Abstract

At the time of deposition, black shales record geochemical information related to the provenance and conditions of deposition for that rock. These initial geochemical conditions are susceptible to alteration and overprinting by the aggressive processes of black-shale diagenesis. In this study, we evaluate a sequence of Ordovician black shales from the southern Welsh Basin for the presence of such a diagenetic overprint. Through use of cathodoluminescence, backscattered electron imaging, reflected-light microscopy, fission tracks, and microprobe elemental mapping, we can discern textural relationships with in the Welsh black shales. The relationships observed constrain a paragenetic sequence and overall diagenetic history including the early diagenetic formation of pyrite, apatite (rare earth element (REE)-rich) and carbonate and later formation of diagenetic monazite (REE-, U-rich) and late-stage carbonate. The chemical reactions necessary to produce this mineral sequence require postdepositional redistribution of REE (including changing cerium anomalies (Ce/Ce*)) and uranium. As a result, REE, U, and their associated isotopic systems (Sm-Nd and U-Pb) can reflect a diagenetic overprint rather than the source area and depositional conditions. The paragenetic sequence described in this study records several postdepositional changes in the whole-rock mineralogy. Among these changes are the formation of REE-rich apatite and REE- and U-rich monazite. The formation of these phases requires that REE and U are being redistributed on at least a mineral scale and that the initial depositional and provenance information recorded by the whole rock has been disturbed. Therefore, the approach used in this study to carefully characterize the paragenetic sequence from the Llandeilo-Caradoc black shales should be considered necessary in order to fully evaluate the usefulness of trace-element proxies susceptible to diagenetic redistribution.

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