Uranium Enrichment in the Lithospheric Mantle: Case Studies from French Massif Central

Abstract

Mantle xenoliths are often enriched in large ion lithophile elements (LILE) relative to heavy REE. This feature is classically attributed to enrichment by melts/fluids infiltrated into the lithosphere. Moreover several recent papers have reported peridotite from world-wide continental occurrences characterized by selective U and Pb __ Sr enrichment (e.g. Jochum et al., 1989; Ionov et al., 1995). However current interpretations of selective U enrichment in mantle xenoliths call upon post-eruption alteration involving Uand Pb-rich fluids. Our ICP-MS study of a large number of mantle xenoliths collected in various localities from French Massif Central confirms the widespread distribution of U-rich peridotites on the scale of this region (e.g. Alard et al., 1996). Virtually all the analysed samples are selectively enriched in uranium with U/ThpM ratios as high as 60 in several samples. It is worthnoting that Uand Pb-rich xenoliths include peridotites which are otherwise depleted in highly incompatible elements, with compositions typical of unmetasomatized 'Depleted MORB Mantle'. The only noticeable exceptions are xenoliths characterized by overall enrichment of LILE and negative anomaly of high field strengh elements, a signature which is commonly ascribed to mantle metasomatism by carbonate-rich melts; they display a normalised U/ Th ratio close to 1. Similar to the results obtained by Ionov et al., 1995, our data indicate that U/Th fractionation is roughly coupled with Pb/Ce and Sr/ Ce positive fractionation. These correlations show that the fractionation of U, Pb and Sr over Th and Ce, respectively, are linked together and are probably the result of the same process. In individual xenolith suites, U/Th and Pb/Ce ratio are well correlated with LREE/MREE. Separated and leached Cpx and olivine were analysed from several typical samples (Fig. 2). U/ ThpM for all the cpx fractions, ranges from 1.6 to 10.4. and Pb/CevM is generally lower than 2. These fractionations are significantly lower than those observed in the whole rock and the U, Pb and Sr contents of cpx are not high enough to account to the whole rock budget. However the cpx U/Th and Pb/Ce ratios display a positive correlation with the whole rock ratio. Nevertheless, all olivine fractions show U