Potassium partitioning into molten iron alloys at high-pressure: Implications for Earth's core

Details Potassium partitioning into molten iron alloys at high-pressure: Implications for Earth's core Potassium partitioning into molten iron alloys at high-pressure: Implications for Earth's core

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmmr44a..07b&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #MR44A-07

Physics

1015 Composition Of The Core, 3924 High-Pressure Behavior, 8124 Earth'S Interior: Composition And State (1212, 7207, 7208, 8105)

Scientific paper

It has long been known that the Earth's mantle is more depleted in potassium, than C1-chondrites [1,2]. Two major mechanisms could explain this depletion: (i) the depletion is due to potassium volatility during the early stages of planetary accretion, or (ii) potassium has been incorporated into the core during segregation of an iron-rich metallic liquid. The potassium isotopic composition data of most solar system material place strong limits of 2% on the quantity of potassium that could have been lost by volatilization, ruling out any considerable volatilization or evaporation of potassium [3]. Thus, at least part of the potassium deficit in the mantle might be due to the incorporation of K into the core. To provide strong constraints on the potassium content of the core at the time of Earth's formation, multi-anvil experiments were carried out to determine the partition coefficients of potassium between molten silicates and iron alloy liquids (S-free and S-bearing alloys), between 5 and 15 GPa, at a temperature of 2173 K, and at an oxygen fugacity about 2 log units below the iron-wustite (IW) buffer. No pressure dependence of the potassium partition coefficients in S-free and S-bearing systems was found within the investigated pressure range. We also shown that the degree of melt polymerization is not a controling factor for K partitioning in compositions relevant to the Earth's mantle. We review all available high-pressure data to obtain reliable partition coefficients for the interaction between molten silicates and Fe-alloy liquids at pressures and temperatures relevant to those of core formation in a terrestrial magma ocean. All the existing data, for an oxygen fugacity of about 2 orders of magnitude lower than the IW buffer, and for compositions relevant to the Earth's mantle, and at 2173 K plot around a constant value of 0.083 ± 0.051 and 0.0063 ± 0.0019 for S-bearing and S-free metal, respectively [4]. The dominant controlling parameters appear to be the temperature and the chemical composition of the metallic phase, with potassium partitioning coefficients significantly increased with temperature, and with the sulphur and oxygen contents of the Fe-alloy liquid. Our considerations distinguish too extreme cases, with an S-free or S-rich iron core, which yield K-contents of ~25 or ~250 ppm, respectively. These two extreme values have very different consequences for thermal budget models of the Earth's core since its formation. [1] Gast, P.W., 1960. J. Geophys. Res. 65, 1287. [2] Wasserburg, G.J. et al. 1964. Science 143, 465. [3] Humayon, M., Clayton, R.N., 1995. Geochim. Cosmochim. Acta 59, 2131. [4] Bouhifd, M.A. et al. 2006. Phys. Earth Planet. Int., (under review).

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