PILLOW basalts metamorphosed to the greenschist fades have been frequently dredged from mid-ocean ridges1-3. These rocks usually exhibit a distinct mineralogical differentiation in which the pillow rim or margin is replaced by chlorite or, less commonly, epidote. Calcic plagioclase and olivine within the pillow interior or core are replaced to varying degrees by albite and chlorite, respectively2. Actinolitic amphibole, pyrite, quartz, sphene, and small amounts of nontronite and saponite are the other alteration phases identified in these rocks1-3. Corresponding to this mineralogical differentiation is a chemical differentiation characterised, in general, by enrichment of MgO and H2O in the pillow rim relative to the core; the core is in turn enriched in Na2O, CaO and SiO 2. Cann2, in his study of the metamorphosed pillow basalts dredged from the Carlsberg Ridge, Indian Ocean, refers to the Mg-enriched, Ca-depleted, chloritised rind of the pillows as hyalospilites, and the Na-enriched cores as orthospilites. Metabasalts similar in chemistry and mineralogy to those dredged from mid-ocean ridges have also been found in ophiolites, obducted portions of oceanic crust now exposed on continents 4,5. The oxygen- and strontium-isotopic composition of these basalts6,7, and the oxygen-isotopic composition of secondary minerals separated from dredged basalts8, suggest that alteration resulted from interaction with seawater at temperatures from about 200 to 350 °C. Passage of seawater through a sequence of pillow basalts would most probably occur along fractures and cracks between individual pillow tubes. In this regard, Spooner et al.6,7 from studies of the extent of hydration, oxidation and oxygen- and strontium-isotopic composition of metamorphosed pillow basalts at Troodos, Cyprus, have concluded that the flow of seawater through basalt pillow piles occurred primarily along fractures and pillow boundaries, whereas grain-boundary diffusion was the principal mechanism of mass transfer accompanying chemical reaction within the solid rock. Because of this variation in permeability, pillow rims would be exposed to a much greater volume of seawater relative to the crystalline cores; consequently, alteration of the rim would proceed at a higher effective water/rock ratio9. We propose here that the water/rock mass ratio during seawater-basalt interaction affects the stability of secondary minerals and therefore accounts for the mineralogical and chemical diversity between the core and rim of metamorphosed pillow basalts. To demonstrate this, we report the change in mineralogy and chemistry of basalt glass reacted with seawater at 300 and 350 °C, 500 bar at water/rock mass ratios of 62 and 10, respectively. The experimental system and analytical procedures are discussed in refs 10, 11.