Chemical bonding and the incorporation of potassium into the Earth's core

Abstract

It has been argued that most of the Earth's potassium was segregated into the outer core and that the radioactive decay of 40K provided the heat source for the geodynamo. This idea rests on the assumption that the affinity of potassium for sulfur or metallic iron is enhanced at high pressure. In this paper, the high pressure electronic structures of K in sulfide, iron sulfide and metallic iron coordination environments were determined from molecular orbital (SCF‐Xα‐SW) calculations on (KS8)15−, (KS8Fe6)3− and KFe14 clusters. It is shown that, even at high pressure, potassium cannot alloy with metallic iron. Although a high‐pressure electronic transition may enhance the potassium‐sulfur chemical bond, the electronic structure of the KS8Fe6 cluster shows that this electronic transition cannot happen in an iron sulfide melt. Consequently, potassium will not have an enhanced affinity for sulfur in planetary interiors. If the lower mantle were more reducing, potassium might be excluded from the silicate phases by more strongly lithophile elements and segregated into a metal sulfide phase in the outer core (cf. the phase assemblages in enstatite chondrites). Given the oxidation state of the Earth, however, it is unlikely that significant quantities of potassium have been incorporated into the outer core. The Earth, like the moon and the eucrite parent body, is depleted in potassium. An alternative heat source (e.g., the radioactive decay of U and Th) must be invoked to explain the geodynamo.