A Comment on 'Postcumulus Processes Recorded in Whole Rock Geochemistry: A Case Study from the Mirabela Layered Intrusion, Brazil' by S.J. Barnes and M. Williams.

This article critically examines the formation of ultramafic rocks in the Mirabela Layered Intrusion, challenging the closed-system crystallization model proposed by Barnes and Williams. It presents evidence that the rocks formed through concurrent crystallization and compaction (CC&C), where early crystallization of plagioclase and later apatite saturation, combined with compaction-driven liquid loss, explain observed stratigraphic geochemical trends in sodium, strontium, and phosphorus. The study highlights how local porosity variations and liquid segregation during compaction influence mineral distribution and incompatible element concentrations, suggesting that the mush initially contained a significant liquid fraction rather than forming a low-porosity "hard ground." This interpretation has implications for understanding magmatic differentiation, mush dynamics, and the evolution of parent magma compositions in layered intrusions.