Lead and strontium isotope relationships in the Oka carbonatite complex, Quebec

Abstract

Abstract Lead isotopic compositions are presented for 8 calcite, 1 pyrite and 1 melilite separates from the Oka, Quebec, carbonatite complex. Pyrite, melilite and two calcite samples with small in situ decay corrections yield similar initial isotope ratios, averaging 206 Pb 204 Pb = 19.65 , 207 Pb 204 Pb = 15.56 , 208 Pb 204 Pb = 39.00 . This composition shows a close affinity for Pb isotope ratios from ocean island basalts (OIB) and indicates an origin of the Pb in large-ion lithophile (LIL)-element depleted mantle sources. The Pb probably results from a mixture from a LIL-element depleted source and possibly, metasomatic fluids. Four calcites fail to yield accurate initial ratio information due to large in situ decay corrections. In addition three samples exhibit high initial 207 Pb 204 Pb ratios indicative of probable crustal contamination. The silicate fraction of one okaite and a sample of country rock gneiss near Oka contain Pb with isotopic compositions that illustrate the kinds of contamination that could be involved. Sr isotope data are reported for 2 whole-rock sovites, plus 1 nepheline, 4 calcite and 2 apatite separates. All samples yield nearly identical isotopic compositions, with a mean 87 Sr 86 Sr ratio of 0.70331 ± 0.00002 at the 2 sigma level. This result verifies an earlier value reported by Bell et al. (1982) and supports their suggestion of a LIL-element depleted mantle source for the Sr. The Sr data do not show the apparent crustal contamination detected by some of the Pb data, presumably because of the high Sr contents of the carbonate samples. The Oka data are compared with results of a similar study by Lancelot and Allegre (1974) on carbonatite complexes from eastern Uganda. We conclude that the African data, as presently known, are compatible with the model developed for the mantle-derived Oka suite.