The Uranium Content of Garnet Lherzolite Xenoliths from Kimberlites

D. A. Carswell and C. M. Rice
Department of Geology, University of Sheffield, Mappin Street, Sheffield, S1 3JD
Department of Geology and Mineralogy, University of Aberdeen, Marischal College, Aberdeen, AB9 1AS

Summary: Uranium contents ranging from 0.03–1.18 ppm have been determined by delayed neutron activation analysis in a suite of 19 ultrabasic xenoliths (mostly of the common garnet lherzolite type) from the kimberlites of southern Africa. Consideration of these and previous data lead us to conclude that the U content of uncontaminated garnet lherzolite in the uppermost mantle is on average roughly 0.04–0.05 ppm, levels which are sufficient to account for the observed U contents of fresh basalts. Higher U contents in mantle-derived ultrabasic xenoliths are the result of secondary alteration processes, most probably related to infiltration metasomatism stemming from contact with kimberlitic or carbonatitic magma fractions. This introduced U is concentrated along the boundaries of the primary mineral grains and is mostly readily removed by acid leaching. A partial analysis is given for a secondary uraniferous calcium phosphate phase observed in kelyphitic rims around garnets.

Mineralogical Magazine; June 1980 v. 43; no. 330; p. 689-693; DOI: 10.1180/minmag.1980.043.330.01
© 1980, The Mineralogical Society
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