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Actinide chemistry in Allende Ca-Al-rich inclusions

Murrell, M. T. and Burnett, D. S. (1987) Actinide chemistry in Allende Ca-Al-rich inclusions. Geochimica et Cosmochimica Acta, 51 (4). pp. 985-999. ISSN 0016-7037. doi:10.1016/0016-7037(87)90110-4.

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The microscale distribution of U and Th has been studied by fission track radiography in a set of Ca-Al-rich inclusions from the Allende meteorite. In the Type B inclusions, generally believed to have been melted, the major phases melilite and fassaite are important actinide host phases with similar Th/U and roughly three times higher concentrations in melilite than fassaite. Significant enrichments are also found on grain boundaries and in some regions of alteration. The distribution is not completely understood in terms of igneous partitioning and may also reflect incomplete assimilation of relict materials during melting. On the rims of Type B inclusions and throughout all other inclusions studied, perovskite is the dominant actinide host phase. However, perovskite does not have a characteristic Th/U value; measured ratios range from 1.4 to 12, both above and below the accepted solar system value of 3.8. Overall, neither alteration nor loss or gain of an actinide-rich phase appears to have been an important Th/U fractionation mechanism; volatility differences may be the dominant factor. Some results, e.g., high Th/U for “fluffy” Type A inclusions, can be interpreted as resulting from incomplete incorporation of the more volatile U during a primary condensation stage, but more complex interpretations are required for inclusions with low Th/U. The Th/U and REE abundance patterns for the spinel plus perovskite rim suggest derivation of this rim by volatilization of interior material. The time scale for this heating is required to be brief (<10^4 sec), but within this constraint spinel-perovskite rim formation by several mechanisms (deceleration heating, passage through nebular hot spots, differential sublimation or vaporization plus recondensation) is possible.

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Burnett, D. S.0000-0001-9521-8675
Additional Information:© 1987 Pergamon Journals Ltd. Received 17 June 1986, Revised 19 January 1987, Available online 14 April 2003. We wish to thank both L. Grossman for generously providing us with a sample of TS23F1 and G. MacPherson for 3529-41. We are grateful for the excellent cooperation of the LBL 88-in. cyclotron staff, in particular Ruth Mary Larimer. We thank A. Hashimoto for discussions and J. Beckett and D. Wark for detailed critical comments on this manuscript. This research was supported by NASA grant NAG 9 94. Editorial handling: H. Y. McSween. Jr. Caltech Division of Geological and Planetary Sciences Contribution Number 4322
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Caltech Division of Geological and Planetary Sciences4322
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Official Citation:M.T Murrell, D.S Burnett, Actinide chemistry in Allende Ca-Al-rich inclusions, Geochimica et Cosmochimica Acta, Volume 51, Issue 4, April 1987, Pages 985-999, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:56967
Deposited By: Ruth Sustaita
Deposited On:24 Apr 2015 22:15
Last Modified:10 Nov 2021 21:06

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