Mineralogical, chemical, and isotopic results from seven drilling legs that visited DSDP/ODP Hole 504B over 14 years are compiled here to present an integrated view of hydrothermal alteration of oceanic crust at Site 504. Hole 504B reaches to 2111 mbsf, through 274.5 m sediment, 571.5 m of volcanic rocks, a 209 m transition zone, and 1050 m into a sheeted dike complex. The volcanic section was altered through a series of processes involving interaction with seawater at low temperatures, with the effects of cold, oxidizing seawater decreasing downward. These processes and their effects on the volcanic section are generally similar to those in other oceanic upper crustal sections.The transition zone and upper dikes were altered in a subsurface mixing zone, where hydrothermal fluids upwelling through the dikes mixed with cooler seawater circulating in the overlying more permeable volcanic rocks. Alteration of the transition zone and upper dikes (down to 1500 mbsf) occurred in a series of stages, reflecting the thermal and chemical evolution of the hydrothermal system from (1) early chlorite, actinolite, albite-oligoclase, and titanite, to (2) quartz, epidote and sulfides, to (3) anhydrite, and finally to (4) zeolites and local calcite. The maximum temperature estimated for the first two stages is 350°-380°C, and the inferred mineral assemblages for these early stages are typical of the greenschist facies.The lower dikes (1500-2111 mbsf) underwent an early, high-temperature (>400°C) alteration stage, resulting in the formation of hornblende and calcic secondary plagioclase, consistent with reactions inferred to occur in deep subsurface reaction zones, where hydrothermal vent fluids acquire their final compositions. Much of the subsequent reactions produced greenschist assemblages at ~300°-400°C. The lower dikes have lost metals and sulfur and are a source of these elements to hydrothermal vent fluids and seafloor sulfide deposits. The lower dikes underwent subsequent alteration stages similar to the upper dikes, with rare epidote + quartz veins recording the presence of upwelling hydrothermal fluids, and limited late off-axis effects (zeolites and prehnite). Anhydrites in the lower dikes indicate more reacted fluid compositions than in the upper dikes.Alteration of the sheeted dikes from Hole 504B is heterogeneous, with recrystallization controlled by fracturing and access of fluids. Defining the position of the seismic Layer 2/3 transition depends upon the scale of observation, but the change at Site 504 occurs within the sheeted dikes and is correlated with progressive changes in porosity and hydrothermal alteration. However, we still do not know the nature of the transition from sheeted dikes to gabbros in in situ ocean crust, or the nature of the inferred fault at the base of Hole 504B and its role in fluid flow and alteration.

Twenty-two young seamounts near the East Pacific Rise (EPR) consist predominantly of OI-Hy normative basalt chemically identical to depleted mid-ocean ridge basalts (MORB). This basalt is systematically more primitive than the EPR lavas and is supplied directly to the seamounts from the mantle under the ridge, ridge-transform intersections, and fracture zones. The seamounts also contain small volumes of differentiated basalt with low Mg # (Mg # = Mg/Mg + Fe 2 +) which fractionate from basalt parents of high Mg # mainly by processes of fractional crystallization. Seamounts also contain Ne normative alkali basalts, which are enriched in light rare earth, other incompatible trace elements, and volatiles, as well as a complete spectrum of transitional basalts of intermediate composition. We suggest that this spectrum of primitive basalt types forms owing to magma mixing during melt segregation in the region of melting of a chemically, isotopically, and perhaps mineralogically heterogeneous mantle. Melting probably takes place at about 100 MPa in the stability field of plagioclase or plagioclase-spinel, and the extent of partial melting is highly variable: relatively low to produce alkalic basalts and relatively high to produce the tholeiitic MORB. All basalt types, however, are probably hybrids produced by mixing in the region of melt segregation. The thermal and mechanical regime which results in the magma diversity observed on young seamounts is evidently also present at very slow spreading ridges but not below fast-spreading ridges. Normal ridge crest segments and fracture zones (seamount volcanism) thus exhibit a range of thermochemical characteristics which differ markedly from plume/hotspot volcanism. 11,235 11,236 BATIZA AND VANKO: PACIFIC SEAMOUNT PETROLOGY genic isotope ratios, enrichmen.t of large ion incompatible trace elements, and silica undersaturation (i.e., nepheline in the CIPW norm) (for example, in the North Atlantic [Schilling et al., [1983]). However, this is not always the case, and this has led to the conclusion that the upper mantle contains numerous reservoirs. For example, Chen and Frey [1983] have shown that on Haleakala volcano an inverse correlation between trace element enrichment and isotope abundances exists. So, there are not universal good correlations between incompatible trace element enrichment, isotope ratios, and normative mineralogy among oceanic basalts.In this paper we demonstrate that, to complicate matters further, many near-ridge volcanoes in the Pacific are composed primarily of depleted N-MORB. The presence of low-K tholeiitic basalt on the off-ridge volcanoes has been known for some time. Engel and Engel [1963] reported a low-K basalt from Cobb seamount. Barr [1974] and Turner et al. [1980] showed that many volcanoes in the northeast Pacific are made of low-K tholeiite. In addition, Schilling and Bonatti [1975] and Bonatti [1967] reported similar basalts from volcanoes near the southern East Pacific Rise. Batiza et al. [1977], Batiza [1977a, 1978], and Gasset al. [1973...

Quartz‐bearing veins in metagabbroic rocks dredged from the Mathematician Ridge, east Pacific, contain abundant fluid inclusions. Heating and freezing data on nearly 400 inclusions from seven samples allow determination of the temperatures, pressures, and fluid compositions in the subseafloor hydrothermal system at the time of quartz growth. Coexisting dense halite‐saturated inclusions and low‐density, low‐salinity vapor‐rich inclusions (average 45 and 2 wt % NaCl equivalent, respectively) attest to an episode of phase separation in some samples. The phase separation occurred at temperatures of about 600°–700°C and pressures of 60–100 MPa (600–1000 bars). The fact that samples that formed at 60–100 MPa are now exposed on the seafloor, where ambient hydrostatic pressure is only 30–35 MPa, suggests that the samples have been tectonically uplifted of the order of 3 km. The fluids could originally have been part of a deep axial hydrothermal circulation cell, or alternatively, they could have been formed in a deep convection cell underlying the off‐axis edges of a magma chamber. Fluids are NaCl‐CaCl2 brines with molar Na: Ca of 4–8. This range of molar Na: Ca is very close to that of the inferred hydrothermal end‐member from various active black smokers, to the measured ratios from basalt‐seawater interaction experiments, and to the ratio calculated during numerical basalt‐seawater interaction calculations. Crushing experiments indicate little or no compressible gas within the fluids. Fluid inclusions in albite suggest trapping temperatures of around 410°–500°C. Those in epidote may have been trapped at around 500°C and 110 MPa (1.1 kbar) pressure, or around 3 km beneath the Mathematician Ridge seafloor.

The lithostratigraphy and alteration of volcanic basement from Holes 504B and 896A, located in different parts of a ridge flank circulation cell, are summarized and compared. The 290-m-thick volcanic section of Hole 896A is located on a basement high that coincides with high heat flow and upwelling fluids. The 571.5-m-volcanic section of Hole 504B is located ~l km away in an area of ambient heat flow. Subtle differences in lithostratigraphy include slightly greater proportions of massive units and fewer pillow basalts in Hole 896A than in Hole 504B. The volcanic sections are geochemically similar, but there is no direct correlation of lithologic or geochemical units between the two sites. Veins are comparable in abundance in the two sections (-30 veins/m, mean vein width < 1 mm), but carbonate veins and thick (>2 mm) saponite and carbonate veins are more abundant in Hole 896A than in Hole 504B. Permeabilty values of the upper basement sections in Holes 896A and 504B are similar (~IO~1 3 to 10~1 4 m 2 ), suggesting that the upper -200 m of basement is sufficiently permeable on a regional scale to support circulation of seawater through basement.Alteration effects in basement from Hole 896A are similar to those in the upper 320 m of volcanic rocks in Hole 504B. These include celadonitic phyllosilicates in fractures and alteration halos and reddish, Fe-oxyhydroxide-rich alteration halos along fractures. Dark gray rocks, characterized by the presence of saponite ± carbonate ± pyrite occur throughout Hole 896A and the entire Hole 504B volcanic section. Alteration reflects evolution from open circulation of cold, oxidizing seawater, to more restricted circulation of seawater caused by burial of the crust by sediments and sealing of fractures with saponite. Latestage carbonates and minor zeolites formed in veins throughout both holes from reacted seawater fluids (decreased fluid Mg/ Ca). Oxygen and strontium isotopic evidence indicate an early generation of carbonates in both holes that formed at relatively low temperatures (~25°-35°C) during open circulation, whereas later carbonates in Hole 896A formed at slightly higher temperatures (~50°-70°C) during more restricted circulation, possibly similar to the present ridge flank hydrothermal upflow conditions (~50°-80°C).Chemical changes in altered upper crust include oxidation, increased alkalis, Mg, CO 2 , and H 2 O; local uptake of P; elevated δ l8 θ, δD, δ"B, and 87 Sr/ 86 Sr; and lower S contents and δ 34 S. The greatest chemical changes occur in alteration halos and breccias, and the smallest chemical changes occur in the lower volcanic section of Hole 504B. Secondary minerals filling fractures and cementing breccias are sites of uptake of Mg, CO 2 , and H 2 O, in addition to changes occurring in altered rocks.

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