LIQUIDUS TEMPERATURES IN THE FE-S SYSTEM: APPLICATION TO EARTH'S CORE

Earth's core is predominantly iron with an approximately 10 wt. % light element component. Sulfur is believed to be an important constituent in Earth's core because of its abundance in iron meteorites (the only samples of planetary cores we have), and its ability to partition into liquid iron at high pressures. In this study we have measured the liquidus temperatures of an iron plus 10 wt. % sulfur sample from 115 GPa to 155 GPa at the GSECARS sector of the Advanced Photon Source, Argonne National Laboratory using x-ray diffraction techniques. Up to this pressure the Fe-Fe₃S system exhibits eutectic behavior. These data suggest a minimum temperature for the core-mantle boundary of 2900 K. Combining this new data with previous studies allowed us to extrapolate the liquidus curve for a single composition down to the inner core boundary. Comparison of the liquidus to the adiabatic temperature gradient expected for iron plus 10 wt. % sulfur core would suggest that the inner core should be liquid and the outer core should be solid; this is in direct contrast with what is known from seismology. Sulfur is not likely to be the major light element component of the outer core based on this result, although volume data suggests that it may be an important component of the inner core.