Isotope Geochemistry of Clays and Clay Minerals from Sedimentary Rocks

Abstract

Authigenic clay minerals are records of a long and complex history of the sediments since their deposition. These minerals can be broadly classified as either syn-depositional or post-depositional, but the distinction between them in an ancient sedimentary rock is not always easy. This distinction is important because only the isotopic records of very early authigenic minerals may be examined to determine the time of deposition of the sediments. As is often the case, the separation of early and late diagenetic clay minerals from a rock is often frustrated by the overwhelming abundance of detrital clay minerals in the rocks. The fundamental requirement for tracing the different events in the genesis of the clays is then a clear understanding of the evolutions of the analyzed clay materials. Equally important is a critical assessment of responses of the different isotope systems of clay minerals in question that might have been subjected to varied depositional and post-depositional forces causing modifications in physical and chemical aspects of a mass of sediment (Clauer et al. 1992c).