Partitioning of potassium between iron and silicate at high pressure and temperature

The presence of potassium in the Earth's core would have profound implications for the thermal history of the Earth's mantle and core, the time of inner core formation, the heat flux at CMB, and the operation of the geodynamo. However, the abundance of potassium in the Earth's core has not been constrained by experimental studies. Previous works show that potassium could be supplied to the Earth's core via chemical reactions between Earth's silicate mantle and liquid iron during core formation in the early Earth. Experimental data on partitioning behaviour of potassium are limited in low pressure ranges. Models of the chemical evolution of the Earth's core should include processes under P--T conditions in shallow and deeper magma oceans and during giant impacts. Knowledge of the chemical exchange of light elements between silicate mantle and/or subducting slab materials and the outer core at the CMB is also important for understanding the core--mantle interaction. In this paper we present results of the experiments on chemical reactions and partitioning of potassium between metallic iron and potassium-silicate of KAlSi3O8 at conditions of the Earth's lower mantle down to CMB. High pressure and temperature experiments were carried out using laser-heated diamond-anvil cell technique up to 134 GPa and 3500 K. Analytical transmission electron microscope (ATEM) analysis of the recovered samples from megabar pressure conditions, prepared with Focused Ion Beam (FIB) techniques, revealed that significant amounts of potassium are dissolved into molten iron, indicating that potassium can readily be dissolved into the core by the chemical reactions of molten iron with K-silicate at the Earth's lower mantle conditions. Our results imply a large partition coefficient of potassium between iron and silicate: i.e. DK = 0.15 at 134 GPa and 3500 K. This DK value suggests that the total potassium concentration in the Earth's core is about 35 ppm, 4×10-3 ppm 40K, assuming that a primitive mantle concentration of potassium is 240 ppm, which could serve as one of the important heat sources in the Earth's core.