Pyrite isotopic composition

Vapor and brine from the Brandon vent of the East Pacific Rise have identical Fe isotope compositions, implying that phase separation does not produce an isotopic fractionation (Beard et al. 2003a). The role that sulfide precipitation plays in controlling the Fe isotope composition of the fluid remains unknown. The precision of the two sulfide analyses reported by Sharma et al. (2001) was not sufficient to resolve if sulfide precipitation would produce Fe isotope fractionation in the vent fluid. In a detailed study of sulfldes from the Lucky Strike hydrothermal field from the mid Atlantic Ridge, however, Rouxel et al. (2004) found that sulfldes span a range in 5 Fe values from -2.0 to +0.2%o, and that pyrite/marcasite has lower 5 Fe values ( l%o) as compared to chalcopyrite. The variations in mineralogy and isotope composition are inferred to represent open-system equilibrium fractionation of Fe whereby... 

In a coal desulfurization study, Narayan et al. (50) were able to extract an appreciable amount of elemental sulfur (36% of total sulfur) with perchloroethylene at 120 °C from weathered coal, but not from fresh coal. Hackley et al. (51) determined the isotopic composition of elemental sulfur extracted by perchloroethylene and obtained results consistent with the interpretation that the elemental sulfur originates from the oxidation of pyrite. 

Earlier work on the isotopic compositions of pyrite, elemental sulfur, and organic sulfur in coals from Japan, Australia, and Germany (107-110) was summarized by Nielsen (102). Smith and Batts (110) showed that organic sulfur in Australian coals ranging in age from Permian to Tertiary has a large isotopic variation (+2.9 to +24%c) in coals with more than % sulfur, whereas organic sulfur in low-sulfur coals (less than 1% sulfur) has a narrow isotopic composition between +4.6 and +7.3%o. The relatively uniform isotopic... 

Figure 4. Comparison of isotopic composition of pyritic and organic sulfur in low- and high-sulfur coals in the Illinois Basin. Data are taken form Westgate and Anderson (113). Samples were collected from the top, middle, and base of the Herrin Coal at each mine.

Basinal Brines as a Source of Sulfur in High-Sulfur Coals. Sulfide minerals, such as pyrite and sphalerite, in coal seams may be deposited from basinal hydrothermal fluids. The occurrence of epigenetic sphalerite in Illinois Basin coals has been described by Hatch et al. (119) and Cobb (120). Whelan et al. (121) studied the isotopic composition of pyrite and sphalerite in coal beds from the Illinois Basin and the Forest City Basin, and suggested that some of the coals were affected by Mississippi Valley-type hydrothermal solutions. 

Previous studies (4-6) have illustrated the difficulties in relating the isotopic composition of the organic and pyritic forms of sulfur in coals. This is particularly so where large additions of secondary pyrite further distort the frail relationship, if... 

Sulfur, carbon and hydrogen stable isotope ratios of pyrite, kerogens, and bitumens of two high-sulfur Monterey formation samples from the onshore Santa Maria Basin in California were determined. Kerogens from these were pyrolyzed at 300°C for periods of 2, 10 and 100 hours in closed systems and the yields and isotopic compositions of S-containing fractions (residual kerogens, bitumens and hydrogen sulfide) were determined. 

Two Type II-S kerogens (as defined by Orr (i)) from the onshore Santa Maria Basin Monterey formation were pyrolyzed in this study to determine (a) the distribution of sulfur and its isotopic composition among the various products formed during artificial maturation, and (b) maturation trends reflected in the sulfur isotopic and elemental S/C ratios of kerogens, and in the variation of C and H isotopes. In addition, S isotopes in pyrites, kerogens and bitumens from the two Monterey shale samples were examined to speculate on the mode of S incorporation into Santa Maria Basin sediments. 

The samples were ground and extracted with the solvent mixture 9 1 dichloromethane/methanol for 48 hours in a soxhlet system. The extracted samples were then demineralized to isolate kerogens using a detailed procedure described in (3). Hie kerogen was next solvent extracted a second time and treated with 0.2 N nitric acid at room temperature to separate pyrite from the organic matter and the solution was kept for determination of pyrite S content and isotopic composition. X-ray diffraction was used to verify the removal of pyrite from the kerogen. 

For example, and 5 0 of SO was used to assess mixing between a vertically stacked aquifer system in contact with a salt dome located in northern Germany (Berner era/., 2002). Two major sulfate pools were identified based upon their isotopic compositions (i) SO from the dissolution of evaporite minerals, and (ii) SO derived from atmospheric deposition and from the oxidation of pyrite. Using both the and of the sol , zones of variable groundwater mixing and significant amounts of bacterial sulfate reduction were identified. The SOl derived from the dissolution of the rock salt in the highly saline deep brine showed nearly constant values between +9.6%c and 11.9%o and between +9.5%c and 12.1%o, consistent with the Permian evaporite deposits. Sulfate in near-surface... 

Anderson T. F., Kruger J., and Raiswell R. (1987) C-S-Fe relationships and the isotopic composition of pyrite in the New Albany Shales of the Illinois Basin USA. Geochim. Cosmochim. Acta 51, 2795-2805. 

Gautier D. L. (1987) Isotopic composition of pyrite relationship to organic matter type and iron availability in some North American Cretaceous shales. Chem. Geol. Isotope Geoset Sect 65, 293-303. 

Wilkin R. T. and Arthur M. A. (2001) Variations in pyrite texture, sulfur isotope composition, and iron systematics in the Black Sea evidence for late Pleistocene to Holocene excursions of the O2-H2S redox transition. Geochim. Cosmochim. Acta 65, 1399-1416. 

S content of total solid-phase sulfur. Spy pyrite sulfur content. 6Spy isotopic composition of pyiite. Sorg organic sulfur. S elemental sulfur. SO4 pore-water sulfate concentration. 5SO4 isotopic composition of pore-water sulfate. H2S pore-water sulfide concentration. 5SH2S isotopic composition of pore-water sulfide. SRR sulfate reduction rate. 

Bioturbation and other physical processes associated with the upper portions of marine sediments may lead to rapid exchange between pore-water and overlying depositional water. Depending on the intensity of bioturbation, sulfate in depth zones 1 and 11 and the uppermost part of zone 111 (Figure 4) may be effectively in contact with an infinite reservoir of seawater sulfate. When this is the case, pore-water SO will have a nearly constant 8 value with depth regardless of the withdrawal of isotopically light sulfur to form H2S. The initial isotopic composition of H2S produced by SRB in zones 1 and 11 will be equal to the instantaneous isotopic separation between seawater sulfate and bacterial sulfide (i.e., up to about Aso -HjS = 45%o). Metastable iron sulfides and pyrite formed from this H2S will have an isotopic composition very close to this initial H2S because of the small fractionation observed during sulfidization of iron minerals. 

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