Amphibole Compositions Record Cold Post‐Emplacement Reequilibration in Plutons

Abstract The mechanisms that govern the storage and assembly of magmatic systems are fundamental to understanding the evolution of the continental crust and volcanic hazards. Despite extensive research, the mechanisms differentiating volcanic and plutonic magmatism remain unresolved, particularly regarding discrepancies between mineral compositions and whole‐rock geochemistry. Here, we consider thermodynamic modeling and statistical geothermometry of a global dataset of amphibole‐bulk rock pairs from arc‐derived igneous rocks. Our analysis reveals that arc plutonic amphibole records systematically colder conditions and more felsic equilibrium melts than their volcanic equivalents, for a given whole‐rock composition. We find that these discrepancies cannot be explained by melt extraction alone, and instead reflect low‐temperature reequilibration during protracted residence at near‐solidus temperatures after emplacement. The absence of such reequilibration in volcanic rocks, despite increasing evidence for pre‐eruptive cold storage in volcanic systems, emphasizes the fundamental differences between volcanism and plutonism.