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 ¹³⁵Ba from short-lived radioisotope ¹³⁵Cs, 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 ¹³⁵Ba excesses that are not accompanied by ¹³⁷Ba anomalies. Calcium-aluminum-rich inclusion 1 displays a ¹³⁵Ba excess that is possibly coupled with a ¹³⁷Ba 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 ¹³⁵Ba and absence of coupled ¹³⁷Ba excesses in CAI 2 and CAI 5 is interpreted to indicate that the anomalies are not purely nucleosynthetic in origin but also contain contributions (16-48ppm) from the decay of short-lived ¹³⁵Cs. The majority of Allende CAIs studied to date may also have similar contributions from ¹³⁵Cs on the basis of higher than expected ¹³⁵Ba excesses if the Ba isotope anomalies were purely nucleosynthetic in origin. The ¹³⁵Ba anomalies appear not to be coupled with superchondritic Cs/Ba, which may imply that the contribution to ¹³⁵Ba did not occur via in situ decay of live ¹³⁵Cs. However, it is feasible that the CAIs had a superchondritic Cs/Ba during decay of ¹³⁵Cs, 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 ¹³⁵Cs was extant, which enabled a radiogenic ¹³⁵Ba signature to develop in some early condensates. The nucleosynthetic source of ¹³⁵Cs can be determined by reconciling the predicted astrophysical ¹³⁵Cs abundance with its measured abundance in meteorites. Further, the currently accepted initial ¹³⁵Cs/¹³³Cs of the Solar System, [¹³⁵Cs/¹³³Cs]₀, may be underestimated because the spread of Cs/Ba among samples is small and the range of excess ¹³⁵Ba is limited thus leading to inaccuracies when estimating [¹³⁵Cs/¹³³Cs]₀. If the initial meteoritic abundance of ¹³⁵Cs 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.