Equations of state of iron sulfide and constraints on the sulfur content of the Earth

Abstract

•S, FeS•., and Fe when constrained to the seismologically obtained density-pressure profiles of the outer core of the earth indicate a systematic decrease of apparent sulfur content from 10 to 6.5% with depth. When the shock data are reduced to isotherms, a nearly constant sulfur content in the range 9-12% is inferred. Using these bounds on the sulfur content of the core, and depending on whether an olivine or pyroxene mantle stoichiometry is assumed, the earth can be modeled as being depleted in S by a factor ranging from 1.7 to 3.2 with respect to the abundances of Si and from 2.8 to 7.5 relative to the abundance of Fe, in CCI carbonaceous chondrites. It is concluded that although th,e shock wave data permit the major light element in the core to be sulfur, the earth can be modeled as being depleted in sulfur, along with other volatile elements. A systematic relation, Co(km/s) = 7.15 -0.4712, was also discovered upon comparison of the inferred densities and bulk sound speeds (Co) of the hpp's of the iron sulfides with other measurements for 12 sulfides and elemental sulfur. Here I? is the volume (in cubic centimeters) per mole of atoms.

Thermal expansion of iron-rich alloys and implications for the Earth's core

“…Geochemical and cosmochemical observations suggest that S is a major component of terrestrial and Martian cores (e.g., refs. [25][26][27][28]. Previous estimates of the amount of S needed to account for the outer core density deficit range from a few weight percent to as much as 18 weight percent (14,16,17,26,29,30).…”

Hugoniot data for pyrrhotite and the Earth's core

“…As noted by Ahrens [1979] phase of troilite. The shock wave data for pyrrhotite in the pressure regime between 30 GPa and 120 GPa are basically concordant with the static data.…”

“…Previous data [Ahrens, 1979] characterized the equation of state for this iron sulfide in the pressure regime below 50 GPa. However, only two closely spaced data were obtained in the pressure regime near 150 GPa, relevant to the core.…”

“…However, arguments based on elemental abundances in the solar system and in C1 carbonaceous chondrites remain somewhat less concordant. Ahrens [1979] noted that for a lower bound of 6.5 to 9 weight percent sulfur in the core, the depletion of sulfur in the earth relative to C1 chondrites is between 1.8 and 3.2. The range depends primarily on the choice of either an olivine or pyroxene stoichiometry for the lower mantle because elemental abundances are referenced to silicon.…”

The Condensed Matter Physics of Planetary Interiors

“…This is best explained by the presence of a light alloying constituent. The most likely candidate is sulphur (Ahrens, 1979), although a significant amount of oxygen might be present, if some of the separation of iron from silicates occured at high pressures (Jeanloz and Ahrens, 1979). A much more exotic possibility is that the earth's core is an alloy of iron and metallic hydrogen (Stevenson, 1977).…”