Lead isotopic disequilibrium between sulfide and plagioclase in the Bushveld Complex and the chemical evolution of large layered intrusions

摘要

The Pb isotopic compositions of coexisting plagioclase and sulfide from the Bushveld Complex were determined by laser ablation multi-collector ICPMS (LA MC-ICPMS). The samples are of the upper Critical Zone in the northeast corner of the Complex and were collected from drill core and underground mine exposures. All the rocks are fresh and exhibit no evidence for alteration, weathering, or disruption of the Ph isotope systematics subsequent to the initial cooling of the intrusion. Furthermore, individual plagioclase and sulfide crystals do not contain enough U to warrant correction for radiogenic in-growth. For these reasons, the measured Ph isotope ratios approximate the initial ones. For plagioclase, Pb-207/Pb-206 ranges from 0.98 to 1.02 and Pb-208/Pb-206 from 2.26 to 2.35. Low Pb-207/Pb-206 and Pb-208/Pb-206 ratios characterize grain boundaries and partially annealed microcracks, some of which contain minute fragments of sulfide and other phases, and this accounts for most, if not all, the heterogeneity exhibited by individual samples. Real compositional differences exist, however, in plagioclase from different lithologic layers. For example, plagioclase Pb-207/Pb-206 values vary from 1.004 in norite beneath the Merensky pyroxenite to 1.009 in the mineralized pyroxenite, and 0.997 in overlying norite. In most samples in which sulfide and plagioclase coexist, the sulfide Pb-207/Pb-206 ratio is lower and Pb-201/Pb-206 ratio higher than the corresponding ones in plagioclase. For example, in a mineralized Merensky reef sample, average sulfide Pb-207/Pb-206 and Pb-208/Pb-206 ratios are 0.993 and 2.313, respectively, while those in plagioclase are 1.000 and 2.292. In one sample, the sulfide is extremely heterogeneous, with Pb-207/Pb-206 and Pb-208/Pb-206 ratios as low as 0.84 and 2.12. In this particular sample, the compositions must represent an isolated occurrence of addition of a young Ph component. [References: 29]