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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, A12O3, 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. In contrast, DSDP Legs 64 and 65 found evidence for local hydrothermal activity at a high level in the ocean crust at several localities in the Guaymas Basin and at the mouth of the Gulf of California. Although most of the basalts recovered contained the assemblage smectite, carbonate, and pyrite and were considered to show typical low-temperature alteration, evidence for intense hydrothermal activity was reported from sediments adjacent to sill-like intrusions in the Guaymas Basin (Einsele et al., 1980) and local alteration of basalt to chlorite and actinolite was observed at Site 482 at the mouth of the Gulf. Although these samples represented the first reported occurrence of hydrothermally altered basalts in DSDP drill holes in "normal" ocean crust, they were not thought to be representative of the alteration processes operating along this section of the East Pacific Rise. Subsequent work suggests that many of the other basalts sampled by Legs 64 and 65 were also affected by hydrothermal alteration. The main purpose of this chapter is to describe the alteration of these basalts in terms of their mineralogy and chemistry and to compare them with samples recovered from other DSDP sites.

Alteration of Basalt: Deep Sea Drilling Project Legs 64 and 65