A Rb Isotopic Shift Due to Nucleosynthesys

Introduction: The isotopic compositions of moderately volatile elements like Li, K, Rb may provide key information to further enhance our understanding of the early history of our Solar System (e.g., [1,2]). Here we discuss technical aspects of our efforts to precisely determine the Rb isotopic composition of meteorites and present a hint for a possible nucleosynthetic effect suggested by correlated observations on Sr. As first shown by the investigation of rare gases and subsequently by analyses of presolar grains that were revealed by the noble gas studies, primitive meteorites contain original and relatively unaltered products of stellar nucleosynthesis (see, e.g., [3] for a recent review). Later work, first on Cr [4,5] and subsequently by high-precision ICP-MS on a variety of other elements (e.g., Zr, Mo, Ba, Nd, Os [6-9]) showed that stepwise dissolution can also lead to identifiable variations in isotopic composition that can be traced back to variable contributions from different processes of nucleosynthesis. We have used a similar approach as part of our study of Rb in meteorites. Measuring Rb isotopes: Rb has two isotopes, 85 Rb and 87 Rb, with the " best measurement from a single source " (IUPAC; [10]) corresponding to 87 Rb/ 85 Rb = 0.385705. Absolute isotopic ratios of Rb, with two isotopes only, are difficult to measure by TIMS and may have an uncertainty of 1 to 2 % because instrumental fractionation cannot be corrected for. The situation is more favorable when using a plasma source mass-spectrometer (ICP-MS), because isotopes of a neighboring element may be used for correction. The suggestion of using Zr for fractionation correction [11] was first realized by [12], but the method has two shortcomings: a) the isotopic composition of Zr is ill constrained [12]; and b) Zr as a high field strength element has significantly different chemical (evaporation , ionization) properties compared to Rb, which is a large ion lithophile element. The same technique was applied by [2], who reported an external reproducibil-ity of 0.02% based on repeated analyses of an artificially enriched standard solution. These authors also found Rb isotopic variations in meteorites. Here we report results of our efforts to measure the isotopic composition of Rb using Sr to monitor in-run frac-tionation. Compared to the pair Rb-Zr, masses and chemical characteristics of Rb and Sr are much more similar. Experimental: Rb isotopic measurements were performed using the NU Instruments Plasma ICP-MS at University of Bern. …