Iron isotopic evolution during fractional crystallization of the uppermost Bushveld Complex layered mafic intrusion

Laura D. Bilenker, Jill A. VanTongeren, Craig C. Lundstrom, Adam C. Simon

Research output: Contribution to journalArticle

7 Scopus citations

Abstract

We present δ56Fe (56Fe/54Fe relative to standard IRMM-014) data from whole rock and magnetite of the Upper and Upper Main Zones (UUMZ) of the Bushveld Complex. With it, we assess the role of fractional crystallization in controlling the Fe isotopic evolution of a mafic magma. The UUMZ evolved by fractional crystallization of a dry tholeiitic magma to produce gabbros and diorites with cumulus magnetite and fayalitic olivine. Despite previous experimental work indicating a potential for magnetite crystallization to drastically change magma δ56Fe, we observe no change in whole rock δ56Fe above and below magnetite saturation. We also observe no systematic change in whole rock δ56Fe with increasing stratigraphic height, and only a small variation in δ56Fe in magnetite separates above magnetite saturation. Whole rock δ56Fe (errors twice standard deviation, ±2σ) throughout the UUMZ ranges from −0.01 ±0.03‰ to 0.21 ±0.09‰ (δ56FeaverageWR = 0.10 ±0.09‰; n = 21, isotopically light outlier: δ56FeWR = −0.15‰), and magnetites range from 0.28 ±0.04‰ to 0.86 ±0.07‰ (δ56FeaverageMgt = 0.50 ±0.15‰; n = 20), similar to values previously reported for other layered intrusions. We compare our measured δ56FeWR to a model that incorporates the changing normative mineralogy, calculated temperatures, and published fractionation factors of Fe-bearing phases throughout the UUMZ and produces δ56FeWR values that evolve only in response to fractional crystallization. Our results show that the Fe isotopic composition of a multiply saturated (multiple phases on the liquidus) magma is unlikely to change significantly during fractional crystallization of magnetite due to the competing fractionation of other Fe-bearing cumulus phases.

Original languageEnglish (US)
Pages (from-to)956-972
Number of pages17
JournalGeochemistry, Geophysics, Geosystems
Volume18
Issue number3
DOIs
Publication statusPublished - Mar 1 2017

    Fingerprint

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology

Keywords

  • Bushveld Complex
  • fractional crystallization
  • iron isotope
  • layered intrusion

Cite this