Electron Microprobe Analyses of Magnesium and Iron Distribution in Carbonate Cements and Recrystallized Sediment Grains from Ancient Carbonate Rocks

Abstract

ABSTRACT Mg2+ and Fe2+ inhomogeneities have been found to exist in carbonate cements of sediments ranging in age from Miocene to Devonian. These cements appear to have remained in disequilibrium with their environment throughout their post-accretional history and have therefore preserved original minor element ratios which reflect the pore fluids from which they were precipitated. The cation ratios of waters accountable for precipitation can be inferred from the distribution patterns of minor elements in a cement and the distribution coefficients of the minor elements. The relation between Mg2+, Fe2+ content and cement morphology is complex and highly variable. In some cases a change in minor element content occurs at a petrographic boundary between cement generations. In other cases there exists little or no minor element variability across the boundary separating petrographic generations. The minor element variability within an inhomogeneous generation can vary from a simple compositional trend to an extremely complex pattern. The minor element approach described herein enables a more thorough reconstruction of the cementation history of a carbonate sediment to be made than that afforded by conventional petrographic techniques alone. Although it is not yet possible to unequivocally relate minor element patterns to the diagenetic environments in which cementation took place, this does not detract from the power of the approach to infer ancient water chemistry accompanying the cementation history of a sediment.