Isotopic search for live 135Cs in the early solar system and possibility of 135Cs–135Ba chronometer

Here we report the isotopic excess of 135Ba studied from Ba isotopic measurements of acid leachates of Allende calcium–aluminum rich inclusions (CAIs) and two primitive chondrites, Beardsley (H5) and Zag (H6 clasts). From the excess 135Ba abundances of CAIs, after subtraction of the r-process nucleosynthetic component in a model-dependent manner, a value for 135Cs/133Cs=(4.8±0.8)×10−4 is proposed as the initial ratio in the early solar system. A correlation between excess 135Ba abundances and Cs/Ba ratios observed in the Beardsley and the Zag meteorites reveals the possible existence of primordial 135Cs in the two meteorite parent bodies. On the assumption of two cases for 135Cs/133Cs initial in the early solar system, the 135Cs–135Ba isochron suggests that primitive (aqueous) alteration in the Beardsley and Zag meteorite bodies occurred at 8.2∼11.9 Ma and 13.9∼17.6 Ma after CAI formation, respectively. Our results are apparently consistent with previously reported data from Pb–Pb model age determinations, and 53Mn–53Cr chronometry for chondrites. This is the first report of isotopic excess of 135Ba in meteorites, which may relate to live 135Cs in the early solar system and the 135Cs–135Ba chronological application.