Ocean Ridges.

Low temperature (<100C) processes which result in oxidation, hydration, and alkali enrichment of the oceanic crust were the first alteration effects discovered through shallow drilling of the oceanic crust. In the past few years, these features have been further described from Jurassic oceanic crust of the western Pacific (Site 801: Alt et al., 1992) as well as basalts from the Hawaiian arch (Hole 843B: Alt, 1993).

New studies continue to confirm that hydrothermal metamorphism occurs in mafic rocks throughout most of the oceanic crust, including beneath sedimented ridges (Stakes and Franklin, 1994). Cores recovered during the re-entry of Hole 504B in the Costa Rica Rift during ODP Legs 137 and 140 have provided new data on hydrothermal metamorphism of mafic dike rocks near the base of upper oceanic crust. In particular, Alt et al. (in press) and Laverne et al. (in press) report on the recognition of a greenschist to amphibolite transition within the sheeted dike complex, characterized by the presence of high-temperature hydrothermal phases including aluminous hornblende, clinopyroxene, and anorthite. This transition has also been described from suites of metabasites dredged from the Mid-Atlantic Ridge (Gillis and Thompson, 1993). Mineralogic and fluid inclusion studies of gabbroic rocks dredged from the Mid-Atlantic (Gillis et al, 1993; Kelly et al., 1993) and the Indian Ocean (Stakes et al., 1991) indicate that hydrothermal fluid-rock interactions within the lower oceanic crust occur over a wide thermal range from essentially solidus conditions to temperatures below 200C.

Ophiolites. Metabasic rocks studied from ophiolite complexes continue to provide the most complete three dimensional picture of hydrothermal metamorphism of the oceanic crust, albeit in tectonic settings which are not exactly analogous to modern mid-oceanic ridge spreading centers. In fact, as discussed by Schiffman et al. (1991), the hydrothermal metamorphism of volcanic rocks from many ophiolite complexes is markedly distinct from that of modern oceanic crust. For example, volcanic successions within remnants of the California Coast Ranges ophiolite are characterized by hydrothermal mineral zonations which serially increase from zeolite through prehnite-pumpellyite through greenschist facies. Conversely, volcanic rocks of the modern oceanic crust are characterized by a marked discontinuity between shallow, low temperature (<100 C) alteration and deeper, greenschist (>300C) alteration.

One other major difference between hydrothermal alteration in ophiolites and modern oceanic crust is that the former invariably exhibit evidence of extensive epidotization. Epidosites are metasomatic assemblages of epidote + quartz chlorite, developed mostly within the sheeted dike complex of ophiolites, and which apparently represent sites of intense fluid-rock interaction at temperatures between 300-400C. Although they have not been found in dredge hauls or drill cores from modern ocean ridges, epidosites are believed to represent the high temperature ``reaction zones'' of some submarine hydrothermal systems (Alt, in press), in which base metals are leached and later precipitated in overlying black smoker-type, base-metal sulfide deposits. Various aspects of epidosite-type alteration have recently been described from the Troodos (Bettison-Varga et al., in press), the Semail (Stakes and Taylor, 1992), and the Josephine (Alexander et al., 1993) ophiolite complexes. Recently, pumpellyite- and prehnite-rich alteration analogies---``pumpellyosites'' and ``prehnitites''---have been described from volcanic rocks in the Josephine ophiolite (Harper, in press). These Ca-metasomatic assemblages apparently form through processes essentially equivalent to those which produce epidosites, albeit at different temperatures and fluid compositions.

Arcs and seamounts. Geochemical evidence indicates that most oceanic-derived, metabasic rocks that are accreted to continental margins are probably not formed at mid-oceanic spreading centers. While normal oceanic crust (i.e, MORB-like) is readily consumed in subduction zones, other pieces of non-MORB-like mafic oceanic crust (e.g., ophiolites and arcs) are either formed, or trapped, in-bound of the active subduction zone or else are not readily subducted (e.g., seamounts or oceanic plateaus). Recent submersible investigations of submarine seamounts and arcs indicate that hydrothermal activity may be as common in these environments as it is at mid-oceanic spreading centers.

The accreted terrains which ``capture'' these non-MORB mafic rocks are themselves often pervasively metamorphosed to low grades (e.g., Himmleberg et al., in press), and until recently, most workers simply assumed that the metamorphism of these accreted terrains was entirely related to the accretionary process. A few recent studies are shedding some light on the nature of the in situ metamorphism in some of these various non-ridge oceanic environments. Volcanoclastic oceanic arc rocks drilled in the Sumisu Rift of the western Pacific (Yuasa et al., 1992) and the North Coast Basin of Puerto Rico (Cho, 1991) are metamorphosed to the prehnite-pumpellyite facies. This form of low temperature metamorphism is distinctly different from that described from modern ocean ridges, and is in fact much more closely akin to that found in many ophiolites (Schiffman et al., 1991) and accreted arc terrains (Beiersdorfer, 1993). Prehnite-pumpellyite facies metamorphism may also be characteristic of oceanic seamounts as well, as indicated by a recent study of the Pliocene basement complex in La Palma, Canary Islands (Schiffman and Staudigel, 1994).