The Origin and Evolution of Nucleosynthetic Sr Isotope Variability in Calcium and Aluminum-rich Refractory Inclusions

Abstract

Nucleosynthetic isotope anomalies in meteorites are useful for investigating the origin of materials in the protoplanetary disk and dynamical processes of planetary formation. In particular, calcium and aluminum-rich inclusions (CAIs) found in chondrites are key minerals for decoding the initial conditions of the solar system before the accretion of small planetary bodies. In this study, we report isotopic analyses for three Allende CAIs, fluffy type A (FTA), type B, and fine-grained spinel rich (FS) inclusions, with a specific emphasis on the measurements of 84Sr/86Sr ratios. It was found that the average μ 84Sr values (106 relative deviations from a standard material) were 175, 129, and 56 ppm for the samples of FTA, type B, and FS inclusions, respectively. Additionally, the FTA samples exhibited heterogeneous μ 84Sr values, while those for the type B and FS inclusions were homogeneous within individual inclusions. The elevated μ 84Sr values were most likely explained by the relative enrichment of r-process nuclides in the CAI formation region. The variation of μ 84Sr values between the FTA and type B inclusions, as well as within the FTA inclusion, suggests the presence of multiple CAI source reservoirs with distinct isotopic compositions, which is either inherited from isotopic heterogeneity in the molecular cloud or caused by the selective destruction of r-process-enriched supernova grains via nebular thermal processing. On the other hand, the reaction between a refractory precursor of the FS inclusion and a gaseous reservoir enriched in Mg, Si, and 16O resulted in the lowest μ 84Sr values for the FS inclusion.

Nucleosynthetic, radiogenic and stable strontium isotopic variations in fine- and coarse-grained refractory inclusions from Allende

“…However, the high precision of our data allows us to resolve significant dispersion in the  84 Sr values (particularly for fine-grained CAIs), reflected in the large 2 s.d. value, in agreement with the recent data from Myojo et al (2018). Earlier studies ( Fig.…”

“…1) yielded more uniform values (albeit with lower precision) and it is not clear from the mineralogical data available whether the differences result from the samples being from contrasting CAI types or reflect analytical differences. Our data and those of Myojo et al (2018)…”

“…Because of this large variability we do not present these data as simple weighted means as many of the values are resolvably distinct at the quoted level of precision. Myojo et al (2018) also found variations well outside of analytical precision for  84 Sr within a single CAI, implying the existence of additional heterogeneity below the scale of whole CAI sampling.…”

Elemental and isotopic variability in solar system materials by mixing and processing of primordial disk reservoirs

“…The ε 84 Sr anomalies of the CAIs are indistinguishable within uncertainty and are well within the anomaly range reported in other studies (Moynier et al 2012;Brennecka et al 2013;Myojo et al 2018). Variations in 87 Sr/ 86 Sr range from 0.699478 in the coarse-grained Type B CAI A-ZH-2 to 0.718868 in the more fine-grained and volatile rich A-ZH-5, and correlate with Rb/Sr as expected for a ~4.567 Ga age ( Fig.…”

“…The first evidence for mass-independent Sr isotope anomalies in material processed within the solar system was reported for FUN CAIs, which show deficits of ~-8 ε 84 Sr and excesses of ~+32 ε 84 Sr (Papanastassiou and Wasserburg, 1978) after normalization to a fixed 88 Sr/ 86 Sr ratio. "Normal" CAIs exhibit anomalies of about +1.3 in ε 84 Sr (Brennecka et al 2013;Hans et al 2013;Moynier et al 2012;Myojo et al 2018;Paton et al 2013;Charlier et al 2017). Nucleosynthetic Sr isotope anomalies at the bulk meteorite scale seem to be restricted to CC bodies (Moynier et al 2012;Paton et al 2013;Hans et al 2013;Yokoyama et al 2015).…”