au partitioning between metal-sulfide and silicate liquids. Experiments were carried out in a "Walker-type" 6-8 multi-anvil device in a 1100 ton press. Pressures from 5 to 14 GPa and 14 to 23 GPa have been investigated using an 8 mm or 3mm TEL assembly, respectively. Single crystal MgO capsules were used in both assemblies. We chose the Richardton H-chondrite as starting material because it has an appropriate sulfide fraction (12 wt% S), compared to the possible light element abundance of S in the Earth's core, and the silicate approaches a Bulk Earth composition after reaction with the MgO capsules. Experiment durations at superliquidus temperatures (1700 - 2400 C) were from 4 to 9 minutes. Au in the metal-sulfide quench products was measured using the Cameca SX-50 electron microprobe at the University of Arizona. Secondary ion mass spectrometry (SIMS) was used to measure Au in the quenched silicate liquid in depth profiling mode. The intersection of microblebs (or "nuggets") of Au-rich material during the analysis could be identified and subtracted before calculation of Au contents of the quenched silicate fraction. D(Au)metal-sulfide/silicate ranges from 19000 to 100 The addition of microbleb contributions to the silicate lowers D(Au) by an order of magnitude or less. D(Au) decreases with increasing pressure. A linear fit to the data intersects the D(Au)core/mantle of Earth (300) at 21GPa. This suggests an average equilibration depth of 570 km of descending metal droplets in a silicate magma ocean. This is a slightly lower pressure than suggested by Righter and Drake (1999) but still within their uncertainty.. Although the contribution of Au-bearing microblebs to D(Au) is less than expected, dissemination of such microblebs in a magma ocean could represent a physical mechanism of inefficient core formation.

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