²⁶Al, ²⁴⁴Pu, ⁵⁰Ti, REE, and trace element abundances in hibonite grains from CM and CV meteorites

Abstract

The ion microprobe was used to measure Ti and Mg isotopes as well as rare earth and other trace elements in ten hibonites from the CM carbonaceous chondrites Murchison, Murray, and Cold Bokkeveld and in two hibonites and Ti-rich pyroxene from the CV chondrite Allende. In hibonites from Murchison and Murray fission track densities were also measured, as were Th and U concentrations. Eight of the hibonites, from all four meteorites, exhibit large Ti isotopic anomalies, particularly in ⁵⁰Ti. Two grains from Murray have ⁵⁰Ti excesses of ~ 10%. At least four nucleosynthetic components are required to account for all the Ti isotopic data. Neutron-rich nuclear statistical equilibrium nucleosynthesis is the most likely process to account for a ⁵⁰Ti-rich component (with ⁵⁰Ti/⁴⁹Ti $20 ). The ion probe Ti isotopic measurements confirm that the solar nebula was isotopically heterogeneous on a small spatial scale and argue for a chemical memory origin of the Ti isotopic anomalies which were probably carried into the solar system in the form of refractory dust grains. However, there is no experimental evidence that such interstellar grains survived the formation of the hibonites. The REE and trace element patterns of the hibonites are similar to those seen in CAIs and can be interpreted in terms of fractionation effects during condensation from a gas of solar composition, thus arguing for a solar system origin of the hibonites. Additional evidence for such an origin is provided by the Pu/Th ratios, which are ~ 10^-4, and by the Mg isotopic compositions which are normal except for ²⁶Mg^∗ due to ²⁶Al. Only three out of ten hibonites exhibit ²⁶Mg^∗, consistent with previous studies which demonstrated the paucity of ²⁶Mg^∗ in hibonites. Because of the refractory nature of hibonite and the presence of large Ti isotopic effects, we conclude that a heterogeneous distribution of ²⁶Al in the early solar system is the most likely reason. In particular, our observations of δ⁵⁰Ti = 15%. and of an isochron with ( ²⁶Al/²⁷Al)₀=5×10^-8 in the FUN inclusion HAL are evidence against both late formation and Mg redistribution to explain the lack of ²⁶Al in hibonites. There are no obvious correlations between the Ti isotopic compositions, the presence of ₂₆Mg^∗ , the presence of ²⁴⁴Pu, and the REE and trace element patterns in individual hibonites. This indicates that the anomalous ₅₀Ti, as well as ²⁶A1 and ²⁴⁴Pu, were not co-produced in a single astrophysical source, and/or that these nuclides were introduced into the solar nebula by different carriers before being incorporated into the hibonites.