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

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

Jun 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004pepi..143..469u&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 143, p. 469-479.

Physics

12

Scientific paper

In situ X-ray diffraction experiments on FeS up to 22GPa and 1600K 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 T(K)=20P (GPa)+170 and between FeS IV and FeS V (NiAs-type phase) to be T(K)=39.6P(GPa)+450. 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 4-12GPa. 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 K0T=62.5+/-0.9GPa at T=600K and (dK0/dT)P=-0.0208+/-0.0028GPa/K for FeS IV (HPP), and K0T=54.3+/-1.0GPa at T=1000K and (dK0/dT)P=-0.0117+/-0.0015GPa/K for FeS V (HPP) with fixed K'=4. Thermal expansion coefficients were α=7.16×10-5+6.08×10-8T for FeS IV (HPP) and α=10.42×10-5 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 20wt.% 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 100km.

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