Iron isotopic evolution during fractional crystallization of the uppermost B ushveld C omplex layered mafic intrusion

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2017-03, Vol.18 (3), p.956-972
• Main Authors: Bilenker, Laura D., VanTongeren, Jill A., Lundstrom, Craig C., Simon, Adam C.
• Format: Article
• Language: English
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1002/2016GC006660

We present δ 56 Fe ( 56 Fe/ 54 Fe 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 δ 56 Fe, we observe no change in whole rock δ 56 Fe above and below magnetite saturation. We also observe no systematic change in whole rock δ 56 Fe with increasing stratigraphic height, and only a small variation in δ 56 Fe in magnetite separates above magnetite saturation. Whole rock δ 56 Fe (errors twice standard deviation, ±2σ) throughout the UUMZ ranges from −0.01 ±0.03‰ to 0.21 ±0.09‰ (δ 56 Fe averageWR  = 0.10 ±0.09‰; n = 21, isotopically light outlier: δ 56 Fe WR  = −0.15‰), and magnetites range from 0.28 ±0.04‰ to 0.86 ±0.07‰ (δ 56 Fe averageMgt  = 0.50 ±0.15‰; n = 20), similar to values previously reported for other layered intrusions. We compare our measured δ 56 Fe WR to a model that incorporates the changing normative mineralogy, calculated temperatures, and published fractionation factors of Fe‐bearing phases throughout the UUMZ and produces δ 56 Fe WR 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. Whole rock and magnetite separates from the uppermost portion of the Bushveld Complex were analyzed for their Fe isotope compositions We find no systematic variation in whole rock or magnetite Fe isotope ratios with stratigraphic height 85% crystallization of a dry tholeiitic multiply‐saturated magma does not significantly fractionate Fe isotopes