Strontium isotope variations in the early solar system revealed by calcium-aluminium inclusions (CAIs) and ureilites

Abstract

Isotope variations attributed to nucleosynthetic heterogeneities are preserved in different solar system materials (meteorites and planets) and provide insights into the origin of the elements, and the formation and evolution of the protoplanetary disk. Strontium isotopes are potentially highly sensitive to such nucleosynthetic variations. The three most abundant strontium isotopes 86Sr, 87Sr, and 88Sr are produced by the s- and r-processes nucleosynthesis, in common stellar environments. In contrast, the lightest isotope, 84Sr, is only produced by the p-process, and requires extreme conditions facilitating either proton-capture or photodisintegration. This study presents high-precision Sr isotope data for a range of early solar system materials, including twelve fine- and coarse-grained calcium-aluminium inclusions (CAIs) (considered the first solid material to have condensed in the solar system) separated from Allende (a CV3 meteorite), and fourteen ureilites (differentiated ultramafic meteorites, though to represent residual mantle material from which silicate and metallic melts have been removed).

Nearly all CAIs possess μ84Sr excesses (measured 84Sr/86Sr relative to terrestrial standards), coarse-grained CAIs show variable μ84Sr, ranging from terrestrial values up to +679 ± 113ppm, whereas fine-grained CAIs show a narrower range with a mean of +68 ± 43ppm. Fine-grained CAIs possess relatively radiogenic 87Sr/86Sr isotope compositions suggesting that the initial μ84Sr variability may have been lost due to open-system behaviour subsequent to their formation. Stable isotope δ84/86Sr and δ88/86Sr compositions largely lie to the right of the terrestrial Mass Dependent Fractionation Line (MDFL) consistent with previous studies, although the data here indicate that fine-grained CAIs generally possess light δ88/86Sr and coarse-grained heavy δ88/86Sr. Overall, the μ84Sr data taken with the δ84/86Sr and δ88/86Sr variations suggest the presence of excess 84Sr, attributable to p-process nucleosynthesis.

In contrast, many ureilites (both main group and polymict ureilites) possess μ84Sr deficits, with a mean value of -101.6 ± 69ppm (2.s.d, n = 8), while others possess terrestrial values. The δ84/86Sr and δ88/86Sr isotope compositions of the ureilites yield a slope of -1.206 ± 0.013 (2 s.e.), that is not easily explained by mass dependent fractionation, but rather by mixing between terrestrial like Sr and material with a low δ84/86Sr. Pre-solar SiC grains possess a low δ84/86Sr composition consistent with such s-process enrichment. The broad covariation of δ88/86Sr with Sr concentration may be due to stable isotope fractionation accompanying melt depletion, where in turn at low Sr concentrations the composition of refractory SiC grains dominates the bulk meteorite, yielding the observed variations in δ84/86Sr - δ88/86Sr space

Overall, the Sr isotope variations see in CAIs are consistent with the Sr isotope compositions observed in many carbonaceous chondrites taken as a signature of material in the outer solar system (Charlier et al., 2017). Whereas the ureilites appear to possess a signature consistent with s-process enrichment, in accord with Nd, Mo and Os isotope anomalies (e.g., Burkhardt et al., 2019; Spitzer et al., 2020; Goderis et al., 2015) taken to indicate the presence of s-process enriched material in the inner solar system.