Partitioning of Potassium Between Fe-S and K-rich Silicate at High Temperature

Partitioning of potassium between liquid iron and silicate melt at high pressure and temperature is important to estimate the amount of potassium in the Earth's core. In the previous works, the partitioning experiments were performed below 26 GPa using a large volume press (e.g. Ito et al., 1993; Gessmann et al., 2002; Murthy et al., 2003), except for the work by Hirao et al. (2006) using a laser heated diamond anvil cell (LHDAC). The aim of this work is to clarify the effects of pressure, temperature and sulfur content on the partitioning coefficients of potassium (DK) in a wide pressure and temperature range using LHDAC. We performed partitioning experiments at 50 GPa and the temperature range of 2500-3000 K using a LHDAC. The starting material was foil of Fe79S21, which was sandwiched by a natural Aduralia (KAlSi3O8). Powder X-ray diffraction experiments were carried out at the ambient pressure to identify the reaction phases at BL13A and BL18C Photon Factory, KEK. Chemical analysis was performed using EPMA and ATEM (Institute for Materials Research, Tohoku Univ.). DK increases from 0.011 to 0.136 with increasing temperature (2500-3000 K) at 50 GPa. This trend is consistent with previous studies (e.g. Gessmann and Wood, 2002; Murthy et al., 2003). Our result indicates that DK decreases with increasing pressure. DK at 50 GPa and 3000 K in the sulfur- bearing system is larger than that reported by Ito et al.(1993) at 26 GPa and 2900 K in the sulfur-free system. This results show that sulfur may increase the solubility of potassium into metallic iron melt. DK of 0.005-0.12 at 135 GPa and 3000-4000 K is estimated by extrapolation of the DK to the condition of the core-mantle boundary (CMB) using the dependence of temperature and pressure observed in this study. This value suggests that the total potassium content in the core is about 1.2-29 ppm assuming that a primitive mantle concentration of potassium is 240 ppm (McDounough and Sun, 1995). It can produce only the radiogenic heat of 0.005-0.12 TW in the core, which is 1.2-6.0 % of the total current heat flux from the core.