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

Abstract

New shock and release wave data for pyrrhotite (Fe_(0.9)S) obtained over the pressure range of 3–158 GPa (0.03–1.58 Mbar) suggest that this mineral undergoes a major shock-induced phase change(s) with an onset in the range 2.7–3.8 GPa. Free-surface velocities of Fe_(0.9)S released from states between 8.9 and 24.7 GPa indicate a maximum postshock density of ∼5.54 g/cm^3. A pressure of ∼25 GPa appears to be required to drive the phase transition(s) to completion. A density for the high-pressure phase (hpp) consistent with present static high-pressure X ray data, ∼5.34 g/cm^3, was used to calculate a zero-pressure, adiabatic bulk modulus of 126–128 GPa for the hpp. Release adiabat measurements centered at 152 and 158 GPa are consistent with the assumption that the Hugoniot curve in the 25- to 158-GPa range reflects the properties of a denser polymorph, possibly with eight-fold coordination, which may be similar to the local bonding of sulfur in the liquid core of the earth. Similar, but less well constrained, reductions are presented for pyrite, FeS_2, based on the three data points of Simakov et al. (1974) in the pressure range 88–320 GPa. These are inferred to represent the behavior of an unknown hpp (approximate zero-pressure density of 5.3 g/cm^3) and indicate that this phase forms at a pressure above ∼29 GPa. Reduction of these data yields a zero-pressure bulk modulus for the hpp in the range 205–244 GPa. The raw Hugoniot data for Fe_(0.9)S, FeS_2, 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 CCl carbonaceous chondrites. It is concluded that although the 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, C_0(km/s) = 7.15 - 0.47 [V bar], was also discovered upon comparison of the inferred densities and bulk sound speeds (C_0) of the hpp's of the iron sulfides with other measurements for 12 sulfides and elemental sulfur. Here inline image is the volume (in cubic centimeters) per mole of atoms.

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