Controls over cesium and rubidium contents of sedimentary rocks

Abstract

In nature, Cs and Rb follow K, which is a much more abundant but chemically similar element. Data for these elements and their mineral carriers were acquired by the combined ICP-OES/ICP-MS (Inductively Coupled Plasma Optical Emission Spectroscopy/Inductively Coupled Plasma Mass Spectrometry) techniques and quantitative XRD (X-ray diffraction), supplemented by multivariate regression analysis, in order to detect controls over cesium and rubidium contents of clastic sedimentary rocks. The sample material comprised clay fractions of bentonites and paleosols, dominated by illite-smectite, and bulk mudstones, sandstones, paleosols, bentonites, basalts, and tuffs from several European basins, ranging from the Proterozoic to Miocene. Results indicate that Rb and Cs contents of clastic sedimentary rocks, and of mudstones in particular, are controlled by the relative concentrations of K-feldspar, micas, and illite-smectite, thus detrital and late diagenetic components of these rocks, and concentrations of Rb and Cs in these minerals. The concentrations of Cs in some illite-smectites, significantly higher than upper continental crust (UCC) averages, indicate hydrothermal alteration or, in case of paleosols and bentonites, can be inherited from anomalous parent rocks. K, Cs, and Rb contents of normal mudstones (unaffected by the hydrothermal fluids or anomalous parent rocks), whether marine or freshwater, are close to the UCC values. In normal mudstones of our collection, feldspars contain 0 to 22 % of Cs and 0 to 41 % Rb, micas 0 to 57 % Cs and 0 to 48 % Rb, and illite-smectite 32 to 100 % Cs and 17 to 100 % Rb, both mostly non-exchangeable and ‘fixed’ in the illite structure. The collected data do not support the use of Rb/K as a paleosalinity proxy and call for careful use of Rb and Cs in chemostratigraphy. The presented relationships indicate that Cs/K and Rb/K ratios in illite-smectite are controlled by the chemistry of fluids, which promote crystallization of illite during burial. Normal diagenetic fluids precipitate illite with, on average, 2.4 times more Cs and 70 % more Rb than the UCC values. In “abnormal” illites, the average Cs and Rb values are 18 and 2.5 times higher than the UCC, respectively. It is confirmed that thin illite crystals have higher Rb/K and Cs/K ratios, tentatively explained by the “supply and demand” mechanism: high ratio between the concentrations of Rb and Cs in pore fluids and the volume of neoformed illite at the beginning of illite crystallization. Major sources of Cs in diagenetic pore fluids in sedimentary basins are detrital micas or hydrothermal fluids invading those basins. The impact of Cs and Rb sorption at illite and mica wedges and/or frayed edges on the total budgets of these elements cannot be evaluated from our data.