17:15 〜 18:45
[MZZ45-P12] Natural variation of rubidium stable isotope from various environmental materials
キーワード：同位体、ルビジウム、粘土鉱物、地球化学
Rubidium stable isotopes (δ87Rb) has been considered useful for elucidating Earth’s surface processes. We have proposed that Rb adsorbs on phyllosilicate layers by forming inner-sphere complex during which fractionation of Rb stable isotope ratio occurs. Our preliminary study of the river and seawater samples indicated increase of δ87Rb from crust (−0.11‰) to seawater (0.21 – 0.37‰). Considering experimentally determined isotope fractionation between aqueous and adsorbed Rb to the phyllosilicate (Δ87Rb=−0.41 to −0.29‰), it is estimated that two step fractionation is required through weathering, river transport, and reaction in the seawater at least. To verify the degree of fractionation in natural system, we collected series of water and particulate matter (or sediment) from lake (Lake Kizaki), river (Tone River), and ocean (Pacific Ocean) to trace the change in Rb speciation and isotope ratio using X-ray absorption fine structure (XAFS) and inductively coupled plasma mass spectrometry. In this presentation, we propose practical aspects of analytical improvements for these materials, as well as speciation data of Rb.
In order to verify the acceptable range caused by non-isobaric interference from K and Na which are difficult to separate by cation exchange process, we test change in δ87Rb with varying Na/Rb and K/Rb mass ratios. Systematic decreasing trend of δ87Rb was observed from both Na/Rb and K/Rb ratios being 1 to 50. Considering analytical error, we set target level of Na/Rb and K/Rb ratios to be <10. However, Na/Rb ratios in rivers and oceans exceed 1000, which is relatively higher than the Na/Rb ratio of rocks. It can be pointed out that this trend makes the separation of matrix elements such as Na in columns more difficult. Using cation exchange resin (Dowex 50W X8, DuPont) could separate majority of Na from Rb, while reduction of K was not sufficient. Additional use of Sr resin (Sr-spec, Eichrom) improved a separation of Rb from K. The optimized condition was using 1 mL Sr resin with >1 μg Rb. Hence, sequential use of cation exchange resin and Sr-spec resin enable us to reduce Na/Rb and K/Rb efficiently.
The result of extended X-ray absorption fine structure (EXAFS) of lake water-sediment systems, in addition to natural marine sediment-seawater and river dissolved-suspended particle, indicated that Rb adsorbed on certain phyllosilicates (vermiculite and illite) by forming inner-sphere complex. The Rb-O bonding distance was about 0.05-0.15 Å longer than that of hydrated ion. The isotope measurement result showed that the lighter isotope (85Rb) was enriched on the solid phase which was consistent with the longer bonding distance. Therefore, Rb stable isotope fractionation is expected to occur in the lake system as well.
Chemical analysis of the lake water, the inflow and outflow rivers showed increasing trend of cations from upstream to downstream. The concentration of Rb on the dissolved state increased toward the downstream plausibly due to the desorption of Rb from the phyllosilicates due to the increase in competitive ions. Therefore, the solid-water partition ratio is expected to change from upstream to downstream, and the δ87Rb of the suspended particles is expected to be lower. We discuss Rb stable isotope ratio variations in the Earth's surface processes by comparing the adsorption structure of Rb and the results of laboratory experiments.
In order to verify the acceptable range caused by non-isobaric interference from K and Na which are difficult to separate by cation exchange process, we test change in δ87Rb with varying Na/Rb and K/Rb mass ratios. Systematic decreasing trend of δ87Rb was observed from both Na/Rb and K/Rb ratios being 1 to 50. Considering analytical error, we set target level of Na/Rb and K/Rb ratios to be <10. However, Na/Rb ratios in rivers and oceans exceed 1000, which is relatively higher than the Na/Rb ratio of rocks. It can be pointed out that this trend makes the separation of matrix elements such as Na in columns more difficult. Using cation exchange resin (Dowex 50W X8, DuPont) could separate majority of Na from Rb, while reduction of K was not sufficient. Additional use of Sr resin (Sr-spec, Eichrom) improved a separation of Rb from K. The optimized condition was using 1 mL Sr resin with >1 μg Rb. Hence, sequential use of cation exchange resin and Sr-spec resin enable us to reduce Na/Rb and K/Rb efficiently.
The result of extended X-ray absorption fine structure (EXAFS) of lake water-sediment systems, in addition to natural marine sediment-seawater and river dissolved-suspended particle, indicated that Rb adsorbed on certain phyllosilicates (vermiculite and illite) by forming inner-sphere complex. The Rb-O bonding distance was about 0.05-0.15 Å longer than that of hydrated ion. The isotope measurement result showed that the lighter isotope (85Rb) was enriched on the solid phase which was consistent with the longer bonding distance. Therefore, Rb stable isotope fractionation is expected to occur in the lake system as well.
Chemical analysis of the lake water, the inflow and outflow rivers showed increasing trend of cations from upstream to downstream. The concentration of Rb on the dissolved state increased toward the downstream plausibly due to the desorption of Rb from the phyllosilicates due to the increase in competitive ions. Therefore, the solid-water partition ratio is expected to change from upstream to downstream, and the δ87Rb of the suspended particles is expected to be lower. We discuss Rb stable isotope ratio variations in the Earth's surface processes by comparing the adsorption structure of Rb and the results of laboratory experiments.