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

作者：Malavergne Valerie; Tarrida Martine; Combes Rossana; Bureau Helene; Jones John; Schwandt Craig

来源：Geochimica et Cosmochimica Acta, 2007, 71(10): 2637-2655.

DOI：10.1016/j.gca.2007.03.011

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 degrees C) and obtained the distribution coefficient D(metal-silicate) and the exchange partition coefficient K(metal-silicate) for these elements between metal and silicates (mineral or liquid). D(metal-silicate)(Pb) and Du(metal-silicate)(U) depend 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. K(d metal-silicate)(Pb) and K(d) (U)(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 D(metal-silicate), of the present study, we obtained that: 0.008 ppm < Pb(in) (the core) <4.4 ppm, and 0.0003 ppb < U(in) (the core) < 0.63 ppb, depending on whether the metal is S-ftee or S-saturated respectively, and if the mantle was molten or solid during the segregation process of the Earth's core around Delta IW-2. For Mars, based on a core segregation process around Delta IW-1, we obtained that: 0.005 ppm < Pbin the-re < 3 pprn, and 0.00002 ppb < U(in 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 Delta IW-3. The values found in the present study for U in the Martian core suggest that the magnetic field activity of Mars before similar to 0.5 b.y. after its formation would be difficult to ascribe to the decay of U alone.

• 出版日期2007-5-15
• 单位中国地震局