Diverse nucleosynthetic components in barium isotopes of carbonaceous chondrites: Incomplete mixing of s- and r-process isotopes and extinct 135Cs in the early solar system

Barium isotopic compositions of chemical leachates from six carbonaceous chondrites, Orgueil (CI), Mighei (CM2), Murray (CM2), Efremovka (CV3), Kainsaz (CO3), and Karoonda (CK4), were determined using thermal ionization mass spectrometry in order to assess the chemical evolution in the early solar system. The Ba isotopic data from most of the leachates show variable Ba-135 excesses correlated with Ba-137 excesses, suggesting the presence and heterogeneity of additional nucleosynthetic components for s- and r-processes in the solar system. The isotopic deviations observed in this study were generally small (- 1 < epsilon < +1) except in the case of the acid residues of CI and CM meteorites. Large deviations of Ba-135 (epsilon = -13.5 to -5.0) and Ba-137 (epsilon = -6.2 similar to-1.2) observed in the acid residues from one CI and two CM meteorites show significant evidence for the enrichment of s-process isotopes derived from presolar grains. Two models were proposed to estimate the Cs-135 isotopic abundances by subtraction of the s- and r-isotopic components from the total Ba isotopic abundances in the three CM meteorites, Mighei, Murchison (measured in a previous study), and Murray. The data points show individual linear trends between Cs-135/Ba-136 ratios and Ba-135 isotopic deviations for the three samples. Considering the different trends observed in the three CM meteorites, the Ba isotopic composition of the CM meteorite parent body was heterogeneous at its formation. Chronological information is unclear in the data for Murchison and Murray because of large analytical uncertainties imposed by error propagation. Only the Mighei meteorite data indicate the possible existence of presently extinct Cs-135 (Cs-135/Cs-133 = (2.7 +/- 1.6) x 10(-4)) in the early solar system. Another explanation of the data for the three CM meteorite is mixing of at least three components with different Ba isotopic compositions, although this is model-dependent. (C) 2011 Elsevier Ltd. All rights reserved.