Correlated morphological, chemical, and isotopic characteristics of hibonites from the Murchison carbonaceous chondrite

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Twenty-four hibonite-dominated refractory grains from the Murchison carbonaceous chondrite have been identified from polished grain-mounts of density separates, and have been morphologically and chemically divided into three groups: (1) 14 colourless PLAty Crystal fragments (PLACs) with less than 2.7% TiO 2; (2) 3 pleochroic Blue AGgregates (BAGs) which are composed of crystal plates and fragments and have TiO 2 concentrations between 5.1% and 6.5%; (3) Six Spinel-HIBonite spherules and crystal aggregates (SHIBs), often with spinel, Fe-silicate, and clinopyroxene rims. The maximum TiO 2 concentrations of hibonite in the SHIBs ranges from 3.5 to 7.6%, but the TiO 2 concentration in one grain was heterogeneous and ranged from 0.5% to 5.0%. Hibonite in all types is stoichiometric with Ti and Mg predominantly in a coupled substitution of Mg 2+ and Ti 4+ for 2Al 3+. One grain with a rounded hibonite core does not clearly fit into the three designated groups. The hibonites have been analysed by ion microprobe mass spectrometry for their Mg and Ti isotopic compositions. Twelve of the PLACs have ( 26Al 27Al ) 0 ratios less than 1 × 10 −5 but the other two PLACs have ( 26Al 27Al ) 0 of 5.5 × 10 −5 and 7.7 × 10 −5. The excess 26Mg may reflect initial Mg isotopic heterogeneities in the source region of the PLACs, or be the result of in situ 26Al decay, in which case the distribution of 26Al was heterogeneous. Three BAGs show substantial positive mass-fractionation of Mg. One grain has excess 26Mg while the other two have 26Mg depletions. The 26Mg depletions indicate the preservation of Mg isotopic anomalies in the BAGs, but the excess 26Mg in one grain could be due to in situ 26Al decay. The six SHIBs have excess 26Mg that is correlated with 27Al 24Mg . The Mg isotopic systematics of the SHIBs are consistent with the in situ decay of 26Al with ( 26Al 27Al ) 0 of ca. 5 × 10 −5. The unclassified hibonite has a clearly-resolved deficit in 26Mg. Titanium isotopic anomalies are common in the hibonites measured here. The largest anomalies are in 50Ti with variations in 50Ti 48Ti ranging from −50 to +16%. relative to terrestrial. Smaller anomalies are present in 49Ti and 47Ti. The Ti isotopic compositions are not directly correlated with morphological type, but the PLACs show the largest variations in isotopic composition, whereas the SHIBs are generally close to terrestrial. The presence of hibonite populations with distinct morphological, chemical and Mg isotopic systematics suggests that there were several hibonite formation episodes within the early solar system. The hibonites probably formed by the melting of refractory dust aggregates during local transient thermal events.