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

Ahrens, Thomas J. (1979) Equations of state of iron sulfide and constraints on the sulfur content of the Earth. Journal of Geophysical Research B, 84 (B3). pp. 985-998. ISSN 0148-0227.

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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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Additional Information:Copyright 1979 by the American Geophysical Union. (Received January 9, 1978; revised July 12, 1978; accepted October 12, 1978.) Paper number 8B1089. I am indebted to Harold A. Richeson for so carefully managing our gun facility; David A. King, with whom I initially began this study; Ian Jackson, whose help was critical in firing the last of these experiments; an d the support of the National Science Foundation (grant EAR77-23156). I appreciate the very helpful critical comments of Raymond Jeanloz, Don L. Anderson, John Verhoogen, Robin Brett, William A. Bassett, and Ian Jackson on the final manuscript. Contribution 3003, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125.
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Caltech Division of Geological and Planetary Sciences3003
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Official Citation:Ahrens, T. J. (1979), Equations of state of iron sulfide and constraints on the sulfur content of the Earth, J. Geophys. Res., 84(B3), 985–998, doi:10.1029/JB084iB03p00985.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:51339
Deposited By: George Porter
Deposited On:06 Nov 2014 16:35
Last Modified:13 Feb 2019 18:06

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