Uranium and thorium abundances in stony meteorites: 1. The chondritic meteorites

Simultaneous determinations of U and Th abundances are obtained in the Orgueil (type I) carbonaceous chondrite, the Mokoia and Karoonda olivine-pigeonite chondrites, and the Hvittis enstatite chondrite by neutron activation methods. The olivinepigeonite chondrite abundances are similar to ordinary (i.e. hypersthene and bronzite) chondrite values reported by other workers, but the Hvittis enstatite chondrite is depleted in both U and Th. The Orgueil carbonaceous chondrite is enriched in U relative to the ordinary chondrites but agrees with new calculations by Hoyle and Fowler of U abundances in primitive solar material. The Th abundance in Orgueil is also enriched but is not as high as the calculated values. Th/U ratios increase and U abundances decrease from type I carbonaceous chondrites through olivine-pigeonite and ordinary chondrites to the Hvittis enstatite chondrite. If the chondrites are related in a similar sequence by way of increasing temperatures and states of reduction, U is apparently lost more readily than Th as a result of these processes. The bearing of these new data on the radioactive heat production in earth models based on the various chondrite types is also discussed.