K-U-Th systematics of matter in the planetary bodies of the solar system

Geochemical data for the content of potassium (K), uranium (U), and thorium (Th) in rock samples from the earth, moon, Venus, and Mars are analyzed. Data for the carbonaceous and ordinary chondrites, the eucrites, and the shergottite meteorites were also examined. The ratio of Th to U in all the samples (with the exception of a few terrestrial sedimentary rocks) was about 3 to 1. Analysis of the correlation of K- and U-content permitted the identification of three different groups of samples. The group with the highest K-content included samples from the largest terrestrial bodies: the earth and Venus. The group with the lowest K-content contained samples from the smaller planetary bodies: the moon and the eucrite parent bodies. The third group consisted of samples from Mars and the SNC meteorites having moderate K-content. On the basis of the geochemical classification, it is concluded that: (1) the primary silicate material in samples from the planetary group was approximately the same as that of the carbonaceous and ordinary chondrites; and (2) the depletion of potassium observed in samples from the small planetary group was probably due to the loss of volatile potassium during the accretion process. The complete geochemical data are given in a table.