Radiogenic Isotopic Study of Diagenesis and Low-Grade Metamorphism of Pelites

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This research is designed to develop applications of radiogenic isotope analysis to pelitic sediments that have been affected by a wide range of fluid-mediated diagenesis through low- grade metamorphism. Although data for clay-rich sediments imply that they have a memory for fluid-rock interactions owing to their limited permeability as compared with other basin sediments, much isotope data lead to ambiguous results in part because analyzed samples are heterogeneous and contain unknown proportions of more than one generation of fine-grained clay mineral. This proposal is a renewal of our current support, in which high resolution scanning electron microscopy (STEM) is combined with isotopic analysis of equivalent pelitic rocks. The ultimate aim of these studies is to determine the timing and extent of water-pelite interactions and thus of basin- to regional-scale fluid/rock interactions. Having previously focused on Rb-Sr and development of Sm-Nk methods, we now propose to develop the 40Ar/39Ar laser technique for pelitic sediments, using well characterized samples (thin sections and separates), and other isotopic techniques to provide supporting data. We intend to determine the limitations of the methods, both with respect to spatial resolution and age. Based on our previous observations of shales and slates from a wide variety of occurrences, we have chosen to obtain data from bentonites (as standards having clays of a single origin) and shales from the Texas Gulf Coast and Welsh Sedimentary Basin, as well as other prograde sequences. The latter is representative of diagenesis in a passive marginal basin, whereas the former represents tectonically-induced dissolution and crystallization over the range of diagenesis to greenschist facies conditions. The samples from each locality represent the complete range of diagenesis through low-grade metamorphism.