Uranium and thorium microdistributions in stony meteorites

Uranium distributions have been determined in seventeen meteorites using fission track techniques. In seven cases, Th was also determined by a new method using fast neutrons. The actinides are generally concentrated in phosphates, usually whitlockite and/or chlorapatite. Whenever whitlockite and chlorapatite coexist, chlorapatite is richer in uranium. U concentrations in a given phosphate phase are highly variable from meteorite to meteorite and sometimes also show large variations in the same meteorite. A clinopyroxene phase enriched in U (approximately 0.2 to 0.3 ppm) is usually found in Ca-rich achondrites. The Th/U ratios of phosphates differ considerably from whole rock values indicating that these elements were fractionated during the meteorite formation. (author)