Equation of state of liquid Fe‐10 wt % S: Implications for the metallic cores of planetary bodies

Abstract

[1] The equation of state (EOS) of molten Fe-10 wt % S was investigated by means of static high-pressure density measurements using a newly developed variation of the sink/ float technique. The method involves the use of tailored density composite spheres made of Pt or WC cores and an Al 2 O 3 (ruby/sapphire) mantle as bracketing markers for comparative density measurements of liquid Fe-10 wt % S at pressures (P) up to 20 GPa and at temperatures (T) up to 2123 K. The results of isothermal compression data confirm the validity of extrapolating to 20 GPa an EOS for pressures up to 6 GPa obtained earlier by Sanloup et al. [2000] using an X-ray absorption technique. The analysis of new data obtained in the 1773-2123 K interval up to 20 GPa resulted in new EOS parameters and also revealed the need for a well-determined melt density value at 1 atm. Application of our results to the identification of composition of liquid metallic cores of Ganymede, Io, and Mars showed Fe-10 wt % S to be a possible candidate. We used different EOS formulations to extrapolate our results to the P-T conditions of the Earth's interior. For temperatures between 4773 K at the core-mantle boundary and 5973 K at the inner core boundary the extrapolation made using the Vinet EOS resulted in density values closest to the density of the core suggested by the preliminary reference Earth model.

The interior structure of Mercury and its core sulfur content

“…The EoS data for pure Fe (Anderson and Ahrens, 1994), the EoS data for Fe − 0.1wtS (Balog et al, 2003), and the low pressure data from Sanloup et al (2002) show that the bulk modulus at reference conditions decreases approximatively linearly with increasing x S for x S < 0.1. The data of Sanloup et al (2002) correspond to measurements of K T for Fe − S liquids with different weight fraction of S in the pressure range 0−5GPa, whereas the K T values of Balog et al (2003) and Anderson and Ahrens (1994) have been obtained by fitting data measured at different pressures on equations of state. By fitting a linear relation to these data, we obtained…”

“…Sohl and Spohn, 1997). See Table ( we use a linear relation derived from fitting data of (Anderson and Ahrens, 1994) for pure Fe and of (Balog et al, 2003) for Fe − 0.1wt%S. We have…”

Thermal Evolution and Magnetic Field Generation in Terrestrial Planets and Satellites

“…If the composition of the core were sulfur-rich, iron sulfide (FeS) would precipitate instead of Fe. Solid FeS has a density lower than that of liquid Fe-FeS in the pressure range of Ganymede's core (Fei et al 1995;Balog et al 2003) and would float upward from the deep core to form a solid FeS layer on top of the core (Hauck et al 2006). Hauck et al (2006) speculate that this process may also drive a dynamo but the model has not been studied in much detail.…”

Presumption of large-scale heterogeneity at the top of the outer core basal layer

“…Laboratory experiments at low pressures ( P < 10 GPa) show that the presence of S in the liquid iron decreases κ, and that the combined effect of density and compressibility results in a V P decrease with increasing S-content (e.g. Balog et al, 2003;Sanloup et al, 2004;Jin et al, 2014). However, shock waves experiments (Huang et al, 2011) and ab initio molecular dynamics (Badro et al, 2014), which better reproduce ICB conditions, show a V P increase with the increase of light element content.…”