New high-pressure and high-temperature metal/silicate partitioning of U and Pb: Implications for the cores of the Earth and Mars

Details New high-pressure and high-temperature metal/silicate partitioning of U and Pb: Implications for the cores of the Earth and Mars New high-pressure and high-temperature metal/silicate partitioning of U and Pb: Implications for the cores of the Earth and Mars

May 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007gecoa..71.2637m&link_type=abstract

Geochimica et Cosmochimica Acta, Volume 71, Issue 10, p. 2637-2655.

Computer Science

12

Scientific paper

In order to quantify possible fractionation of U and Pb into a metallic core, we have performed piston cylinder and multi-anvil press experiments at high pressure (up to 20 GPa) and high temperature (up to 2400 °C) and obtained the distribution coefficient Dmetal silicate and the exchange partition coefficient Kmetal silicate for these elements between metal and silicates (mineral or liquid). DPbmetal silicate and DUmetal silicatedepend strongly on the S content of the metallic phase, and also on the oxygen fugacity, in agreement with an effective valence state of 4 for U in silicates and 2 for Pb in silicates. KPbd metal silicate and KUd metal silicate show no discernable pressure and temperature trend. U remains lithophile even at high pressure and high temperature but its lithophile nature decreases at very low oxygen fugacity. From our experimental data, it was possible to calculate the U and Pb contents of the cores of Mars and Earth under core-mantle equilibrium conditions at high pressure and high temperature. From the Dmetal silicate of the present study, we obtained that: 0.008 ppm < Pbin the core <4.4 ppm, and 0.0003 ppb < Uin the core < 0.63 ppb, depending on whether the metal is S-free or S-saturated respectively, and if the mantle was molten or solid during the segregation process of the Earth’s core around ΔIW-2. For Mars, based on a core segregation process around ΔIW-1, we obtained that: 0.005 ppm < Pbin the core < 3 ppm, and 0.00002 ppb < Uin the core < 0.05 ppb, depending on the metallic composition: S-free or S-saturated respectively. Our results suggest that the low concentration of Pb in the terrestrial mantle could not be explained by an early Pb sequestration in the Earth’s core even if S is the dominant light element of the core. If we assume a magma ocean scenario, U might produced a maximum value of 1.5% of the total heat budget of the core with a segregation occurring below ΔIW-3. The values found in the present study for U in the Martian core suggest that the magnetic field activity of Mars before ˜0.5 b.y. after its formation would be difficult to ascribe to the decay of U alone.

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