Low Temperature Alteration of Oceanic Basalts, DSDP Leg 37

During Leg 37 deep penetration of acoustic basement was achieved at four sites on the west flank of the MidAtlantic Ridge at approximately 37°N latitude. These sites (332, 333, 334, and 335) were drilled along a seafloor-spreading flow line in rocks ranging in age from about 3.5 to 13 m.y. The maximum subbasement penetration (583 m) was achieved at Hole 332B; depth of penetration at the other holes ranged from about 180 to 330 meters. These holes provide an excellent opportunity to investigate the nature and intensity of alteration of sea-floor basalts in relation to lithology, age, and depth within layer 2 of the ocean crust. Previous studies of alteration of sea-floor basalts, dealing mainly with dredged samples, have shown that alteration is due largely to low temperature chemical interchange between seawater and rock or magma. This type of alteration is most pronounced along fractures and open channelways and involves the growth of smectite, carbonate, and phillipsite. The resulting chemical changes, as recently summarized by Scott and Hajash (1976), generally involve notable increases in H2O, Fe2θ3, K2O, Rb, Cs, U and smaller increases in total iron, B, Ce, Sr, V, Zr, and Pb. Tiθ2 and P2O5 are generally considered to be little affected by low temperature alteration, but small increases in these oxides have been noted by Hart (1970), Miyashiro et al. (1969), Melson (1973), and Shido et al. (1974). Several workers have noted increasing alteration with age of the sea floor (Hart, 1970; Aumento, 1971).