Hibonite-bearing microspherules: A new type of refractory inclusions with large isotopic anomalies

Abstract

Reported are petrographic descriptions, major and trace element chemistry and, Mg, Ca, and Ti isotopic compositions of a new class of refractory inclusions that consist of spherules composed of hibonite and a silicate glass. The distinctive features of these inclusions are excesses in 48Ca and 50Ti in both glass and hibonite, and 26Mg depletions relative to terrestrial isotopic compositions. Three spherules have been examined and analyzed, one from the Lancé CO3 meteorite and two from the Murchison CM2 meteorite. Lancé 3413-1/31 (LA3413-1/31) and Murchison 7-228 (MUR7-228) have euhedral to subhedral hibonite crystals enclosed within glass. Murchison 7-753 (MUR7-753) has a rounded hibonite core with several small inclusions of perovskite. A small fragment of glass is attached to the hibonite and an Fe-silicate rim is imperfectly preserved around the grain. LA3413-1/31 has a Group II REE pattern; MUR7-228 a refractory pattern with depletions in the relatively volatile elements Sr, Ba, Nb, V and Eu; and MUR7-753 a pattern characterized by the prior removal of an ultrarefractory component and overall fractionation of all REEs. The partitioning of the LREEs between hibonite and glass in MUR7-228 is consistent with equilibrium hibonite-liquid partition coefficients previously determined; LA3413-1/31 shows much less partitioning, while MUR7-753 shows no evidence for partitioning and preserves an unequilibrated refractory component highly enriched in Gd. All spherules have initial magnesium depleted in 26Mg by around 3‰ relative to terrestrial Mg, but only MUR7-228 shows evidence for in situ decay of 26Al, with an initial 26Al 27Al of (1.7 ± 0.7) × 10 −5. Both hibonite and glass in all three spherules show excesses of 48Ca and 50Ti, ranging up to +40 and +20‰, respectively, relative to terrestrial Ca and Ti. Spherules such as these are rare and the only other occurrence is in the unique chondrite ALH85085. The hibonite in the spherules shows similarities to isotopically anomalous hibonite crystal fragments (PLACs), but it is unlikely that the spherules formed by remelting of PLACs. The precursors include isotopically anomalous Ca-Ti carriers but also isotopically normal refractory components that probably formed as condensates. The spherules formed by melting of these precursors under disequilibrium conditions and rapid cooling after hibonite crystallization. These inclusions must have formed early, prior to the dilution of isotopic anomalies by mixing processes and in an area characterized by excesses of 48Ca and 50Ti, depletions of 26Mg and lack of 26Al.