Anatomy of an Allende coarse-grained inclusion

Abstract

Inclusion 5241 is a petrographically complex, coarse-grained CAI from the Allende meteorite, containing a core with two petrographically and chemically distinct units. The core is surrounded by a melilite mantle similar to that described for Allende Type B1 inclusions, which consists of large, millimetersized crystals. The two different units of the core are (1) spinel-free islands consisting of melilite, fassaite, and minor anorthite (2) spinel-rich areas with major fassaite and reversely zoned melilite. In this unit, spinel occurs in fassaite, melilite, and in densely packed belts surrounding the spinel-free islands. Mineral chemistries of melilite and fassaite in the spinel-free islands are different from those in the spinel-rich areas: melilite in the spinel-free islands is unzoned and displays a very narrow chemical variation (<10 mol %) within individual crystals, ranging between Åk_(52) and Åk_(68) in all islands. They also contain considerable amounts of Na-melilite molecule (as much as 2.7 mol %). In contrast, melilite in the spinel-rich areas is reversely zoned and barren of Na-melilite: the composition varies between Åk_(43) and Åk_(62), with the lowest Åk-content within a zone decorated with fassaite and spinel inclusions. Zoning in individual melilite crystals in the mantle is not continuous (as in Allende Type B1 inclusions), but is rhythmic outwards, from Åk_(30) to Åk_(35) near the core/mantle boundary followed by a continuous decrease with the lowest value near the center of the melilite crystals (Åk_(25)). The Åk-content rises again and reaches a value of Åk_(28) at the rim of the inclusion. Fassaite in the spinel-free islands is typically concentrically zoned, with Ti-Al-rich cores and Ti-Al-poor rims. The concentration of Ti^(3+) in some fassaite crystals in the islands appears to be lower than in the spinelrich areas. Zoning of fassaite in the spinel-rich areas is nonsystematic and the Tio_2 variations are considerable (2–12wt%). Bulk compositions of the spinel-free islands plot inside the Type B CAI field but considerably below the Fo-An-Ge plane because they are spinel deficient. In contrast, compositions of the spinel-rich areas plot outside the Type B CAI field close to pyroxene-saturated compositions (STOLPER, 1982). Textural relations and mineral chemistries in the spinel-free islands, spinel-rich areas, and melilite mantle cannot be explained by gas solid condensation, simple stage crystallization from a refractory silicate liquid, or evaporative loss by distillation, but suggest a series of complex events including the capture of solid spinel-free bodies by a refractory liquid; the melilite mantle may have formed by crystallization from a liquid which enveloped the complex core.

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