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

Here we report the isotopic excess of Ba-135 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 Ba-135 abundances of CAIs, after subtraction of the r-process nucleosynthetic component in a model-dependent manner, a value for (CS)-C-135/Cs-133 = (4.8 +/- 0,8)x 10(-4) is proposed as the initial ratio in the early solar system. A correlation between excess Ba-135 abundances and Cs/Ba ratios observed in the Beardsley and the Zag meteorites reveals the possible existence of primordial Cs-135 in the two meteorite parent bodies. On the assumption of two cases for Cs-135/Cs-133 initial in the early solar system, the Cs-135-Ba-135 isochron suggests that primitive (aqueous) alteration in the Beardsley and Zag meteorite bodies occurred at 8.2 similar to 11.9 Ma and 13.9 similar to 17.6 Ma after CAI formation, respectively. Our results are apparently consistent with previously reported data from Pb-Pb model age determinations, and Mn-53-Cr-53 chronometry for chondrites. This is the first report of isotopic excess of Ba-135 in meteorites, which may relate to live Cs-135 in the early solar system and the Cs-135-Ba-135 chronological application. (C) 2001 Elsevier Science B.V. All rights reserved.