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

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‰ (δ⁵⁶Fe_averageWR = 0.10 ±0.09‰; n = 21, isotopically light outlier: δ⁵⁶Fe_WR = −0.15‰), and magnetites range from 0.28 ±0.04‰ to 0.86 ±0.07‰ (δ⁵⁶Fe_averageMgt = 0.50 ±0.15‰; n = 20), similar to values previously reported for other layered intrusions. We compare our measured δ⁵⁶Fe_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 δ⁵⁶Fe_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.