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 6x10-3 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 1010 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.