STRATIGRAPHIC AND INTER-MINERAL PB ISOTOPIC HETEROGENEITY IN THE BUSHVELD COMPLEX, AND THE EVOLUTION OF PARTIALLY MOLTEN ROCKS

The characteristics of cumulate rocks are conventionally taken to reflect processes in the overlying magma chamber. Variations in composition through certain stratigraphic sequences indicate, however, modifications of the rocks by postcumulus processes. One such sequence is the top of the Critical zone of the Bushveld Complex. Here the Merensky and overlying Bastard units each consist of a basal pyroxenite overlain by norite and anorthosite. The present study is of a continuous, 55 m long drill core from the Atok mine that penetrated both units.

Pb isotopic compositions of plagioclase and sulfide were determined in individual thin sections by laser ablation MC-ICP-MS. Repeated analysis of NIST standard glass 614, which nominally contains 2.32 ppm Pb, yielded 1s reproducibilities of 0.04% for 207Pb/206Pb and 0.03% for 208Pb/206Pb. The Pb content of Bushveld plagioclase ranges from about 2 ppm in the pyroxenites to 0.1 ppm in the leucocratic rocks. At the lower concentrations, the 1s errors on 207Pb/206Pb and 208Pb/206Pb ratios increase to an estimated 1% due to decreasing signal sizes (ca. 1mV on 207).

The Pb isotopic composition of plagioclase exhibits significant and irregular stratigraphic variations. For example, plagioclase 207Pb/206Pb=0.9795±0.0037 (1 s absolute error) in norite beneath the Merensky pyroxenite, 0.9936±0.0008 in the pyroxenite 1.5 m above, 1.0099±0.0004 in mineralized pyroxenite 0.5 m above that, and 0.9939±0.0038 in overlying norite. Plagioclase and sulfide in the same thin section are commonly not in isotopic equilibrium, and the sulfides themselves may exhibit enormous heterogeneity. Thus, in a sample of Bastard pyroxenite, plagioclase 207Pb/206 is 1.0095, but this ratio ranges from 0.90830 to 0.99737 in 7 co-existing sulfide grains. No evidence for isobaric interferences has been found but is the subject of ongoing investigation.

The data imply reaction between interstitial melt and crystalline assemblages with different Pb isotopic compositions. Some Pb must have been introduced by fluid from the country rocks at temperatures at which the composition of the sulfide but not the plagioclase could be modified. Published H isotope data indicate that this temperature must have exceeded 8-900°C. Such a process may account for the reported heterogeneity of Os isotopes.