Metal-silicate partitioning of iodine at high pressures and temperatures: Implications for the Earth's core and ^129*Xe budgets

The partition coefficients of iodine D_met/sil between molten metal and molten silicate were investigated using a Laser Heated Diamond Anvil Cell (LHDAC) at pressures between 2 and 20GPa and at ~2800K. No pressure dependence of D_met/sil was observed within this range, but the composition of the Fe-Ni alloy liquid phase was shown to have an effect. When the metallic liquid was alloyed with S, O and Si, there was an increase in iodine solubility in the metal. Iodine exhibited mildly siderophile behaviour across all the investigated conditions, with D_met/sil=1.25±0.65 (2 s.d.) (Fe metal system) and D_met/sil=4.33±1.41 (2 s.d.) (Fe-alloy). In conjunction with a revised bulk silicate Earth (BSE) concentration, it is calculated that the core could be a significant reservoir for iodine, with up to 82% of the bulk Earth's iodine budget in the core, depending on the light element content of the metal phase and the process of core formation. The composition of the metal phase appears to have a greater effect on the partitioning and sequestration of iodine than the style of core segregation.As the core likely formed while ¹²⁹I was still extant, the core can also be a reservoir for radiogenic ¹²⁹Xe from the decay system ¹²⁹I-¹²⁹Xe (T_1/2=15.7Myr). Preliminary modelling indicates that the decay of ¹²⁹I in the core has the potential to generate radiogenic ¹²⁹Xe concentrations that are at least two orders of magnitude greater than what has been estimated for the depleted mantle. While this may have a significant impact on the isotopic signatures of the overlying mantle, it is not