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Published April 1987 | public
Journal Article

Actinide chemistry in Allende Ca-Al-rich inclusions

Abstract

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.

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

Additional details

Created:
August 22, 2023
Modified:
October 23, 2023