Lead isotope variations within the Bushveld complex, Southern Africa: a reconnaissance study

• Published in: Journal of African earth sciences (1994) 1995-11, Vol.21 (4), p.595-606
• Main Authors: Harmer, R.E., Auret, J.M., Eglington, B.M.
• Format: Article
• Language: English
• ISSN: 1464-343X; 1879-1956
• DOI: 10.1016/0899-5362(95)00109-3

Variations in initial 87 Sr 86 Sr through the mafic cumulates of the Bushveld complex have long been used to trace the nature and relative influence of different parental liquids in the development of the magma chamber. The authors report the results of the first systematic investigation of the lead isotopic variation in the complex and provide information on both silicate and sulphide phases through the cumulate succession. Marginal and cumulate lithologies indicate that the initial lead isotopic composition (represented by model source 238 U 204 Pb ratios) varies systematically in a manner analogous to that detected with strontium isotopic ratios. Low μ2 ratios are evident in the Lower Zone and increase up the cumulate pile to the Main Zone, which apparently had homogeneous initial lead isotopic ratios. Unlike 87 Sr 86 Sr , however, considerable μ2 variation is detected in the Upper Zone. Cumulate lithologies also exhibit a greater range of μ2 values than those noted in the Marginal suite representatives, comparable to the situation for strontium isotopes. Most of the model μ2 results are higher than values expected for mantle-derived magmas, suggesting the incorporation of material with a crustal pre-history. Analyses of a detailed sampling of the Merensky and Bastard cyclic units at the Atok mine, eastern Bushveld are also presented and those reveal extreme values of μ2 while the variation in μ2 do not mirror the 87 Sr 86 Sr variations reported for the same samples by Lee and Butcher (1990). Elevated μ2 in some cumulate samples are impossibly high for a magmatic origin and presumably reflect the introduction of extra-magmatic components during sub-solidus hydrothermal alteration. Les variations du rapport initial 87 Sr 86 Sr dans les cumulats mafiques du complexe du Bushveld ont depuis longtemps été utilisés pour tracer la nature et l'influence relative des différents liquides parentaux dans le développement de la chambre magmatique. Nous apportons ici la première étude systématique des variations isotopiques du plomb dans ce complexe ainsi que des informations sur les phases silicatées et sulfurées de la succession cumulative. Les lithologies cumulatives et de bordure indiquent que les compositions isotopiques initiales du plomb (représentées par les rapports 238 U 204 Pb modèles de la source) varient systématiquement d'une manière analogue aux rapports isotopiques du strontium. Les rapports μ2 sont bas dans la Zone Inférieure et augmentent vers la Zo