Sulfur‐Bearing hcp Iron for Earth's Inner Core

Abstract Earth's iron core is believed to contain light impurities which should have been included during core formation. In order to place constraints on core composition, here we report the effect of sulfur on density of hexagonal close‐packed (hcp) iron which was not previously examined, using laser‐heated diamond anvil cells with in situ synchrotron X‐ray diffraction. Results show that the unit‐cell volume for 2 wt% sulfur‐bearing hcp iron is indistinguishable from that for pure iron at 300 K and smaller at high temperatures under core pressures. This gives rise to a moderately dense hcp phase which can explain the observed large density jump across the inner core‐outer core boundary (ICB). A self‐consistent core model was established with Fe‐5.1 ± 0.6wt%Si‐2.0 ± 0.6wt%S‐0.7 ± 0.6wt%O for the outer core and Fe‐5.1 ± 0.2wt%Si‐2.0 ± 0.6wt%S for the inner core with an ICB temperature of 5,730 K, which is geochemically defensible and supports the equilibrium core formation scenario.