Equation of State of Fe3S and Limits on the Sulfur Content of Earth's Core

Sulfur is a common component of protoplanetary cores, as represented by iron meteorites, and it is likely to be a significant alloying component with iron in Earth's core as well, along with silicon, oxygen, and perhaps other light elements. Cosmochemical limits on the sulfur content of Earth's core, based on the relative volatilities of sulfur and other elements, are weakened by the observation that this approach fails to accurately predict the sulfur content in iron meteorite parent bodies. To better understand the geophysical consequences of sulfur addition to Earth's core, we report equation of state measurements of Fe3S to pressures and temperatures exceeding 140 GPa and 2000 K, using synchrotron X-ray diffraction in a laser heated diamond anvil cell. New room temperature measurements are also reported, improved by the use of a neon pressure medium. With this P-V-T equation of state for Fe3S, along with an assumed 2% density change upon melting and a 4000 K core-mantle boundary temperature, the PREM density in the outer core can be matched with 14 wt% sulfur, which should be considered an upper bound because of the likely additional presence of other light elements.