Isotopic search for live ~(135)Cs in the early solar system and possibility of ~(135)Cs-~(135)Ba chronometer

摘要

Here we report the isotopic excess of ~(135)Ba 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 ~(135)Ba abundances of CAIs, after subtraction of the r-process nucleosynthetic component in a model-dependent manner, a value for ~(135)Cs/~(133)Cs = (4.8 ± 0.8) * 10~(-4) is proposed as the initial ratio in the early solar system. A correlation between excess ~(135)Ba abundances and Cs/Ba ratios observed in the Beardsley and the Zag meteorites reveals the possible existence of primordial ~(135)Cs in the two meteorite parent bodies. On the assumption of two cases for ~(135)Cs/~(133)Cs initial in the early solar system, the ~(135)Cs-~(135)Ba 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 ~(53)Mn-~(53)Cr chronometry for chondrites. This is the first report of isotopic excess of ~(135)Ba in meteorites, which may relate to live ~(135)Cs in the early solar system and the ~(135)Cs-~(135)Ba chronological application.