Isotopic composition of pyrite: Relationship to organic matter type and iron availability in some North American cretaceous shales
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Abstract
The S isotope composition of pyrite in Cretaceous shales from the Western Interior of North America is related to organic C abundance, kerogen type and Fe availability. Both calcareous and noncalcareous rocks show a correlation between S and C, but noncalcareous rocks are relatively enriched in S with a higher S/C ratio. This higher ratio probably shows that pyrite formation was Fe limited in the calcareous rocks. Organic-carbon-rich noncalcareous shales accumulated slowly beneath anoxic bottom waters. The anoxic bottom waters allowed hydrogen-rich organic matter to be preserved. Such shales have a narrow range of 34S-depleted sulfide and have Fe/S ratios like stoichiometric pyrite, suggesting that pyrite formation in organic-rich shales was also limited by Fe availability. Conversely, organic-poor shales commonly accumulated at comparatively high rates, contain hydrogen-poor and refractory organic matter, and have a wide range of pyrite-S isotopic compositions. These organic-poor shales contain post-sulfidic authigenic minerals such as siderite and have excess reactive Fe rather than pyrite stoichiometry. Evidently Fe played a large role in early diagenesis and determined the course of post-sulfidic diagenesis. Fe availability was, however, mainly controlled by provenance, by the rates of sediment accumulation, and by the oxygen content of the depositional environment.
Publication type | Article |
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Publication Subtype | Journal Article |
Title | Isotopic composition of pyrite: Relationship to organic matter type and iron availability in some North American cretaceous shales |
Series title | Chemical Geology: Isotope Geoscience Section |
DOI | 10.1016/0168-9622(87)90009-1 |
Volume | 65 |
Issue | 3-4 |
Year Published | 1987 |
Language | English |
Publisher | Elsevier |
Description | 11 p. |
First page | 293 |
Last page | 303 |
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