Sr and Pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the Bushveld Complex, South Africa: microdrilling and progressive leaching evidence for sub-liquidus contamination within a crystal mush

Progressive leaching of plagioclase for Sr isotopes and microdrilling for Sr and Pb isotopes from grains of plagioclase and orthopyroxene from the Critical Zone and the Lower Zone indicates that these minerals are not in isotopic equilibrium. Leaching suggests Critical Zone plagioclase either lost Rb or had a more radiogenic Sr_i rim relative to the core, whereas plagioclase from an Upper Zone sample is isotopically homogeneous for Sr_i. Microdrilling analyses of plagioclase from the Lower and Critical Zones consistently have a higher initial ⁸⁷Sr/⁸⁶Sr (Sr_i) and a less radiogenic modeled ²³⁸U/²⁰⁴Pb composition (μ₂) than coexisting orthopyroxene. The range of calculated Sr_i for plagioclase and orthopyroxene is 0.70506–0.70662(34) and 0.70290–0.70654(36), respectively. The average difference in Sr_i between mineral pairs was 0.00095. The range of calculated μ₂ for plagioclase and orthopyroxene is 9.42–10.30 (average 9.7) and 9.83–15.75 (average 10.1), respectively. The range of measured ²⁰⁸Pb/²⁰⁶Pb for plagioclase and orthopyroxene is 34.757–36.439(33) and 36.669–41.845(85), respectively. One orthopyroxenite without evidence for more than one population of crystal size distribution, nonetheless had Sr_i = 0.70654 (36) with calculated μ₂ of 10.32 for larger grains as compared with Sr_i = 0.70290 (32) and calculated μ₂ of 9.97 for smaller grain-size fractions. Isotopic results from this study demonstrate that whole-rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the Bushveld Complex. However, systematic changes have the potential to elucidate the timing of contamination with regard to other processes (crystal aging, compaction-driven recrystallization, and mineral exsolution) occurring within a slowly cooled crystal–liquid–vapor mush system.