Systematic variations of [formula omitted] ratios, Sr compositions, selected major-oxide concentrations, and mineral abundances in piston cores from the Red Sea

Abstract

A mixing model is presented which relates the concentrations of Sr to the 87Sr86Sr ratios and concentrations of other elements contained in a mixture of two components. The model is applied to the measured 87Sr86Sr ratios, Sr, CaO, Fe2O3 and SiO2 concentrations of the detrital fractions of sediment from three piston cores representing different depositional environments in the Red Sea. The sediment is shown to be a mixture of two components derived from old sialic rocks and young volcanic rocks of the surrounding land masses. One of the cores contained a third component which is identified as authigenic Fe2O3. Removal of this contaminant is shown to return the system to its original detrital constituents. The Sr concentrations of the mixing components were determined from mixing equations derived from the analytical results. The calculated Sr concentrations of the volcanogenic detritus range from 654 to 995 ppm and are consistent with reported values for alkali basalts. The range of Sr concentrations for the sialic detritus, 64–68 ppm, is lower than expected for rocks of granitic composition, indicating that this component consists of highly weathered material. The amount of basaltic detritus contained in each sample was calculated using its measured Sr concentration. The systematic variation of the basaltic compositions as a function of depth defines a stratigraphic profile for each core. Sediment layers either enriched or depleted in volcanogenic detritus are correlated over a distance of as much as 200 km. Layers enriched in the basaltic component are more pronounced in the upper portions of the cores, suggesting increased input of this material in more recent times. Layers enriched in sialic detritus represent pluvial periods in the Red Sea region associated with increased continental runoff during interstadials of the Würm Glaciation. The concentrations of quartz, feldspar, amphibole, illite, kaolinite and chlorite are shown to vary systematically as a function of the calculated basaltic composition of the sediment. The basaltic component consists primarily of unweathered amphibole, feldspar and volcanic glass fragments whereas the sialic component is composed of poorly-ordered clay minerals and amorphous metal oxides. The systematic variations of the Sr results, major-oxide concentrations and mineral abundances are internally consistent, indicating that the application of the two-component model to this mixing regime is valid.