Thermal expansion of liquid Fe-S alloys at high pressure

Sulfur (S) is classically considered to be the major light element alloyed to iron (Fe) in the core of small planetary bodies such as the Moon, Ganymede, Mercury and Mars. Geophysical and/or geodetic observations support the liquid nature of at least a portion of their metallic cores. With respect to other light elements, S drastically modifies liquid Fe properties (e.g. density, compressibility, sound wave velocity), which may be related to the local structure of the liquids. Therefore, knowledge on local atomic arrangement and its relation with macroscopic physical properties of liquid Fe-S alloys at high pressure is fundamental to infer planetary core composition, interior structure and dynamics.

The density and thermal expansion are most basic but indispensable properties for modeling core dynamics. Few studies have been carried out to determine densities of liquid Fe-S alloys based on different methods (e.g. sink/float, X-ray absorption, and X-ray radiography), with results not always in agreement. In particular, thermal expansion has been reported at high pressure only for liquid FeS (Nishida et al., 2011, Am. Miner., 96, 864-868).

In this work, we present measurements of the local structure and density of liquids in the Fe-FeS binary system at high pressure, over a wide temperature and compositional range. High-pressure experiments were performed using a DIA-type multi-anvil press installed at the beamline PSICHE at Synchrotron SOLEIL, France. Six different compositions were studied: FeS, Fe-25wt.% S, Fe-20wt.% S, Fe-10wt.% S, Fe-5wt.% S and Fe. Upon complete melting, liquid diffuse scattering were recorded at the pressures of 5-9 GPa and temperatures up to 2500 K by combining angular and energy dispersive X-ray diffraction (CAESAR) technique. Local structure and density as well as thermal expansion were extracted from the obtained data for each composition.

The link between the evolution of the local atomic arrangement and changes in the macroscopic liquid properties will be discussed. More in general, these new thermal expansion data will significantly improve our knowledge of thermo-elastic properties of cores forming materials, with direct implications for modeling core composition, crystallization and internal dynamics (and hence magnetic field generation) of small telluric planetary bodies.