K, Mg, Ti and Ca isotopic compositions and refractory trace element abundances in hibonites from CM and CV meteorites: implications for early solar system processes

Abstract

Potassium and magnesium isotopic compositions of hibonites from the Murchison (CM) and Allende (CV) meteorites are determined by an ion microprobe to look for possible presence of the short-lived nuclides ⁴¹Ca and ²⁶Al at the time of their formation. Abundance anomalies in the neutron-rich isotopes ⁵⁰Ti and ⁴⁸Ca as well as REE and additional refractory trace element abundances have also been determined to infer a plausible formation environment of the hibonites in the early solar system. The results obtained in this study suggest a widespread distribution of the short-lived nuclide ⁴¹Ca in the early solar system. Observation of correlated presence of the two short-lived nuclides ⁴¹Ca and ²⁶Al on a microscopic scale in refractory hibonites and silicates from CM and CV meteorites suggests these nuclides to be cogenetic and support a stellar source for these and several other short-lived nuclides in the early solar system. Moderate to high enrichments in the neutron-rich isotopes ⁵⁰Ti and ⁴⁸Ca are seen in some of the analyzed hibonites. Our data for radiogenic and stable isotopic anomalies as well as refractory trace element abundance patterns in isolated CM hibonites with platelet morphology show trends similar to those reported earlier. They are devoid of ⁴¹Ca and ²⁶Al, show large enrichment in ⁵⁰Ti and ⁴⁸Ca and have typical Group III REE patterns. However, hibonites present within hibonite-spinel inclusions show variations in their REE patterns. One of them shows the normally expected Group II REE pattern, while another has an ultrarefractory REE pattern and a third has affinities towards platelet hibonites that appear to suggest a link between these two groups of hibonites with distinct morphological characteristics. We propose a new model to explain the absence of the short-lived nuclides ⁴¹Ca and ²⁶Al in some of the hibonites, whose REE abundance patterns and stable isotopic anomalies suggest that they are some of the first solar system solids, assuming a stellar origin for these nuclides. We attribute this absence to the very early formation of these hibonites near the central region of the collapsing protosolar cloud prior to the arrival of the short-lived nuclides injected into the cloud from a stellar source.