[M-IS14] Aqua planetology

[MIS14-P17] Possibilities of the rubidium stable isotope ratio as geochemical tools

*Rimi Konagaya¹, Hiroyuki Tsuboi¹, Takaaki Itai¹, Yoshio Takahashi¹ (1.The University of Tokyo)

Keywords:rubidium, stable isotope, clay mineral, geochemistry

One of important aspects of molecular geochemistry is to develop new isotope tools of various processes by focusing on physico-chemical properties of elements from their atomic scales. There have been no studies so far on the isotope fractionation of rubidium (Rb) caused by Rb behavior during water-rock interaction, possibly due to general knowledge that the reactivity of Rb⁺, or alkaline metal ion, is very low. This is not the case, however, if we think about its interaction with phyllosilicates. It has been clarified that cesium (Cs) is adsorbed on 2:1 type phyllosilicates as inner-sphere (IS) complex by analyzing extended X-ray absorption fine structure (EXAFS). It is considered that the specific adsorption structure is caused by the ionic radius of Cs. Therefore, it was predicted that Rb which is close to Cs in terms of ionic radius also forms IS complex on phyllosilicates. Therefore, in this study, it was aimed to clarify whether the concentration and isotopic ratio of Rb change by the formation of IS complex to phyllosilicate. Rb⁺ was adsorbed on vermiculite, montmorillonite, and illite under various pH and ionic strength conditions, and the adsorption structure was analyzed by EXAFS. For comparison, the same experiment was carried out for the strongly acidic cation-exchange resin (SCR). After these adsorption experiments, Rb stable isotope ratios (⁸⁷Rb/⁸⁵Rb) in liquid and solid phases were measured with a multi-collector type inductively coupled plasma mass spectrometry (MC-ICP-MS) and the degree of the isotope fractionations were analyzed. The host phase of Rb⁺ in marine sediment in the Pacific Ocean and the standard rock samples were also analyzed with EXAFS and their Rb stable isotope ratios were measured with MC-ICP-MS. In addition, dissolved Rb⁺ and Rb⁺ in suspended particles in river water samples collected in the Tone River were also analyzed with EXAFS and their Rb stable isotope ratios were measured with MC-ICP-MS. In this study, it was verified experimentally that Rb forms IS complexes in specific phyllosilicates (vermiculite and illite) and Rb stable isotope fractionation occurs during its adsorption. The lighter isotope (⁸⁵Rb) was concentrated in the solid phases in this isotope fractionation. On the other hand, Rb was mainly adsorbed as outer-sphere complexes in montmorillonite and strongly acidic cation-exchange resin, and the isotope fractionations were small. These results showed that the fractionation was clearly observed for the adsorption system with formation of IS complex (vermiculate and illite). EXAFS analysis also revealed that Rb in sedimentary rocks is also dominated by IS complexes with phyllosilicates. In the ocean system, it was revealed that the fractionation has occurred between marine sediment and seawater. The lighter isotope was concentrated in the solid phases in these systems. Hence, the Rb isotope fractionation observed in the laboratory experiments was also found in the marine environment. However, the mean Rb isotope ratio of seawater-marine sediments is significantly higher than the crustal average, suggesting that the Rb isotope ratio is higher in rivers before its inflow into the ocean. Actually, δ⁸⁷Rb of the Tone River water was fractionated between dissolved and suspended particle phases and this fractionation was consistent with the fractionation observed in the laboratory experiments for illite and vermiculite. Finally, δ⁸⁷Rb of seawater and marine sediment can be explained by considering adsorption reaction on phyllosilicates in both rivers and oceans in this study. Also, Illite and smectite are typical clay minerals in pelagic sediments, and these minerals are also ubiquitous in planetary materials such as meteorites. Therefore, I proposed that possible geochemical applications of the ⁸⁷Rb/⁸⁵Rb isotope ratio can be estimation of the record of sea water fluctuation and water-rock ratio. It has now been widely recognized that various planets had initially water, and the water-rock ratio is important for the study on the evolution of planets and asteroids within the solar system. Measurement of Rb isotope in clay minerals in meteorites and halite, which represents Rb remaining aqueous phase, may provide constraints on the water-rock ratios in the parent body of the meteorites. If this method can be applied to the “Ryugu” return sample provided by the spacecraft “Hayabusa2”, it can be used to estimate the water-rock ratio of the parent body.