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.