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 (μ 2) 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 μ 2 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 μ 2 of 10.32 for larger grains as compared with Sr i = 0. 70290 (32) and calculated μ 2 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. © 2011 Springer-Verlag.

Duke Scholars

Author Alan E. Boudreau Earth and Climate Sciences