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

摘要

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 ~(135)Ba from short-lived radioisotope ~(135)Cs, 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 ~(135)Ba excesses that are not accompanied by ~(137)Ba anomalies. Calcium-aluminum-rich inclusion 1 displays a ~(135)Ba excess that is possibly coupled with a ~(137)Ba 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 ~(135)Ba and absence of coupled ~(137)Ba 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 ~(135)Cs. The majority of Allende CAIs studied to date may also have similar contributions from ~(135)Cs on the basis of higher than expected ~(135)Ba excesses if the Ba isotope anomalies were purely nucleosynthetic in origin. The ~(135)Ba anomalies appear not to be coupled with superchondritic Cs/Ba, which may imply that the contribution to ~(135)Ba did not occur via in situ decay of live ~(135)Cs. However, it is feasible that the CAIs had a super-chondritic Cs/Ba during decay of ~(135)Cs, 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 superchondrit-ic Cs/Ba in the early Solar System while ~(135)Cs was extant, which enabled a radiogenic ~(135)Ba signature to develop in some early condensates. The nucleosynthetic source of ~(135)Cs can be determined by reconciling the predicted astrophysical ~(135)Cs abundance with its measured abundance in meteorites. Further, the currently accepted initial ~(135)Cs/~(133)Cs of the Solar System, [~(135)Cs/~(133)Cs]o, may be underestimated because the spread of Cs/Ba among samples is small and the range of excess ~(135)Ba is limited thus leading to inaccuracies when estimating [~(135)Cs/~(133)Cs]_0. If the initial meteoritic abundance of ~(135)Cs 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.