| Summary: | Isotopic studies of meteorites suggest that the early Solar System was rich in the short-lived radionuclide 26Al and that small nucleosynthetic isotopic heterogeneities existed (mostly <1‰). However, rare subtypes of meteoritic calcium-aluminum-rich inclusions (CAIs) have larger nucleosynthetic anomalies (up to 100‰ or more), but lack evidence for having incorporated significant amounts of 26Al. The common interpretation is that in the earliest stages of Solar System history, larger isotopic heterogeneity existed and that fresh 26Al may have arrived in the Solar System after a first generation of solids had formed. However, it remains poorly understood how the solar nebula evolved before and during addition of 26Al, e.g., if relationships existed between different anomalies and what the degree of isotopic heterogeneity was when 26Al arrived.
For this study, a large number of hibonite-rich CAIs were recovered from the Murchison meteorite. Among these types of CAIs, 26Al-poor isotopically anomalous objects are common. With the improved precision and spatial resolution achievable with the CAMECA IMS-1280, a multielement isotopic study was performed in a large number of these CAIs to better characterize their formation reservoirs and to yield constraints on the relationship between 26Al incorporation and nucleosynthetic signatures in different elements.
This study shows that the 26Al-poor nebular reservoir was characterized by large anomalies in 48Ca and 50Ti and that the magnitudes of these anomalies are linked to the relative abundance of 16O. This result could suggest a physical link between presolar nucleosynthetic carriers and a 16O-poor reservoir. This study further shows that all analyzed CAIs with convincing evidence for incorporation of significant 26Al are uniformly enriched in 16O. Six of these CAIs were studied for calcium and titanium isotopes and showed no resolvable anomalies in those isotopes. This suggests that the CAI formation region may have been highly uniform in oxygen isotopes (and possibly also in calcium and titanium isotopes) when 26Al arrived. Importantly, no CAIs with supracanonical 26Al/27Al ratios were found, which may favor a scenario in which 26Al was mixed with 26Al-poor material before arriving in the CAI formation region. But it is also possible that a phase with larger fluctuations in 26Al/27Al was not sampled by the CAIs studied here.
A limited number of analyzed CAIs show clear evidence for mass-dependent heavy isotope enrichment, which is interpreted in the context of melt evaporation. These types of CAIs appear to have sampled both the 26Al-poor, heterogeneous isotopic reservoirs and the 26Al-rich, highly homogenized reservoirs. If the relationship between 26Al-rich and 26Al-poor CAIs is temporal, these results would suggest that CAIs formed by at least two distinct processes over an extended period of time.
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