Hibonite-hercynite inclusion HH-1 from the Lancé (CO3) meteorite: The history of an ultrarefractory CAI

Abstract

We studied the petrography, major and trace element chemistry, and magnesium, calcium, titanium and iron isotopic compositions of a hibonite-rich refractory inclusion from the Lancé CO3 meteorite. The inclusion, 290 × 230 μm in size, consists of a core of hibonite crystals containing small perovskite grains, surrounded by a hercynite mantle and a thin diopside rim. The hercynite contains many small perovskite, ilmenite and sulfide grains. Trace element abundances show an ultrarefractory signature in all measured phases with an increasing depletion of the REEs heavier than Tb with increasing mass in hibonite, perovskite and ilmenite, but not hercynite. Calcium and titanium are isotopically anomalous with large deficits in 48Ca and 50Ti. Deficits are 60%. in 50Ti and 11.5%. in 49Ti in hibonite, hercynite, perovskite and ilmenite. In addition, the titanium isotopes show a mass-dependent isotopic fractionation of 8.5%./amu in favor of the heavy isotopes. Calcium isotope deficits in hibonite are 6.9%. in 42Ca, 5.0%. in 43Ca and 32.5%. in 48Ca relative to 40Ca- 44Ca. The deficits are smaller in the rim diopside ( δ 48 Ca = − 21%.) and a neighboring pyroxene grain( δ 48 Ca = − 8%.). Magnesium shows excesses in 26Mg, but the lack of an isochron relationship between 26Mg 24Mg and 27Al 24Mg indicates redistribution of radiogenic 26Mg. Magnesium in hibonite is isotopically light ( F Mg = − 5.3%./amu). The iron isotopes in hercynite are normal. The data show that inclusion HH-1 experienced a complex metasomatic history including Fe-metasomatism, sulfurization and formation of nepheline and sodalite. The isotopic data provide not only a record of nucleosynthetic processes and nebular inhomogeneity but also a tracer of the relative movement of elements which may have occurred during alteration.