Evidence for extinct 135Cs from Ba isotopes in Allende CAIs?

The abundance and distribution of isotopes throughout the Solar System can be used to constrain the number and type of nucleosynthetic events that contributed material to the early nebula. Barium is particularly well suited to quantifying the degree of isotope heterogeneity in the Solar System because it comprises seven stable isotopes that were synthesized by three different nucleosynthetic processes (s-, r-, and p-processes), all of which contributed material to the Solar System. There is also potential contribution to Ba-135 from short-lived radioisotope Cs-135, conclusive evidence for which is yet to be reported. Four Allende (CV3) Ca, Al-rich inclusions (CAI 1, CAI 2, CAI 4, CAI 5) and one Allende dark inclusion (DI) were analyzed for Ba isotope variability. Two CAIs (CAI 2 and CAI 5) display Ba-135 excesses that are not accompanied by Ba-137 anomalies. Calcium- aluminum-rich inclusion 1 displays a Ba-135 excess that is possibly coupled with a Ba-137 excess, and the remaining refractory inclusions (CAI 2 and DI) have terrestrial Ba isotope compositions. These Ba isotope data are presented in conjunction with published whole rock Ba isotope data from individual Allende CAIs. The enrichment in Ba-135 and absence of coupled (137)Baa excesses in CAI 2 and CAI 5 is interpreted to indicate that the anomalies are not purely nucleosynthetic in origin but also contain contributions (16-48 ppm) from the decay of short-lived Cs-135. The majority of Allende CAIs studied to date may also have similar contributions from Cs-135 on the basis of higher than expected Ba-135 excesses if the Ba isotope anomalies were purely nucleosynthetic in origin. The Ba-135 anomalies appear not to be coupled with superchondritic Cs/ Ba, which may imply that the contribution to Ba-135 did not occur via in situ decay of live Cs-135. However, it is feasible that the CAIs had a superchondritic Cs/ Ba during decay of Cs-135, but Cs was subsequently removed from the system during aqueous alteration on the parent body. An alternative scenario is the potential existence of a transient high-temperature reservoir having superchondritic Cs/ Ba in the early Solar System while Cs-135 was extant, which enabled a radiogenic Ba-135 signature to develop in some early condensates. The nucleosynthetic source of Cs-135 can be determined by reconciling the predicted astrophysical Cs-135 abundance with its measured abundance in meteorites. Further, the currently accepted initial Cs-135/Cs-133 of the Solar System, [Cs-135/Cs-135](0), may be underestimated because the spread of Cs/ Ba among samples is small and the range of excess Ba-135 is limited thus leading to inaccuracies when estimating [ Cs-135/Cs-135](0). If the initial meteoritic abundance of Cs-135 was indeed higher than is currently thought, the most probable stellar source of short-lived radioisotopes was a nearby core-collapse supernova and/ or the Wolf-Rayet wind driven by its progenitor. (C) 2013 Elsevier Ltd. All rights reserved.