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
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  • 作者：N. I. Chutas (1)
    E. Bates (1)
    S. A. Prevec (2)
    D. S. Coleman (3)
    A. E. Boudreau (1) boudreau@duke.edu
  • 关键词：Bushveld Complex &#8211 ; Sr and Pb isotopes &#8211 ; Isotopic disequilibrium
  • 刊名：Contributions to Mineralogy and Petrology
  • 出版年：2012
  • 出版时间：April 2012
  • 年：2012
  • 卷：163
  • 期：4
  • 页码：653-668
  • 全文大小：1.4 MB
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  • 作者单位：1. Division of Earth and Ocean Sciences, Duke University, Box 90227, Durham, NC 27708-0227, USA2. Department of Geology, Rhodes University, P.O. Box 94, Grahamstown, 6140 South Africa3. Department of Geological Sciences, University of North Carolina, 325 Mitchell Hall, CB#3315, Chapel Hill, NC 27599-3315, USA
  • 刊物类别：Earth and Environmental Science
  • 刊物主题：Earth sciences
    Geology
    Mineral Resources
    Mineralogy
  • 出版者：Springer Berlin / Heidelberg
  • ISSN：1432-0967
文摘
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 Sri rim relative to the core, whereas plagioclase from an Upper Zone sample is isotopically homogeneous for Sri. Microdrilling analyses of plagioclase from the Lower and Critical Zones consistently have a higher initial 87Sr/86Sr (Sri) and a less radiogenic modeled 238U/204Pb composition (μ2) than coexisting orthopyroxene. The range of calculated Sri for plagioclase and orthopyroxene is 0.70506–0.70662(34) and 0.70290–0.70654(36), respectively. The average difference in Sri 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 Sri = 0.70654 (36) with calculated μ2 of 10.32 for larger grains as compared with Sri = 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.

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