Another Look at Actinide Chemistry in Ca-Al-Rich Inclusions

Th/U values for Allende inclusions are often high relative to the accepted solar system average (3.8). In order to understand this fractionation, we are using fission track radiography to measure Th and U distributions in a group of Allende inclusions. We previously reported (Murrell and Burnett, 1984) on TS-23, a type Bl inclusion. Th and U were found to be enriched greater than 100 x's Cl with Th/U = 3. 8 ± 0.l in the spinel + perovskite portion of the rim on this inclusion. The Ti-Al-rich clinopyroxene band in this rim is also enriched in Th and U but with Th/U = 1. In the interior of TS-23, fassaite and melilite were found to contain 40 ppb and 90 ppb U, respectively, with little Th-U fractionation. Also, enrichments of Th and U to about 80 x's Cl were observed along grain boundaries which contained alteration products.