Evidence for submarine hydrothermal activity along oceanic spreading centers has been accumulated from a large and diverse body of geochemical and geophysical data (e.g., Boström and Peterson, 1966; Wolery and Sleep, 1976; Humphris and G. Thompson, 1978), and the alteration of oceanic rocks by seawater has been the subject of intense study for the last few years. Previous studies of Deep Sea Drilling Project (DSDP) samples have found little evidence of high-ternperature hydrothermal activity in the upper part of the oceanic crust. Instead, most of the alteration described appears to be the result of low-temperature interchange between basalt and seawater under ambient conditions on the seafloor. This type of alteration is most pronounced along fractures and open channelways and involves the growth of smectite, carbonate, and occasionally pyrite and phillipsite. Olivine crystals and interstitial glassy material within the basalts are frequently replaced by smectites and more rarely, carbonate, but the plagioclase feldspars and clinopyroxenes are normally unaffected. The resulting chemical changes usually involve notable increases in H2O, Fe2O3, K2O, Rb, Cs, and U and smaller increases in total iron and Sr. TiO2, Al2O3, P2O5, Nb, Zr, and Y are usually considered to be little affected by alteration (e.g., Bass, 1976; Robinson et al., 1977). Several workers (e.g., Pritchard et al., 1979) have noticed an increase in the degree of alteration with age.

Supplement to: Morrison, M Ann; Thompson, Robert N (1983): Alteration of basalt: Deep Sea Drilling Project Legs 64 and 65. In: Lewis, BTR; Robinson, P; et al. (eds.), Initial Reports of the Deep Sea Drilling Project (U.S. Govt. Printing Office), 65, 643-660