ISOTOPIC SEARCH FOR LIVE 135CS IN THE EARLY SOLAR SYSTEM AND POSSIBILITY OF 135CS-135BA CHRONOMETER

Abstract

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)x10-4 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.