HIBONITE-HERCYNITE INCLUSION HH-1 FROM THE LANCE (CO3) METEORITE - THE HISTORY OF AN ULTRAREFRACTORY CAI

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 Lance CO3 meteorite. The inclusion, 290 X 230 mu m 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 Ca-48 and Ti-50. Deficits are 60 parts per thousand in Ti-50 and 11.5 parts per thousand in Ti-49 in hibonite, hercynite, perovskite, and ilmenite. In addition, the titanium isotopes show a mass-dependent isotopic fractionation of 8.5 parts per thousand/amu in favor of the heavy isotopes. Calcium isotope deficits in hibonite are 6.9 parts per thousand in Ca-42, 5.0 parts per thousand in Ca-43, and 32.5 parts per thousand in Ca-48 relative to Ca-40- Ca-44. The deficits are smaller in the rim diopside (delta(48)Ca = -21 parts per thousand) and a neighboring pyroxene grain (delta(48)Ca = -8 parts per thousand). Magnesium shows excesses in Mg-26, but the lack of an isochron relationship between Mg-26/Mg-24 and Al-27/Mg-24 indicates redistribution of radiogenic Mg-26. Magnesium in hibonite is isotopically light (F-Mg = -5.3 parts per thousand/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.