RbSr studies of CI and CM chondrites

RbSr whole rock analyses have been performed on 2 CI and 3 CM chondrites. Four of these stones (Ivuna, Orgueil, Cold Bokkeveld and Erakot) were previously studied in this laboratory and were shown to be discordant from a 4.6 Gyr isochron. The fifth, Murchison, was not previously studied. The new data support the discordance of the first four stones, and indicate that Murchison is also discordant. Studies of Sr isotope ratios in unspiked Orgueil show that the discordance is not due to inhomogeneities in the $\text{Sr}{}^{84}\text{Sr}{}^{86}$ ratio caused by incomplete mixing of nucleosynthesis products.In order to gauge the effects of weathering, two leaching experiments were performed on fresh, interior samples of Murchison; one for a period of 1.5 hr and the other for 117 hr. The results indicate that the relative solubility of nonradiogenic Sr is approximately twice that of Rb and radiogenic Sr is more soluble than the nonradiogenic Sr. This gives the residue a lower model age than the whole rock both by increasing the $\text{Rb}\text{Sr}$ ratio and by decreasing the $\text{Sr}{}^{87}\text{Sr}{}^{86}$ ratio. This result is in contrast to that expected from studies of ordinary chondrite finds, which generally show higher model ages than falls. The constancy of $\text{K}\text{Rb}$ and $\text{K}\text{Sr}$ ratios between the two leaching experiments, and their difference from the unaltered whole rock ratios suggest that the bulk ratios are produced by dissolution of a single phase, and the higher radiogenic Sr content by selective leaching of other phases.