Clinopyroxene precursors to amphibole sponge in arc crust

The formation of amphibole cumulates beneath arc volcanoes is a key control on magma geochemistry, and generates a hydrous lower crust. Despite being widely inferred from trace element geochemistry as a major lower crustal phase, amphibole is neither abundant nor common as a phenocryst phase in arc lavas and erupted pyroclasts, prompting some authors to refer to it as a “cryptic” fractionating phase. This study provides evidence that amphibole develops by evolved melts overprinting earlier clinopyroxene – a near-ubiquitous mineral in arc magmas. Reaction-replacement of clinopyroxene ultimately forms granoblastic amphibole lithologies. Reaction-replacement amphiboles have more primitive trace element chemistry (e.g. lower concentrations of incompatible Pb) than amphibole phenocrysts, but still have chemistries suitable for producing La/Yb and Dy/Yb “amphibole sponge” signatures. Amphibole can fractionate cryptically as reactions between melt and mush in lower crustal “hot zones” produce amphibole-rich assemblages, without significant nucleation and growth of amphibole phenocrysts.