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Anatomy of an Allende coarse-grained inclusion

El Goresy, A. and Armstrong, J. T. and Wasserburg, G. J. (1985) Anatomy of an Allende coarse-grained inclusion. Geochimica et Cosmochimica Acta, 49 (11). pp. 2433-2444. ISSN 0016-7037. doi:10.1016/0016-7037(85)90243-1.

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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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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1985 Pergamon Press Ltd. Received March 25, 1985; accepted in revised form August 7, 1985. We acknowledge Brian Mason for loan of PTS 5241. We are grateful to the Fairchild Distinguished Visiting Scholar’s Program at CALTECH for support of the senior author during this study. The help of Sharon Miller (CALTECH; now JFL) and H. Urmitzer (MPI) in editing and typing the manuscript is gratefully appreciated. The manuscript benefitted from thorough reviews by Drs. G. MacPherson and A. Kornacki.
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Caltech Fairchild Distinguished Visiting Scholar’s ProgramUNSPECIFIED
Issue or Number:11
Record Number:CaltechAUTHORS:20131021-131558858
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Official Citation:A. El Goresy, J.T. Armstrong, G.J. Wasserburg, Anatomy of an Allende coarse-grained inclusion, Geochimica et Cosmochimica Acta, Volume 49, Issue 11, November 1985, Pages 2433-2444, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:41994
Deposited By: Tony Diaz
Deposited On:22 Oct 2013 16:09
Last Modified:10 Nov 2021 04:36

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