UPb isotopic geochemistry of komatiites and pyroxenes from the southern Abitibi greenstone belt, Canada

Abstract

New U and Pb concentrations and Pb isotopic data are reported for several komatiitic flows and for primary magmatic clinopyroxenes (CPX) from flows and sills of basaltic compositions from the southern Abitibi greenstone belt, for which RbSr, SmNd and ReOs isotope data were previously published. A suite of komatiites from Pyke Hill displays a narrow range of 206Ph/ 204Pb (17.91–19.20) when compared to previously reported values of ∼ 16–30 for komatiites in the same area. Whole-rock samples leached with HCl underwent up to 40% weight loss. The leachates display essentially the same isotopic composition as the bulk samples but the residues have higher 206Pb/ 204Pb (19.9–24.0). The CPX display 206Pb/ 204Pb (13.96–17.98) lower than whole rocks and leaching resulted in either higher or lower Pb ratios for the residues compared to bulk samples. For both whole rocks and mineral separates, leaching induced a significant decrease of Pb concentrations in the residues, which contained < 0.04 ppm compared to 1–2 ppm for the bulk samples. The U concentrations did not vary systematically with the degree of leaching but the residues always had higher 238U/ 204Pb than the bulk samples. Whole rocks and CPX exhibit U loss relative to Pb. All the samples define a ∼ 2.73-Ga PbPb isochron, identical to that previously reported for komatiites in the same area. However, our isochron is displaced toward slightly more radiogenic 207Pb/ 204Pb compared to the previously reported isochrons and this is interpreted as reflecting mantle source heterogeneities for komatiites. These mantle sources are depleted and the most radiogenic one is similar in composition to that of carbonatites and other alkaline complexes in the Superior Province. We also show that the Pb concentration in komatiites from Pyke Hill displays a coherent variation with respect to MgO, FeO, Re and Os, and that Pb behaves like Os in all the samples, suggesting a common bearing phase controlling the distribution of these two elements. The “hydrothermal Pb removing” inferred by B. Dupré and N.T. Arndt to account for large variations of the 206Pb/ 204Pb ratios of some komatiite flows is not universal. If our samples were affected by any hydrothermal remobilisation, it did not mask the primary magmatic distribution of Pb.