Geochemical Anatomy of the Main Magnetitite Layer, Bushveld Complex, South Africa.

The article focuses on the origin and compositional variations of the Main Magnetitite Layer (MML) within the Upper Zone of the Bushveld Complex, South Africa, a mafic layered intrusion known for its extensive magnetitite layers. Detailed vertical and lateral sampling reveal steep upward decreases in chromium (Cr) content in magnetite, punctuated by sharp reversals and lateral heterogeneities, with the MML sometimes splitting into multiple sub-layers separated by magnetite–plagioclase rocks. The study argues against models involving grain settling, magma injection, or liquid immiscibility for magnetitite formation, instead proposing that magnetite crystallized by heterogeneous nucleation triggered by seismic shock waves at the chamber floor, inducing convection and chemical inhomogeneities in Cr distribution. The authors conclude that convection-driven processes and episodic erosion shaped the MML's compositional profiles, with no evidence for magma addition based on consistent plagioclase compositions below and above the layer.