The Earth's core composition from high pressure density measurements of liquid iron alloys

•Density measurements of liquid iron alloys up to megabar conditions.•An Earth's core containing 6 wt% S and 2 wt% Si best matches the geophysical constraints.•Silicon cannot be the only light element in the Earth's core.

High-pressure, high-temperature in situ X-ray diffraction has been measured in liquid iron alloys (Fe–5 wt% Ni–12 wt% S and Fe–5 wt% Ni–15 wt% Si) up to 94 GPa and 3200 K in laser-heated diamond anvil cells. From the analysis of the X-ray diffuse scattering signal of the metallic liquids, we determined density and bulk modulus of the two liquid alloys. Comparison with a reference Earth model indicates that a core composition containing 6% of sulfur and 2% of silicon by weight would best match the geophysical data. Models with 2.5% of sulfur and 4–5% of silicon are still consistent with geophysical constraints whereas silicon only compositions are not. These results suggest only moderate depletion of sulfur in the bulk Earth.