Melting relations in the iron-sulfur system at ultra-high pressures: Implications for the thermal state of the Earth

The melting temperatures of FeS-troilite and of a 10 wt % sulfur iron alloy have been measured to pressures of 120 and 90 GPa, respectively. Our results document that FeS melts at a temperature of 4100 (± 300) K at the pressure of the core-mantle boundary. Eutectic-like behavior persists in the iron-sulfur system to the highest pressures of our measurements, in marked contrast to the solid-solution-like behavior observed at high pressures in the iron-iron oxide system. Iron with 10 wt % sulfur melts at a similar temperature as FeS at core-mantle boundary conditions. If the sole alloying elements of iron within the core are sulfur and oxygen and the outer core is entirely liquid, the minimum temperature at the top of the outer core is 4900 (± 400) K. Calculations of mantle geotheims dictate that there must be a temperature increase of between 1000 and 2000 K across thermal boundary layers within the mantle. If D″ is compositionally stratified, it could accommodate the bulk of this temperature jump.