Evidence for Rapid Growth of Melilite in an Unusual Compact Type a Refractory Inclusion from Allende

Compact Type A (CTA) inclusions are extremely melilite-rich (typically ~90 vol%), with moderate amounts of spinel and minor fassaite and perovskite. Because CTAs are one of the major types of coarsegrained inclusions found in CV3 chondrites but have not been thoroughly studied as a group, we have undertaken a comprehensive petrographic, mineralchemical, and trace element study of a large suite of CTAs from the CV3 chondrites Allende, Axtell, Efremovka, and Leoville. A preliminary report of our results for some representative members of the suite was given by [1]. Here we report the results of a detailed study of a unique member of the suite, Allende inclusion TS2. Petrography and mineral chemistry. TS2 is round in cross-section and 5.2 mm in diameter. It is dominated by very large (0.1–2 mm) melilite crystals and, as noted by [2], chains and clusters of coarse (50– 150 μm) spinel grains, with ~1 vol% each of perovskite and fassaite. Digital X-ray mapping of the entire inclusion shows that melilite in the outer ~500 μm of the inclusion tends to be more aluminous (Åk15–50) than that in the interior (Åk30–70). A distinctive, major feature of this inclusion is that many melilite crystals have no dominant core-rim zoning but instead consist of 50– 200 μm patches of aluminous melilite (Åk 25–53, median Åk38, with a normal distribution of compositions) (Fig. 1) set in, and optically continuous with, relatively Mgrich melilite (Åk32–62, median Åk51, with a distribution that is skewed towards aluminous melilite) (Fig. 2). Within many individual crystals, especially in the interior of the inclusion, the aluminous patches enclose numerous 0.5–5 μm spinel grains while the relatively Åk-rich patches are either spinel-free or adjacent to coarse spinel. The spinel-bearing, Åk-poor melilite is never in contact with melilite grain boundaries. Toward the edge of the inclusion (the outer ~500 μm), Åk-poor patches are not associated with fine-grained spinel. The distributions of coarse and fine spinel are quite different. Coarse spinel occurs in clumps and chains, whereas the fine spinel grains are never clumped together. Both rastered beam X-ray microanalysis and digital analysis of backscattered electron images show that there is about 5 vol% spinel, 1 vol% fassaite, and 0.05 vol% perovskite in the spinel-bearing Åk-poor zones. Both the coarse and fine spinel have 0.2–0.4 wt% Cr2O3 and 0.4–0.7 wt% TiO2. The fine spinel has lower V2O3 contents than the coarse spinel (~0.3 vs. 0.3–0.6 wt%). The coarse spinels are unzoned. Both the bimodal size distribution of spinel and the patchy zoning of melilite are unique to this inclusion. Mapping also reveals that one of the curved chains of coarse spinel marks the boundary of a spinel-free palisade body, 1.7×0.85 mm, that consists of relatively Åk-rich (45–65 mol%) melilite and minor perovskite. Several melilite crystals adjacent to the spinel wall are oriented radially and may have grown inward from it. An electron probe traverse along the length of one such grain shows that, with increasing distance from the spinel wall, its Åk content drops from 49 to 44 mol% over the first 10 μm and then increases to 62 mol% over the remaining 250 μm of the crystal, with several narrow (10–20 μm) bands of reverse zoning. In this crystal, Na2O contents are positively correlated with Åk contents, a relationship expected for igneous melilite [3] but not apparent in the patchy melilite. The palisade body also contains two grains of unusual, Nb-rich (~4– 6 wt% Nb2O5) perovskite enclosed in “normal”, Nbpoor perovskite.