Potassium solubility in metal: the effects of composition at 15 kbar and 1900°C on partitioning between iron alloys and silicate melts

Abstract

To determine the role of radioactive heating as an energy source in planetary cores, the solubility of K in metal has been examined experimentally. All experiments were conducted at 15 kbar and 1900°C and involved K partitioning between Fe alloys and silicate melts. Experiments conducted with different concentrations of S in the metallic liquid indicate that S increases the solubility of K in metal. Unlike S, the presence of C in the metallic liquid does not increase K solubility in metal to a level detectable with the electron microprobe. The silicate composition significantly affects the solubility of K in S-rich metal, with the metal/silicate partition coefficient for K increasing by nearly two orders of magnitude with increasing depolymerization of the silicate melt. Using an appropriate silicate composition for the early, differentiating Earth and assuming that S is a significant light element in the core, the metal/silicate partition coefficient for K is 6×10 −3 at 15 kbar and 1900°C. Such a partitioning value, if representative of the behavior of K at core formation conditions, suggests the presence of less than 1 ppm K in the Earth's core with a present-day heat generation of 10 10 W, which is 2–3 orders of magnitude lower than estimates of the power necessary to drive the Earth's geodynamo. Other thermodynamic variables, namely pressure, temperature, and oxygen fugacity may also affect the solubility of K in metal.