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Hoff, J.A., 1992

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Hoff, J.A., 1992, Pb isotopic systematics in dolomitic rocks and their application to geochronology, provenance, and fluid-rock interaction: Stony Brook, New York, State University of New York, Ph.D. dissertation, 168 p.


Although the diagenetic history of ancient carbonates is often complex, Pb isotope data can provide useful information concerning the sources of diagenetic fluids and the timing of diagenetic events in carbonate sequences. Presented in this dissertation are two case studies into the behavior of the U-Th-Pb system in dolomitic carbonates. In the Burlington-Keokuk Formations (Miss.; Iowa - Illinois) the two major dolomite generations (dolomites I and II) exhibit a difference in initial 206Pb/204Pb. This difference is used to constrain the sources of dolomitizing fluids. The composition of dolomite I is consistent with a marine source for Pb, which had an ultimate derivation from rocks uplifted along eastern North America during the Acadian Orogeny. In contrast, dolomite II has a significantly higher 206Pb/204Pb character. Fluid-rock modeling is used to show that dolomite II could have formed by interaction of dolomite I with a saline fluid that had previously interacted with underlying sedimentary units. The Pb isotopic data indicates that this saline fluid did not derive its Pb from the more radiogenic (basement?) sources sampled by the ore solutions that precipitated the MVT deposits in the region. Whole-rock dolostones from both the Burlington-Keokuk Formations and the Wahoo Formation (Penn.; Prudhoe Bay, AK) are enriched in U (up to 67 ppm) relative to their constituent dolomites (less than 2 ppm). The enrichment of U, high µ's (238U/204Pb), and low ?'s (232Th/238U) exhibited by whole rocks from both sequences suggest that U was mobile relative to Th and Pb during diagenesis. The timing of U-enrichment/mobility are constrained using Pb isotopic data. Despite high µ's, Burlington-Keokuk Formations dolostones are relatively unradiogenic (206Pb/204Pb less than 21). Calculations require U-enrichment to be relatively recent (i.e. less than 5 Ma). In contrast, dolostones from the Wahoo Formation are relatively uranogenic (206Pb/204Pb = 32 to 78) and the spread of isotopic data yields Pb-Pb and U-Pb ages of approximately 260 Ma. These ages are consistent with U enrichment concurrent with the development of a regional unconformity during the Permian. In both the Burlington-Keokuk and Wahoo Formations, U was probably transported as U(VI) complexes in near surface oxidized waters and enriched in local reducing zones adjacent to exposure surfaces.

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