Abstract: Pumpellyite has been found in doleritic basalt of a sheeted dyke complex drilled from 2072.1 m below sea floor in DSDP/ODP Hole 504B, south of the Costa Rica Rift, eastern Pacific. It occurs as fine-grained crystal aggregates accompanied by albite, chlorite and chalcopyrite, which partially replace a plagioclase phenocryst (An85–88) that is also associated with primary magnetite. Chemical compositions of the pumpellyite vary antithetically in relation to Fe* and Al as well as Fe* and Mg, indicating the dominant substitution of Fe3+ by Al with the minor substitution of Fe2+ by Mg. Such compositional variations overlap with those of prehnite-pumpellyite facies rocks dredged from other oceanic ridges and intra-oceanic ares, and those of similar facies rocks from ophiolites, but are aluminous compared with those of zeolite facies metabasites in ophiolites. These observations suggest that the breakdown of the plagioclase phenocryst and magnetite in the presence of a Cu- and S-bearing fluid phase led to the formation of pumpellyite + albite + chlorite + chalcopyrite during oceanic ridge hydrothermal alteration.