Orthopyroxene chemistry excludes deep-sourced crystal slurries in the Bushveld Complex

Abstract

The Bushveld Complex is commonly interpreted as an open-system magma chamber that grew incrementally through several major and numerous minor replenishment events, with magma sourced from deep magma reservoirs. A central point of debate is whether these replenishing magmas were crystal-rich (i.e., crystal slurries) or effectively crystal-free (ranging from normal to superheated melts). We address this question using the chemistry of cumulus orthopyroxene throughout the Bushveld Complex. Recent seismic and gravity imaging has identified a deep staging chamber at depths of ~40–45 km, corresponding to pressures of ~1.0 GPa, thereby defining the pressure conditions under which any deep-sourced crystal slurries must have formed. Experimental crystallization of Bushveld-type magmas at ~1.0 GPa produces orthopyroxene enriched in Al₂O₃ (~3.5–7.0 wt%) and Cr₂O₃ (~1.7–2.2 wt%), providing a quantitative benchmark for high-pressure orthopyroxene. In contrast, cumulus orthopyroxene across the Bushveld Complex is systematically low in Al₂O₃ (