Phase relationships and equations of state for FeS at high pressures and temperatures and implications for the internal structure of Mars

In situ X-ray diffraction experiments on FeS up to 22 GPa and 1600 K were carried out using large volume multianvil apparatus, combined with synchrotron radiation at SPring-8. We investigated phase stability relationships of FeS and determined the straight phase boundaries between FeS III (monoclinic phase) and FeS IV (hexagonal phase) to be <F>T (K)=20P</F> <F>(GPa)+170</F> and between FeS IV and FeS V (NiAs-type phase) to be <F>T (K)=39.6P (GPa)+450</F>. We also found anomalous behavior in the c/a ratio, thermal expansion, and isothermal compression of FeS V as well as FeS IV, in the pressure range 4–12 GPa. These anomalies in FeS can be attributed to the spin-pairing transition of Fe, and divides FeS IV and FeS V into the high-spin low-pressure phase (LPP) and the possibly low-spin high-pressure phase (HPP). In order to investigate the internal structure of Mars, we evaluated the equations of state for FeS IV (HPP) and FeS V (HPP). A least square fit to the experimental data yielded <F>K0T=62.5±0.9</F> GPa at <F>T=600</F> K and <F>(dK0/dT)P=-0.0208±0.0028</F> GPa/K for FeS IV (HPP), and <F>K0T=54.3±1.0</F> GPa at <F>T=1000</F> K and <F>(dK0/dT)P=-0.0117±0.0015</F> GPa/K for FeS V (HPP) with fixed <F>K′=4</F>. Thermal expansion coefficients were <F>α=7.16×10-5+6.08×10-8T</F> for FeS IV (HPP) and <F>α=10.42×10-5</F> for FeS V (HPP), respectively. Using these equations of state, we examined the internal structure of Mars that has a model mantle composition [Meteoritics 20 (1985) 367] and Fe–FeS core. Our models show that an Mg-silicate perovskite-rich lower mantle is stable only with the Fe-rich core having less than 20 wt.% sulfur. The polar moment of inertia factor C derived from Mars Pathfinder data [Science 278 (1997) 1749] is consistent with any compositions between Fe and FeS for the Martian core, but it excludes the presence of a crust thicker than 100 km. [Copyright &y& Elsevier]