A possibility of rubidium isotope as a proxy of water-rock ration at surface of aqua planets

Abstract:

In geochemistry, many studies have been conducted to develop new isotope tools based on the chemical properties of various elements that can cause fractionation of their stable isotope ratios. In this study, we focus on the interaction of alkaline elements and phyllosilicate minerals. It has been clarified that cesium (Cs) ion is adsorbed on 2:1 type phyllosilicates as inner-sphere (IS) complexes by EXAFS. It is considered that the IS complexation caused by the large ionic radius of Cs⁺ leads to isotopic fractionation, but is not observable due to the absence of multiple isotopes for Cs. Thus, we studied rubidium (Rb) isotope, since Rb with ionic radius close to Cs also forms IS complexes on phyllosilicates. In this study, Rb⁺ was adsorbed on vermiculite, montmorillonite, and illite under various pH and ionic strength conditions, and the adsorption structure was analyzed by EXAFS and Rb stable isotope ratios (87Rb/85Rb) in aqueous and solid phases were measured with MC-ICP-MS to analyze the degree of the isotope fractionation. It was verified experimentally that Rb+ forms IS complexes in specific phyllosilicates (vermiculite and illite), for which Rb stable isotope fractionation occurs during the adsorption (Δ87Rbsolid-liquid = −0.41±0.15‰ and −0.29±0.05‰ for vermiculite and illite, respectively). On the other hand, Rb+ mainly adsorbed as outer-sphere complexes in montmorillonite and strongly acidic cation-exchange resin did not show large isotope fractionation (Δ87Rb < 0.05‰). The results of the adsorption experiments suggested that the Rb concentration in seawater is dominated by the partition between seawater and phyllosilicates. EXAFS analysis revealed that Rb in natural sediments is also dominated by IS complexes with phyllosilicates. In the natural system, it was revealed that the fractionation (Δ87Rb= −0.17±0.07‰) is found between sediment and seawater in the ocean. Moreover, Rb isotope fractionation in river-estuary system was also conducted, showing that the light and heavy Rb isotopes were found in Rb in particulate matter and river water, respectively, which is expected from the laboratory studies. Based on these results, we propose here geochemical application of the 87Rb/85Rb isotope ratio to estimate the water-rock ratio in the system in various systems such as at the surface of aquaplanets.