Uranium distribution in separated clinopyroxenes from four eclogites

Uranium concentrations and distributions in separated clinopyroxenes of eclogites from four different sites have been studied by the fission track method. Types of uranium occurrences included (i) inhomogeneously distributed clusters, (ii) line concentrations at cracks, and (iii) uniform distributions in the body of the mineral. Different grains from the same sample generally vary by factors of 2 to 4 in uniformly distributed uranium concentration. Samples from different locations vary in their average uniformly distributed uranium concentration by a factor of approximately 1000 (that is, from 0.01 to 20 parts per billion of uranium). The degree of homogeneity also varies markedly from one site to the next. The clinopyroxenes from the only oceanic locality studied (Salt Lake Crater) contained both the largest concentration of uranium (15 to 20 parts per billion) and the most uniform uranium distribution. This study suggests that, if total uranium concentrations in eclogitic clinopyroxenes are less than a few parts per billion, much if not most of the uranium may occur as contaminants along cracks or in microinclusions of unknown origin.