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Abstract

Diagenetic calcite, dolomite, pyrite, marcasite, and anhydrite of the Upper Devonian Nisku Formation in central Alberta, Canada, were analyzed for trace elements and sulfur isotopes by ion microprobe with sample spots <20 nm in size. Calcite samples exhibit significant trace element variations (of up to about 3 orders of magnitude), and disseminated sulfides display large sulfur isotope variations (834S of up to about 60%o CDT) on a scale of < 100 nm. These variations cannot be resolved using conventional analysis of powdered samples. The data suggest that most of the analyzed calcites formed from marine pore fluids that evolved toward lower redox-potential. The sulfur isotope variations indicate intensive bacterial sulfate reduction and closed-system Rayleigh fractionation as the dominant diagenetic processes leading to iron sulfide formation. Together, carbonate and sulfide data indicate that the system was at least partially closed during shallow to intermediate burial diagenesis (up to about 1100 m) in the Late Devonian to Early Carboniferous. During the subsequent 300 my, the Nisku reefs in the study area were buried to more than 4000 m, with maximum burial in the Late Cretaceous to Early Tertiary. The existence and preservation of the observed geochemical variations further imply that water-rock interaction during this prolonged and deep burial was quite limited (i.e., the analyzed calcites and sulfides are remarkably resistant to compositional modification via recrystallization).

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