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Mineralogy and petrography of HAL, an isotopically-unusual Allende inclusion

Allen, John M. and Grossman, Lawrence and Lee, Typhoon and Wasserburg, G. J. (1980) Mineralogy and petrography of HAL, an isotopically-unusual Allende inclusion. Geochimica et Cosmochimica Acta, 44 (5). pp. 685-699. ISSN 0016-7037. doi:10.1016/0016-7037(80)90158-1.

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A detailed mineralogical study of HAL was initiated to elucidate the origin of this inclusion because LEE et al. (1979) had found large mass fractionation effects and small nuclear effects in its Ca isotopic composition, but no ^(26)Mg excesses in samples of it with very high ^(27)Al/^(24)Mg ratios. HAL's 1.0 mm radius interior consists almost entirely of three hibonite crystals and is surrounded by a 2.0 mm thick, multi-layered rim. The first layer, called the black rim, is black and compact, resembles a devitrified glass and contains an anisotropic Al-Fe-oxide similar to hercynite in composition. This is followed by a friable rim sequence, layer I of which is predominantly feldspathoids with minor anorthite, Ti-Fe-oxide and Al-Fe-oxide. Layer II contains abundant perovskite, plus grossular, andradite and pyroxene in addition to the minerals of layer I. Layer III is mostly Ca-phosphate, possibly hydroxy-apatite and perovskite. Layer IV is rich in hibonite, Al-Fe-oxide, perovskite, nepheline and the two garnets, lacks Ca-phosphate but contains traces of a Ti-Sc-Zr-oxide. Layer V is rich in Al-Fe-oxide, pyroxene, nepheline, the two garnets and olivine whose crystals display peculiar rectangular cross-sections. The black rim does not completely surround the hibonite core. Sectors of the friable rim exist where layer I is missing and where the mineralogy of adjacent layers is no different from that of the same layers in other sectors. Pentlandite, nickel-iron and barrel-shaped olivine crystals, minerals typical of the matrix of Allende and found nowhere else in HAL, are found in layer V and increase in abundance toward its exterior, as if grains of these phases accreted together with the other minerals of layer V. This layer also contains alternating olivine-rich and garnet-, pyroxene-rich bands, resembling rhythmic layering. For these reasons, we conclude that each of the layers of the friable rim formed by the accretion of an assemblage of condensate grains rather than by complete reaction of a precursor to HAL with a nebular gas. Thus, the unusual isotopic characteristics of HAL are thought to have been inherited from a nebular reservoir which was isotopically distinct from that which gave rise to the bulk of the material in Allende. HAL's mineralogical peculiarities indicate that its formation reservoir was also chemically distinct from the latter one.

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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1980 Pergamon Press Ltd. Received 17 July 1979; accepted in revised form 9 January 1980. Helpful discussions with R. C. Aller, R. N. Clayton, A. M. Davis, I. D. Hutcheon, G. MacPherson, R. C. Newton and T. Tanaka are gratefully acknowledged. We thank J. BROWN for preparing polished thin sections of superior quality. Mr Eric Lin of the University of Toronto is thanked for Figs 8 and 13. The original manuscript was improved significantly through thoughtful reviews by A. El Gorsey, J. W. Larimer and G. J. Taylor. This work was supported by funds from the National Aeronautics and Space Administration through grants NGR 14-001-249 (L.G.) and NGL 05-002-188 (G.J.W.), the Alfred P. Sloan Research Foundation (L.G.), the National Science Foundation through grant PHY76-83685 (G.J.W.), the Natural Sciences and Engineering Research Council of Canada (J.M.A.), and the Enrico Fermi Institute and Robert R. McCormick Trust (T.L.).
Funding AgencyGrant Number
NASANGR 14-001-249
NASANGL 05-002-188
Alfred P. Sloan FoundationUNSPECIFIED
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Enrico Fermi InstituteUNSPECIFIED
Robert R. McCormick TrustUNSPECIFIED
Issue or Number:5
Record Number:CaltechAUTHORS:20150116-131325523
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Official Citation:John M. Allen, Lawrence Grossman, Typhoon Lee, G.J. Wasserburg, Mineralogy and petrography of HAL, an isotopically-unusual Allende inclusion, Geochimica et Cosmochimica Acta, Volume 44, Issue 5, May 1980, Pages 685-699, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:53834
Deposited By: Tony Diaz
Deposited On:17 Jan 2015 03:09
Last Modified:10 Nov 2021 20:07

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