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

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 (<1.5 wt%) and Cr₂O₃ (<0.5 wt%), irrespective of stratigraphic position or the presence of chromite. Reduction of Al₂O₃ in high-pressure orthopyroxene during decompression can occur by solid-state plagioclase exsolution, a process documented in massif anorthosites, but no such exsolution textures are observed in Bushveld orthopyroxene. Post-emplacement re-equilibration by interaction with co-existing melt is likewise ineffective: diffusion kinetics and thermal constraints show that melt–crystal exchange cannot reset bulk orthopyroxene compositions and would be restricted to thin crystal rims. Extensive dissolution–reprecipitation is also unsupported by preserved zoning patterns and the absence of diagnostic reaction textures. Mass-balance considerations further demonstrate that even complete equilibration between high-pressure orthopyroxene and basaltic melt would yield Cr contents substantially higher than those observed. We therefore conclude that orthopyroxene—and by inference, all other co-crystallizing minerals—formed at low pressure within the Bushveld magma chamber itself. These results impose a fundamental mineral-chemical constraint on petrogenetic interpretations and effectively rule out models invoking deep-sourced crystal-rich slurries. They further imply that the chromite, platinum, and magnetite reefs formed from crystal-free melts within a shallow, melt-dominated Bushveld magma chamber.