Rare earth elements and sulfur and strontium isotopes of upper Cretaceous phosphorites in Egypt

Abstract

Upper Cretaceous Phosphorites from different localities in Egypt were analyzed for their rare earth elements (REEs) contents and sulfur and strontium isotopes to examine the effect of depositional conditions versus diagenesis on these parameters. The negative Ce and Eu anomalies of the study phosphorites suggest its formation under reducing conditions. However, chondrite-normalized REEs patterns show relative enrichments of LREEs over the HREEs, which is obviously different from the seawater REEs pattern suggesting post-depositional modifications on the REEs distributions during diagenesis. The difference in the REEs concentrations and Ce anomalies among the study localities as well as the similarity between the REEs patterns of these phosphorites and associated black shales might support this interpretation. The concentration of structural SO 4 2− (0.6–3.7%) and their δ 34S values (+0.5 to –20‰) in the upper Cretaceous phosphorites in Egypt suggest the formation of these phosphorites in the zone of sulfate reduction. On the other hand, the sulfur isotopes in the pyrite from the study phosphorites (δ 34S = +4.6‰ − 23‰ with an average of −7.7‰) are attributed to the influence of seawater from which pyrite was formed during diagenesis. The difference between the δ 34S values in the phosphorites (all are positive values) and those in the associated pyrite (mostly negative values) reflect an asymmetric sulfate and sulfide sulfur isotopic composition due to the formation of francolite (source of sulfate) and pyrite (source of sulfide) in different conditions and/or process. The 87Sr/ 86Sr values of the upper Cretaceous phosphorites in Egypt are very close to the marine values during the Campanian–Maastrichtian time and their average (0.707622) is more or less comparable to the average 87Sr/ 86Sr values of the Cretaceous–Eocene Tethyan phosphorites. This suggests no post-depositional alteration (i.e. diagenetic effect) on the Sr isotopic composition of these phosphorites.