Mineralogical control on the chlorine isotope composition of altered oceanic crust

► Chlorine geochemistry is a rough indicator of metamorphic grade and mineralogy. ► High chlorine concentrations are found in samples with high amphibole contents. ► High δ37Cl values (≥ ~ 0.5‰) are reported in samples with high amphibole content. ► Low δ37Cl values (< ~ 0‰) are found in samples dominated by clays. ► AOC is a major Cl host in the subducting oceanic lithosphere.