Fe Isotope Systematics of the Upper and Upper Main Zones of the Bushveld Complex, South Africa

Abstract:

The Rustenberg Layered Suite of the Bushveld Complex, South Africa is the world’s largest exposed mafic magma chamber. The uppermost ~2.5 km of the 8-9 km thick intrusion are represented by the Upper and Upper Main Zones (UUMZ) and are thought to denote the final pulse of magma into the chamber. The basaltic-basaltic andesite parent magma to the UUMZ produced orthopyroxenites and gabbronorites at the base of the UUMZ. ~800 m above the base, the rocks contain cumulus magnetite (5-10 modal%), and magnetite layers occur throughout the remaining stratigraphy. Fa-olivine joins the cumulus mineral assemblage ~1300 m above the base, and cumulus apatite joins ~1800 m above the base. Mineral compositions evolve systematically from the base to the roof, making the UUMZ the best place to investigate extreme magmatic fractionation.

We report Fe isotope data (δ⁵⁶Fe) throughout the UUMZ in the eastern Bushveld in order to investigate differentiation processes and geochemical evolution during crystallization. Preliminary measurements of magnetite (δ⁵⁶Fe_mt = 0.16 to 0.47‰) show two important features: (1) In the lower part of the UUMZ (0 – ~1800 m above the base), δ⁵⁶Fe_mt decreases with increasing stratigraphic height, such that samples prior to cumulus magnetite saturation show high δ⁵⁶Fe, and samples above magnetite saturation show lower δ⁵⁶Fe. (2) In the top 600 m of the UUMZ, however, we observe an increase in δ⁵⁶Fe_mt that is inconsistent with the fractional crystallization trend below, suggesting this section formed by silicate liquid immiscibility, consistent with the previous conclusions for this interval on the basis of apatite REEs. Our ongoing work on the δ⁵⁶Fe of whole rock and mineral separates throughout the UUMZ stratigraphy will provide an important benchmark for Fe isotope fractionation during magmatic differentiation.