Chilled margins were recovered from the sheeted dike complex (SDC) of superfast (>200 mm/y)-spreading East Pacific Rise–spread crust during drilling of Integrated Ocean Drilling Program Hole 1256D on the Cocos Plate. The chilled margins contain stretched spherules, oriented plagioclase laths, grain-size segregation, and color banding. These rheomorphs locally crosscut veins but are elsewhere crosscut by veins. Electron microprobe investigations found that the chilled margins comprise dispersed micrometer-scale minerals and veins including chlorite, actinolite, quartz, anhydrite, sphene, calcite, sphalerite, K-feldspar (adularia and/or orthoclase), magnetite, pyrite, and chalcopyrite. Though many of these phases are present throughout the SDC, anhydrite and calcite have not been previously recognized >100 m below the SDC–lava transition zone, and, with one exception, K-feldspar has not been previously identified in Hole 1256D core. Microstructures include quartz clasts surrounded by anhydrite, K-feldspar veins and clasts cut or surrounded by chilled margin material, and lenses of ductily deformed sphene. Some of the crosscutting relationships and distribution of mineral phases could be explained by hydrothermal alteration that occurred roughly simultaneously with dike intrusion.