Origin of Thorium/Uranium Variations in Carbonaceous Chondrites

Thorium-, U-, and Pb-isotopic analyses of a wide variety of planetary materials show that Th/U ratio (by weight) varies from 3.5 to 4.2. It is generally believed that chondritic meteorites contain refractory lithophile elements in a relative proportions close to solar, i.e., CI chondrites [1]. Surprisingly, a number of analyses of different types of carbonaceous chondrites show a large (at least a factor of 3) scatter in Th/U measurements [2]. The widest spread in Th/U is observed in the most primitive materials, CI-type chondrites. Cosmochronological models rely on the precise knowledge of the average solar system Th/U, therefore it is important to achieve a better understanding of the actinide chemistry in chondrites, e.g., what causes the variations in Th/U ratio