The importance of detrital clay mineralogy in controlling geochemical fluxes during shale diagenesis

The Upper Ordovician shales of the Moroccan High Atlas contain large, early diagenetic concretions. The chemistry of the non-carbonate fraction from several concretion-centre/host-shale pairs was determined and the results used to quantify the overall changes in shale chemistry during burial. Si is quantitatively the most important element released from the shales. К and Ca were also released. There was an uptake of Na by the shales. The fluxes can be explained in terms of the mineralogical transformations which occurred during shale diagenesis. The large Si efflux was due mainly to pressure solution of silt-sized quartz. The Na influx and the effl'ux of К and Ca result from albitisaticn reactions. The main control on the availability of the various species was the detrital clay mineralogy.Evans Jonathan. The importance of detrital clay mineralogy in controlling geochemical fluxes during shale diagenesis. In: Proceedings of the 9th international Clay Conference, Strasbourg, 1989. Vol IV : Clays in sediments. Diagenesis and hydrothermalism. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1990. pp. 163-172. (Sciences Géologiques. Mémoire, 88