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

Potassium and magnesium isotopic compositions of hibonites from the Murchis on (CM) and Allende (CV) meteorites are determined by an ion microprobe to look for possible presence of the short-lived nuclides Ca-41 and Al-26 at t he time of their formation. Abundance anomalies in the neutron-rich isotope s Ti-50 and Ca-48 as well as REE and additional refractory trace element ab undances have also been determined to infer a plausible formation environme nt of the hibonites in the early solar system. The results obtained in this study suggest a widespread distribution of the short-lived nuclide Ca-41 i n the early solar system. Observation of correlated presence of the two sho rt-lived nuclides Ca-41 and Al-26 On a microscopic scale in refractory hibo nites and silicates from CM and CV meteorites suggests these nuclides to be cogenetic and support a stellar source for these and several other short-l ived nuclides in the early solar system. Moderate to high enrichments in th e neutron-rich isotopes Ti-50 and Ca-48 are seen in some of the analyzed hi bonites. Our data for radiogenic and stable isotopic anomalies as well as r efractory trace element abundance patterns in isolated CM hibonites with pl atelet morphology show trends similar to those reported earlier. They are d evoid of Ca-41 and Al-26, show large enrichment in Ti-50 and Ca-48 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 ultrarefr actory REE pattern and a third has affinities towards platelet hibonites th at appear to suggest a link between these two groups of hibonites with dist inct morphological characteristics. We propose a new model to explain the a bsence of the short-lived nuclides Ca-41 and Al-26 in some of the hibonites , whose REE abundance patterns and stable isotopic anomalies suggest that t hey are some of the first solar system solids, assuming a stellar origin fo r 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 fr om a stellar source. Copyright (C) 2000 Elsevier Science Ltd.