Most submarine greenstones recovered from the North Mid-Atlantic Ridge range in δ18O from 2.8 to 6.8‰ (SMOW). An extremely light rock. (δ18O = −7.1‰), was also analyzed, but it might be an ice rafted erratic. The heavier ones, up to 8‰, are probably weathered. On the average, the δ18O of 14 greenstones is slightly lower than that of fresh unmetamorphosed submarine basalt. The 18O fractionations between coexisting metamorphic minerals of the greenstones are consistent with the independently inferred range of metamorphic temperatures of 200–300 °C. The δ18O of the possible metamorphic water can be estimated from this temperature range and the δ18O of the metamorphic minerals to be − 2 to + 2‰. This argues strongly that little or no 'juvenile' water (water outgassed from the mantle) participated directly in the metamorphism. The δ18O data are consistent with the participation of sea water in the metamorphism. Similar conclusions can be drawn from the 18O analyses of a submarine diabase and two gabbros. Disseminated carbonates in the greenstones have δ18C values typically found in carbonatites or partially decarbonated marine limestones.