Constraint of sulfur influence on bulk modulus of liquid Fe

Sulfur has been widely considered as a candidate of light element to account for the density deficit in the Earth's core relative to pure liquid iron because it is depleted in the Earth's mantle, easily alloys with iron, and moderately siderophile during iron-silicate interaction at high pressures. Knowledge about density change caused by alloying S into Fe is therefore critical for estimating the required partitioning and understanding the reality of existence of sulfur in the Earth's core. Currently available data of sulfur influence on bulk modulus of liquid iron are very limited, and due to technical challenges, the pioneer experiments have produced quite devise results indicating that sulfur content can reduce the bulk modulus by 1.7GPa/1wt%S to 3.5GPa/1wt%S. This will result in a 5wt%S uncertainty in estimating the core composition. Using in situ synchrotron x-ray radiograph technique, we measured equation of state of liquid Fe-36wt%S system in a multi-anvil press up to 6 GPa. Fitting the experimental data to a second order Birch- Murnaghan equation of state with a fix K'= 4 produces a bulk modulus of 15.4 GPa which corresponds to an influence of sulfur content of 1.8 GPa/1wt%S on the bulk modulus of the system. This value is consistent with the lower bound of previously defined range of sulfur influence on the bulk modulus of liquid iron. Impact of this result on the core composition modeling will be discussed.