Hydrothermal processes at mid-ocean ridges

REVIEWS OF GEOPHYSICS, SUPPLEMENT, PAGES 71-80 , JULY 1995 U.S. N A T I O N A L REPORT TO INTERNATIONAL U N I O N OF G E O D E S Y A N D GEOPHYSICS 1991-1994

Introduction Submarine hydrothermal systems are an integral component of

crustal construction along the global system of mid-ocean ridges. Thermally-induced circulation of seawater through the permeable parts of the crust and upper mantle has been estimated to account for 34% of the predicted global oceanic heat flux, which in turn comprises close to 25% of the total heat flux of the Earth [Stein and Stein, 1994, and references therein]. Discharge of hydrothermal fluids is manifest along mid-ocean ridges as high temperature (~200-400°C) focused and lower temperature (<200°C) diffuse fluid flow. Off-axis fluid flow may extend out to the crustal sealing age of 65±10 million years, and may be responsible for more than 70% of the hydrothermal heat flux [Stein and Stein, 1994].

Circulation of seawater through the oceanic crust and upper mantle gives rise to a complex series of physical and chemical reactions that lead to the formation of seafloor mineral deposits and to the existence of unique biological communities. In addition, alteration of the basement rocks and mineral precipitation within fractures dramatically influence the thermal structure and physical properties of the oceanic lithosphere, and change the chemistry of the crustal material that is returned to the mantle by subduction. On longer time scales, these water-rock interactions play a role in regulating the chemistry of seawater, although the magnitude of the associated elemental fluxes are not well constrained.

The distribution and characteristics of hydrothermal circulation within the oceanic crust are controlled by the thermal regime and the permeability structure, both of which are strongly influenced by magmatic and tectonic processes. Understanding the spatial and temporal relations between these variables is key to the development of quantitative models of hydrothermal systems and assessment of their global scope and importance New and improved remote sensing technologies, such as multibeam and sidescan sonar systems, real-time acoustic monitoring, and sensors to detect physical and chemical anomalies in the water column, have enhanced our ability to locate hydrothermal fields, thereby expanding our knowledge of their tectonic and volcanic settings, their characteristics, and the temporal variability of active hydrothermal systems. Studies of hydrothermally altered material from the sea floor and from ophiolites have documented the heterogeneity of water-rock interactions and the variations in the mineralogical and geochemical changes, which often reflect more than one hydrothermal event. Recent experimental work has examined the mobilities of elements under different physical and chemical conditions, provided thermodynamic data for important mineral

Copyright 1995 by the American Geophysical Union.

phases and aqueous complexes, and begun to investigate the impact of fluid phase separation within the reaction zone. Mathematical models have been used to constrain the conductive heat transfer from a magma chamber and the temporal evolution of high temperature vents, as well as to examine the relations between fluid flow and permeability within axial and off-axis hydrothermal systems.

This review attempts to highlight some of the research published during the last four years that has significantly advanced our understanding of mid-ocean ridge hydrothermal systems. The focus is on the geological aspects of these systems; a complete discussion of the impact of hydrothermal circulation on the water column and on biological production is beyond the scope of this review. I have selected specific topics under three broad categories: (i) distribution and temporal variability of submarine hydrothermal systems; (ii) rock-water interactions within the crust; and (iii) the evolution of permeability and its relation to off-axis hydrothermal systems. Portions of the mid-ocean ridge that are discussed in these sections are illustrated in Figure 1. To conclude, I enumerate some of the outstanding problems and directions for future research on submarine hydrothermal processes.

Distribution and Temporal Variability of Submarine Hydrothermal Systems

Less than 1% of the sea floor where hydrothermal systems are likely to be found has been systematically investigated; thus, our knowledge of the distribution of hydrothermal activity is very limited and is biased towards sites of active venting of high temperature, focused hydrothermal fluids. Submarine hydrothermal systems have now been found along fast, intermediate and slow-spreading mid-ocean ridges, at intraplate volcanic centers, and in island arc settings, both in back-arc basins and in fore-arc areas; a summary of their locations has been published by Rona and Scott [1993]. However, our ability to develop predictive models of both the distribution and temporal variability of hydrothermal systems has been limited by the lack of high-resolution datasets that can be used to compare salient features of hydrothermal fields in different tectonic settings. This has been further complicated by evidence from time-series sampling of hydrothermal fluids, observations of venting in the immediate aftermath of a volcanic eruption, and isotope dating of hydrothermal deposits, that the characteristics of hydrothermal systems, ranging from individual vents to entire hydrothermal fields, can change over time-scales of days to tens of thousands of years. Despite these limitations, some distinctive relationships are beginning to emerge from several recent studies along fast, intermediate, and slow spreading ridges.

East Pacific Rise at 9-10°N

Detailed studies along the fast spreading East Pacific Rise between 9° and 10°N (full spreading rate of -11 cm/year),

71

Paper number 95RG00296. 8755-1209/95/95RG-00296$ 15.00

H y d r o t h e r m a l processes at mid-ocean r idges

Susan E. Humphris Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

72 HUMPHRIS: HYDROTHERMAL PROCESSES AT MID-OCEAN RIDGES

Figure 1. Sketch map of the global mid-ocean ridge system (and insets showing the Juan de Fuca Ridge and a section of the Mid-Atlantic Ridge) illustrating the areas that are discussed in this review.

coupled with the serendipitous discovery of a volcanic eruption in this area in 1991, have provided the opportunity to investigate the temporal evolution of a hydrothermal system immediately following an eruption. A 1989 deep-towed photographic survey along 83 km of the spreading axis documented the distribution of hydrothermal vents and their relation to volcanic and tectonic features [Haymon etal., 1991]. Active hydrothermal areas are on average spaced about 2 km apart along the ridge axis, and the maximum separation between vents in each area is <100 m. Hydrothermal features (active and inactive chimneys up to 20 m high and sulfide mounds) are numerous, but volumetrically small. High-temperature vents are concentrated along the shallow portion of the ridge where the axial magma chamber reflector shoals to < 1.7 km beneath the sea floor [Detrick et al., 1987]. In addition, many of the vents are located along the margins of the narrow (40-300 m wide) linear axial summit caldera. This suggests that both the depth and location of the heat source and the enhanced permeability along the bounding scarps of the axial summit caldera regulate the geometry of fluid circulation in these regions. Based on these observations, Haymon et al. [1991] proposed a model for hydrothermal circulation at fast-spreading ridges that invokes three-dimensional circulation in the volcanic section superimposed on axis-parallel circulation through the sheeted dike complex.

Following a 1991 volcanic eruption at 9°45'N to 9°52'N, dramatic changes in the distribution and nature of hydrothermal activity were observed [Haymon et al., 1993]. These included

widespread, disorganized discharge of high-temperature (up to 403°C) fluids directly from the new lava flow. The fluid chemistry varied over periods of days possibly owing to subcritical liquid-vapor phase separation at depths of -200 m beneath the sea floor. Haymon et al. [1993] suggested that intrusion of dikes to shallow depths beneath the sea floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor-rich fluid through the overlying lavas. The eruption also buried many of the pre-existing animal communities; however, rapid and extensive growth of flocculent white bacterial mats in areas of diffuse venting was observed.

A year later, diffuse venting was considerably reduced, and hydrothermal discharge was more focused at newly developed black smoker spires [Haymon et al., 1992]. The fluid temperature at one vent had decreased from 403°C to 332°C, and chlorinity had increased from 35 mmoles/kg to 250 mmoles/kg [Von Damm et al., 1992]. Bacterial mats were greatly reduced, and an abundant and diverse megafauna, including new tubeworms up to 30 cm in length, was present. Recent observations of this area in March 1994 indicated that the hydrothermal system is more organized, but has not yet reached a stable state (K.Von Damm, personal communication).

These studies have revealed for the first time the magnitude and rapidity of changes during the early stages of the development of a hydrothermal system in response to a volcanic event. In contrast, time-series studies at 21°N on the East Pacific Rise and at other vent sites have revealed virtually no temporal

HUMPHRIS: HYDROTHERMAL PROCESSES AT MID-OCEAN RIDGES 73

variability in temperature and fluid chemistry over periods of several years [e.g. Lawrence and Edmond, 1992] suggesting that, as the hydrothermal system matures, it approaches a stable state.

Juan de Fuca Ridge

Recent high-resolution geological and hydrothermal studies along the intermediate spreading Juan de Fuca Ridge (full spreading rate of 5-6 cm/yr) in the northeast Pacific have indicated that individual segments are at different stages of magmatic and tectonic evolution, resulting in differences in the observed hydrothermal activity. Studies have focused on three areas: the Endeavour segment, Axial Seamount, and the northern part of the Cleft Segment. The Endeavour hydrothermal site is located at the along-axis high within the wide (0.5-1 km) axial valley of the Endeavour segment, which appears to be in an extensional rather than volcanic phase of spreading. Recent submersible studies [Delaney et al, 1992] have confirmed earlier suggestions that the boundary fault network at the base of the western wall of the axial valley is the primary control on the locations of active and relict vent sites. The main active vent field (-180 m wide and 350 m long), which is in a region of older basaltic pillow and lobate flows, includes more than 15 large (up to 30 m in diameter and >20 m in height), actively venting structures and numerous smaller, less active or inactive structures. This within-field variation in size and level of activity of the structures argues for a secondary control on the distribution of hydrothermal flow channels. Delaney et al [1992] suggest that localization of mineral deposition sufficient to build the large structures involves intersecting ridge-parallel normal faults and other fractures and fissures trending oblique or perpendicular to the ridge.

At Axial Seamount, a large, axial volcano located on the central part of the Juan de Fuca Ridge, the distribution and style of venting is controlled by a combination of localized Assuring and faulting and lava morphology. Although there are a number of low temperature vents within the caldera, the major sites of venting are associated with fractures and fissures along the margin of the caldera [CASM, 1985; Embley et al, 1990; Johnson and Embley, 1990]. The location of the ASHES vent field, which contains the only high temperature vents in the caldera, is controlled by a fault zone that defines the western boundary of the summit caldera [Hammond, 1990]. Compared to the Endeavour structures, the actively venting chimneys in the ASHES vent field are small (several meters in diameter and 4-5 m in height) and are confined to an 80 m x 80 m area. Diffuse venting is more widespread. Areas of focused venting appear to correlate with flow surface roughness and tend to be concentrated within smooth sheet and lobate flows, suggesting that, on a local scale, the effective permeabilities of lavas of different morphologies determine where conduits can be formed [Hammond, 1990].

In contrast with the Endeavour segment to the north, recent observations along the southern Juan de Fuca Ridge indicate that it appears to be in a stage of renewed active volcanism. The discovery of "megaplumes" (radially symmetric hydrothermal plumes >20 km in diameter) in 1986 and 1987 in the water column overlying the northern end of the Cleft Segment and the southern end of the Vance Segment [Baker et al, 1987,1989] led to a broad research effort in this area. Megaplumes are thought to result from sudden voluminous discharges of fluids from preexisting hydrothermal systems, which would require a significant

increase in the local permeability. This could be caused by intrusion and tectonic fracturing [Baker et al, 1989; Cathles, 1993], or by hydrofracturing of a sealed cap on the pre-existing hydrothermal system [Cann and Strens, 1989]. A comparison of two high-resolution sea floor mapping and acoustic imaging surveys of the area revealed that a fissure eruption occurred sometime between 1983 and 1987 [Chadwick et al, 1991; Embley et al, 1991; Fox et al, 1992]. Two distinct types of very recent lava flows were observed: isolated mounds of pillow lava (up to 4 km long, 0.5 km wide and tens of meters high) and a broad sheet flow covering 3.5 k m 2 [Chadwick and Embley, 1994], both of which were fed from dikes intruded along different parts of the same fissure system [ Embley et al, 1991; Embley and Chadwick, 1994]. The highest density of diffuse venting and the high temperature black-smokers occur along this fissure system on one side of the sheet flow, and geochronological studies of the mineral deposits indicate the development of at least one new high temperature site as well as the rejuvenation of older, lower temperature structures [Koski et al, 1994]. The lack of hydrothermal activity on the younger pillow mounds is interpreted to be a reflection of their being fed by lateral dike injection from a magma reservoir underlying the sheet flow region [Embley and Chadwick, 1994]. The magma reservoir acts as the heat source for the long-lived, well-organized hydrothermal system, but only short-lived venting is associated with the lateral dike intrusion event beneath the pillow mounds.

Time-series submersible observations at the Cleft Segment between 1988 and 1991 have demonstrated that the hydrothermal activity changed considerably. Some vents have died out, while others have developed; diffuse hydrothermal fluids have changed from being low salinity and depleted in metals to fluids enriched in chloride and metals [Butterfield and Massoth, 1994]. In 1988, widespread diffuse flow at temperatures of 60°C was associated with extensive bacterial mats. By 1991, venting was less extensive and had decreased in temperature, and the abundance of vent fauna had dramatically decreased. A significant reduction in venting was also indicated by time-series measurement on the intensity and distribution of hydrothermal plumes [Baker, 1994]. The waning of hydrothermal activity along the northern Cleft Segment suggests that the impact of diking events on the overall hydrothermal system may be localized and short-lived. If the megaplume events in 1986 and 1987 represented the onset of seawater circulation through newly opened cracks and fissures, then the waning of activity observed in 1991 suggests time cycles on the order of 5 years. This contrasts with the vent field centered at ~44°40'N in the southern Cleft segment, where temperature and vent fluid chemistry have been stable over a 7-year period [Massoth et al, 1994], and supports the suggestion of Embley and Chadwick [1994] that long-lived hydrothermal activity reflects the presence of an underlying magma reservoir.

Mid-Atlantic Ridge

Our knowledge of the distribution of hydrothermal systems along the slow-spreading Mid-Atlantic Ridge (full spreading rate -2 -4 cm/year) is still in its infancy and, with only four deep-water active hydrothermal fields known, of which to date only two —TAG and Snake Pit — have been studied in any detail, a generalized model of controls on their occurrence cannot yet be derived.

The TAG hydrothermal field at 26°08'N on the Mid-Atlantic


Ridge is one of the largest sea floor deposits and has been intensively studied over the past few years. It is located at the mid-point of a 40 km long ridge segment at the base of the eastern median valley wall. The hydrothermal field extends over an area of at least 5 km x 5 km and consists of presently active low and high temperature zones, as well as a number of relict deposits [Rona et al, 1993a,b]. High temperature activity is confined to a mound that lies on crust at least 100,000 years old. It is distinctly circular in plan view with a diameter of about 200 m, and is mineralogically zoned. The venting fluids have a wide range of temperatures (up to 363°C) and two distinct chemistries, which can be related through processes of conductive cooling, mixing with entrained seawater, and precipitation and dissolution of various mineral phases within the mound [Tivey et al., 1994]. Geochronological studies indicate that hydrothermal activity on the mound has been intermittent over at least the past 20,000 years with a periodicity of 5-6,000 years, and that the current activity began about 50 years ago after a hiatus of about 4,000 years [Lalou et al., 1990, 1993]. Ages of about 102,000 years from one of the relict zones and about 125,000 years from the low temperature field higher on the eastern median valley wall indicate that hydrothermal processes have been active since the formation of the underlying crust [Lalou et al, 1988, 1993]. However, the mechanism by which upflow of hydrothermal fluids can be focused episodically in one area over such long periods of time is unclear. It has been suggested that listric faults associated with the median valley wall provide the pathways for fluids being heated by a source at the zero-age neovolcanic axis [e.g. Thompson et al, 1985]. Alternatively, discrete volcanic centers may act as the heat source for localized activity and exert some structural control. This is supported by observations of very recent volcanics on the volcanic dome associated with the presently active mound [Zonenshain et al, 1989]. More recently, based on observations of east-west faults high on the eastern wall in the vicinity of the low temperature field, Karson and Rona [1990] suggested that the intersection of these transfer faults with ridge-parallel faults may concentrate hydrothermal activity. However, no direct evidence exists for the extension of east-west faults from the low temperature field to the presently active TAG mound.

The Snake Pit hydrothermal field at about 23°22'N covers an area of 150 m x 300 m along the shallowest portion of an intensely fissured neovolcanic ridge. It consists of three mounds aligned east-west and elongated in a direction parallel to the ridge axis. The eastern one is the most active with black smoker chimneys; the central one exhibits only diffuse, low temperature flow and relict chimneys; and the western one is highly tectonized but with black smoker fluids emanating from "beehive" structures [Thompson et al, 1988; Fouquet et al, 1993]. Geochronological studies suggest two major episodes of activity at Snake Pit, the first one between 2,000-4,000 years ago, and the current one beginning about 80 years ago [Lalou et al, 1990,1993]. Fouquet et al [1993] have proposed a model for the formation of Snake Pit that involves an initial hydrothermal event related to faulting and Assuring at the summit of the neovolcanic ridge which produced the elongate mounds. A later tectonic event related to graben formation faulted the western mound, and the hydrothermal circulation was rejuvenated after a volcanic episode, with the upflow zones more focused at the western and eastern mounds.

The other two Mid-Atlantic Ridge sites — Broken Spur and Lucky Strike — were cursorily investigated by submersible in

1993, with further studies being carried out in 1994. At Broken Spur, which is located at about 29°N in the axial graben of the neovolcanic ridge in a similar setting to Snake Pit, three discrete black smoker sites consisting of structures similar to those observed at Snake Pit have been identified [Murton et al, 1993]. In contrast, the setting of the Lucky Strike hydrothermal field at ~37°N is more similar to Axial Seamount on the Juan de Fuca Ridge than to the other Mid-Atlantic Ridge sites. It is located at a depth of -1600 m in a depression between three cones that make up the summit of the Lucky Strike Seamount. Active vent sites are dispersed over an area at least 700 m long and 300 m wide, although the bounds of the field are not yet determined [Langmuir et al, 1993]. The dispersed nature of the distribution of hydrothermal activity at this site contrasts with the more localized distribution seen at Snake Pit and TAG, although the presence of both active chimneys and extensive relict sulfide deposits attest to episodicity in venting similar to the other Atlantic sites.

In spite of the differences in volcanic and tectonic setting of Mid-Atlantic Ridge sites, they all share two characteristics in common: the volume of the hydrothermal deposit at each site is on average larger than observed at most other mid-ocean ridge hydrothermal sites, and there is evidence for episodicity in hydrothermal activity over long time periods. This suggests there are fundamental differences in the persistence or availability of heat sources and the maintenance of effective fluid flow conduits at slow-spreading ridges compared with fast-spreading ridges.

Working Models Relating Hydrothermalism to Volcanic and Tectonic Activity

The distinctive characteristics of hydrothermalism along fast and slow-spreading ridges must be related to differences in the cycling and relative importance of volcanic, tectonic and hydrothermal processes along ridge systems with varying magma supply rates. It is therefore instructive to attempt to reconcile the observations with the Sinton and Detrick [1992] models of magma chambers, since they represent the heat source that drives the hydrothermal circulation. Along fast-spreading ridges, a melt lens, < l -2 km wide and 1-2 km below the surface, overlies a zone of crystal mush that is in turn surrounded by a transition zone of solidified crust with some isolated pockets of magma [Sinton and Detrick, 1992]. The melt lens provides a shallow heat source to drive hydrothermal circulation cells in the overlying crust, with fluid flow focused along shallow faults and fissures. Dike injections and fissure eruptions from the melt lens have two important effects. First, the newly injected material is cooled rapidly by circulation of large volumes of seawater resulting in widespread, diffuse flow, which decreases dramatically within a short period of time. Second, the permeability structure of the upper crust will be modified with every volcanic event, resulting in frequent reorganization of the fluid flow pathways, and thereby limiting the size of hydrothermal structures that can be constructed. In this scenario, cycling between volcanic, tectonic and hydrothermal processes is rapid, and the hydrothermal activity is dominated more by volcanic, than by tectonic, processes.

In contrast, at slow-spreading ridges, there is no evidence for a steady-state magma body; rather, Sinton and Detrick [1992] envision a deeper, dike-like mush zone and broad transition zone. Volcanic eruptions are intermittent and widely dispersed, and are coupled in time to injection events of magma from the mantle. In addition, listric faults bordering the rift valley may extend into


the brittle-ductile transition within the partially molten mush. In this situation, maintenance of hydrothermal circulation is likely to be fault-controlled rather than volcanically-controlled, since construction of large hydrothermal deposits requires episodic renewal of hydrothermal activity in the same location over long periods of time. In one working model, those normal faults which remain active and hence maintain their high permeability act as the preferred conduits for fluid flow beginning at the neovolcanic zone and continuing throughout the median valley as the crust spreads. Even though these channels may become sealed due to mineral precipitation, flow is rejuvenated each time there is movement along the fault. At the border of the rift valley, activity along selected faults that form the median valley walls extends down into the molten mush zone, thereby providing access to the heat source to continue driving the circulation system.

Such models are based on extremely limited datasets and will need considerable refinement as more information becomes available, especially for slow-spreading (low magma supply) ridges. In the next few years, acoustic mapping and imaging surveys using improved high-resolution systems will provide valuable information to better define the detailed relations between hydrothermal, volcanic and tectonic processes.

Rock-Water Interactions in the Oceanic Crust Venting fluids and sea floor mineralization represent the end

result of a complex series of physical and chemical reactions that occur between crustal rocks and seawater in the subsurface portion of active hydrothermal systems. The fluids are generally considered to achieve their characteristics at the base of the hydrothermal cell in a reaction zone where temperatures are 375°-425°C and water/rock ratios are low [e.g. Seewald and Sey fried, 1990; Sey fried et al., 1991]. However, the link between the subsurface alteration assemblages and the fluid chemistry of active hydrothermal vents is not yet well constrained [ Gillis and Thompson, 1993]. As the oceanic crust is transported away from the ridge, the water-rock reactions evolve as the characteristics of the hydrothermal circulation respond to cooling, changes in the physical properties of the crust (in particular porosity and permeability), and reorganization of the pathways of fluid flow. Consequently, the mineralogy and chemistry of hydrothermally altered rocks recovered from the sea floor and from ophiolites reflect the integrated effects of all axial and ridge flank hydrothermal processes and provide a record of the extent of chemical exchange between the crust and seawater, the nature of the reactions that have taken place, and the variation in the composition of the circulating fluids.

Over the last fifteen years, studies of altered rocks from the sea floor and from ophiolites have been combined with experimental and theoretical studies to investigate the variations in water-rock interactions in different portions of submarine hydrothermal systems. These studies have recently been the subject of two detailed reviews: one that describes the alteration reactions within the recharge, reaction and discharge zones [Alt, 1994], and one that addresses the physical and chemical characteristics of hydrothermal reaction zones based on experimental and theoretical considerations [Saccocia et al, 1994]. Perhaps one of the most significant areas of progress in the last four years has been the investigation of alteration processes within lower crustal rocks. These studies have been enhanced by drilling into oceanic crust, which has recovered stratigraphic sections from the lower sheeted dikes and upper gabbros. In combination with studies of

ophiolite sequences and experimental work, these investigations have resulted in the recognition that the reaction zone (where hydrothermal fluids are thought to acquire their final composition) may well lie within the lower part of the sheeted dikes and in the upper gabbros.

Based on geochemical and isotopic data from altered oceanic and ophiolitic rocks, as well as modeling studies, hydrothermal convection in the oceanic crust is most likely layered, with large volumes of seawater circulating through the upper volcanics and reacting at temperatures of <150°C, and only a small percentage penetrating into the sheeted dikes and upper gabbros [e.g. Alt et al, 1986; Gillis and Robinson, 1990; Hart et al, 1994; Rosenberg et al, 1993]. This is supported by the observation that the transition from volcanics to sheeted dikes at Deep Sea Drilling Project(DSDP)/Ocean Drilling Program (ODP) Hole 504B, located on the flank of the Costa Rica Rift, and in many ophiolites coincides with a change from low temperature alteration to greenschist facies metamorphism [Alt et al, 1986; Gillis and Robinson, 1990]. In 1991, Hole 504B was extended to a depth of 2000 meters below seafloor, and seismic and petrographic evidence indicates that the bottom lies near the base of the sheeted dike complex [Dick et al, 1992]. The mineralogy and chemistry of the lower 500-600 m of the sheeted dikes include secondary Ca-rich plagioclase (which is locally replaced by anhydrite) and hornblende, increasing Al and Ti contents of amphibole, and lower 8 1 8 0 values, suggestive of high temperatures of alteration (400-500°C) consistent with those generally attributed to the reaction zone [Alt et al, 1994]. However, a different mineral assemblage that includes Na-rich plagioclase, amphibole (actinolite-magnesio-hornblende), chlorite, and sphene, has been described in metabasalts recovered from the Mid-Atlantic Ridge south of the Kane Fracture Zone (MARK) and is also interpreted to have formed within the reaction zone [Gillis and Thompson, 1993]. The most problematic difference between these two assemblages is the composition of the plagioclase. Experimental studies suggest that the Ca/Na 2 concentration ratio of vent fluids are consistent with plagioclase-quartz-fluid equilibrium under pressure-temperature conditions resembling those in the reaction zone for a plagioclase composition rich in Ca [Berndt and Sey fried, 1993; Saccocia et al, 1994]. Consequently, the existence of Na-rich plagioclase in the MARK metabasalts is inconsistent with this equilibrium model. This indicates that either (i) a non-equilibrium process must be invoked to explain these variations, (ii) that magmatically-derived fluids (which can undergo phase separation) play an important, but as yet unquantified, role in reactions within the reaction zone, or (iii) the MARK metagabbros are not representative of the mineral assemblages in the reaction zone. Another important observation is the scarcity of epidote in hydrothermally altered oceanic rocks when compared with ophiolites and experimental predictions [e.g. Seyfried et al, 1991; Bettison-Varga et al, 1992; Nehlig et al, 1994]. However, it must be emphasized that these comparisons are based on a very limited sampling of rocks from the lower sheeted dikes of the oceanic crust, and that wider variations in their mineralogy and chemistry than are presently recognized should be anticipated as additional material is recovered.

Hydrothermally altered plutonic rocks from the sea floor collected by dredging, drilling and from submersibles indicate that the reaction zone extends into the upper gabbros. The recovery of a 500 m-long section of gabbro from ODP Hole 735B in the Southwest Indian Ocean has enabled reconstruction


of its alteration history and has provided an excellent comparison with hydrothermally altered gabbros sampled elsewhere on the sea floor [e.g. Vanko et al, 1992; Gillis et al, 1993; Hekinian et al, 1993]. The distribution and degree of alteration observed in both suites of rocks are clearly related to permeability and deformation on a local scale, and the drill core shows no depth-related metamorphism that reflects an increase of temperature along a fixed gradient [Stakes etal, 1991]. Metamorphism in the upper portion of the core is spatially related to deformation. Ductile shear zones exhibit extensive dynamic recrystallization that began at temperatures of granulite grade (900-700°C) and continued to amphibolite grade (450°C). Penetration of seawater into the lower crust occurred along zones of brittle-ductile deformation and through networks of cracks adjacent to them, and is evidenced by depletions in 8 1 8 0 and increasing abundance of both amphibole of variable composition and metamorphic plagioclase of intermediate composition. Downcore correlations of contemporaneous mineral assemblages, oxygen isotopic compositions, and vein abundance indicate that seawater was introduced by way of small cracks and veins produced at the end of the phase of ductile deformation and resulted in a hydrous ductile-deformation assemblage in the upper parts of the section. In the lower parts of the section, circulation of fluids is controlled by the distribution of highly permeable magmatic hydrofracture horizons. The enhanced permeability of these zones produced lower temperature greenschist and zeolite facies assemblages as larger volumes of water penetrated the crust [Stakes et al, 1991; Vanko and Stakes, 1991].

This general sequence of events — plastic deformation and metamorphism at high temperatures, followed by brittle deformation and the circulation of fluids, and then cooling with further alteration at lower temperatures — is observed in other sea floor metagabbros [e.g. Mevel and Cannat, 1991; Vanko et al, 1992; Alt, 1994], although the details vary in response to the magmatic and tectonic interactions within each area. For example, the MARK metagabbros show a similar relation of hydrothermal alteration to deformation, although deformation was initiated at lower temperatures (700-550°C) of the amphibolite facies. Brittle fracturing on a variety of scales then provided pathways for more pervasive penetration of seawater and alteration to greenschist and lower amphibolite facies (up to 550°C). Alteration ceased at temperatures of between 80-300°C, suggesting that the lower crust became impermeable as it was transported off-axis [Gillis etal, 1993; Kelley et al, 1993].

Calculations of pressures based on a combination of fluid inclusion and isotopic analyses suggest that the gabbro section sampled by Hole 735B may have originated at a depth of ~2 km below the sea floor near the top of the plutonic section [Vanko and Stakes, 1991]. This is consistent with depths estimated using quartz geobarometry on axial hydrothermal fluids (summarized in Von Damm [1990]), and with exposures in ophiolites, where only the top few hundred meters of the gabbro section are altered [e.g. Nehlig et al, 1994]. However, the timing of fluid penetration, the pathways of fluid flow, and the depth of penetration of hydrothermal fluids into the gabbros are not yet well constrained and are questions that are just beginning to be addressed by deep crustal drilling.

Evolution of Permeability and its Relation to Off-Axis Hydrothermal Circulation

Hydrothermal circulation of fluids within the oceanic crust is not restricted to regions in the immediate vicinity of spreading

centers, but continues off-axis for possibly tens of millions of years. Although relatively little is known about this process, it is now recognized that fluid circulation and the associated alteration reactions and mineral precipitation play a profound role in the evolution of the physical properties of the oceanic crust. For example, based on a review of drilling and downhole logging results, lab measurements of the physical properties of oceanic rocks, and data from ophiolites, Carlson and Herrick [1990] concluded that the seismic velocity structure of the oceanic crust is strongly influenced by changes in porosity and the degree of alteration. Most studies have demonstrated a general decrease in porosity with age [Wilkens et al, 1991; Jacobson, 1992] as would be expected from mineral precipitation within veins, vesicles and cracks. However, a recent compilation of available physical properties data from all DSDP/ODP sites showed no simple trend, although problems with the sparse distribution of drill sites in terms of crustal age, as well as possible bias in recovery rates in different lithologies, may well have affected the results [Johnson and Semyan, 1994].

Permeability is the most critical variable controlling patterns of fluid flow in hydrothermal systems both in space and time. Early observations of the wavelengths of elongate, off-axis heat flow anomalies combined with numerical models of fluid flow through homogeneous porous media and assumptions of high crustal permeability down to depths of >1 km, implied that hydrothermal circulation systems reached depths of several kilometers (references to all these studies are not included here for brevity, but a summary can be found in Fisher et al [1990]). However, measurements of the permeability of the ocean crust made using drill-string packers indicate that the permeability structure of the oceanic crust is strongly layered and only the top few hundred meters are highly permeable [e.g. Anderson et al, 1985; Becker, 1989, 1991; Larson et al, 1993]. Little is known about the lateral heterogeneity of the crustal permeability structure or the spatial and temporal persistence of layers with distinct hydrogeological properties. Precipitation of secondary mineral phases and thermoelastic stresses cause narrowing and closure of cracks; analytical models that attempt to quantify these processes are just being developed [e.g. Germanovich and Lowell, 1992; Lowell et al, 1993; Lowell and Germanovich, 1994],

Despite the difficulties associated with using sea floor measurements to infer the existence and character of subsurface hydrothermal circulation cells, considerable progress has been made in the last few years by combining detailed geophysical/geochemical data from field surveys and drilling with increasingly sophisticated numerical modeling techniques. At the Juan de Fuca Ridge, results from drilling an axial hydrothermal system can be combined with off-axis geophysical and geochemical studies to further our understanding of how hydrothermal systems change as they are transported away from the ridge crest. The circulation of the axial hydrothermal system at Middle Valley — a sediment-covered spreading rift on the northern Juan de Fuca Ridge —is currently being modeled using downhole physical properties data collected during ODP Leg 139. Permeability measurements made proximal to an active vent site indicate the presence of several discrete zones of exceptionally high hydraulic conductivity which, if typical of oceanic basement at spreading centers, must dominate circulation patterns in axial hydrothermal systems [Becker et al, 1994].

About 50 km to the south on the eastern flank of the Juan de Fuca Ridge, a comprehensive program of detailed heat flow studies, seismic surveys, and sediment coring of sea floor ranging


in age from 0.6-6.0 Ma has been conducted as part of the FlankFlux program to investigate off-axis hydrothermal circulation [Davis et al., 1992; Wheat and Mottl, 1994]. Pleistocene turbidites bury the ridge flank beyond a crustal age of about 600,000 years (-18 km from the ridge axis) and form a sharp boundary between sediment-free and sediment-covered oceanic crust. Further to the east, the buried basement varies from being very smooth in some areas to extremely rough in areas where occasional volcanic edifices penetrate through the sediment. These structures are believed to provide permeable pathways for discharge of fluids to the sea floor [Davis et al, 1989] — a conclusion consistent with other studies of both relatively young (<1 Ma in age) crust near the ridge axis and older crust on the ridge flanks that suggest the geometry of fluid circulation is forced by thermal perturbations caused by basement topography [Fisher et al., 1990; Johnson et al, 1993]. In areas of extensive basement outcrops and discontinuous sediment, measured heat flow is only about 15% of that predicted for a cooling lithospheric plate [Davis et al, 1992]. Vigorous flow of seawater cools the basement to temperatures of 10-20°C, and the high water/rock ratios and short residence time of the fluid in the crust results in little change in its chemistry. As the sediment thickness increases to tens of meters, direct access of bottom seawater is limited and the vertical flux of fluids through the sea floor decreases to a few mm/year. This results in a hydrothermal system with a lower water/rock ratio, longer residence times, higher temperatures and greater chemical exchange. In addition, heat and fluid can be advectively transported in the basement over large distances (up to 20 km) in these areas. Finally, the sediment cover becomes thick enough (about 160 m in the case of the turbidites) to effectively prevent exchange of seawater between the basement and the ocean, although active circulation within the crust maintains uniform temperatures at the basement-sediment interface [Davis et al, 1992; Wheat and Mottl, 1994] . Subtle heat flow variations in an area where the sediment thickness and basement topography are relatively uniform suggest a horizontal cell dimension of about 700 m. If an aspect ratio of one is assumed [Lister, 1990; Rosenberg et al, 1993], then the depth of penetration of the circulation system corresponds to a strong upper crustal seismic reflector in the region [Davis et al, 1992].

The most extensive dataset relating to off-axis hydrothermal circulation is from the flanks of the Costa Rica Rift where a variety of geophysical, geochemical and geothermal data have been collected, and a number of holes drilled, including DSDP/ODP Hole 504B located on - 6 Ma old crust (comparable to the oldest area studied on the Juan de Fuca Ridge). Heat flow varies with topography and has a wavelength of 4-7 km, and the agreement between the observed mean and the predicted value suggest that conduction dominates heat transfer and convection is weak [Langseth et al, 1988]. Further modeling has suggested that heat flow is more strongly correlated with basement relief and differential sediment thickness, both of which are reflected in the bathymetry [Fisher et al, 1990, 1994]. Vertical advective flow through sediments of a few mm/year continues to sediment thicknesses of 310 m, and indicates that the physical properties of the sediments influence the duration of the vertical fluid flux [Mottl, 1989]. Porosity and permeability measurements in Hole 504B indicate that the upper 200 m of crust, which contains many open fractures, has permeabilities of 10" 1 4 - 1 0 1 3 m 2 , which decrease with depth as the fractures become filled with secondary minerals [Pezard, 1990]. In the low porosity sheeted dikes, permeabilities decrease by several orders of magnitude to 10" 1 8-

10" 1 7 m 2 [Becker et al, 1989]. The permeability structure and refined modeling of heat and fluid flow in this area suggest that circulation within the basement is restricted to an upper, permeable layer of volcanics extending down to depths of only 200-300 m, and that much of the circulation is concentrated along narrow high permeability zones [Becker et al, 1989; Fisher et al, 1990, 1994]. Moreover, numerical simulations of convection in porous media using the same data suggest that the convection occurs in numerous cells with aspect ratios of less than one within the upper permeable layer [Rosenberg et al, 1993].

These studies demonstrate that hydrothermal circulation can persist for long periods of time as the crust is transported away from the ridge axis. In addition, they suggest that active circulation in the crust can continue once advective exchange of fluids between the crust and the ocean is prohibited by a thick sediment cover. Understanding the influence of the permeability structure of the upper crust on the geometries of the circulation cells will require a better knowledge from field measurements of the three-dimensional permeability field and its temporal evolution, and application of increasingly sophisticated models that numerically simulate changes in the physical properties of the crust in off-axis hydrothermal systems.

Outstanding Problems and Future Directions In the preceding sections, I have attempted to highlight some

of the studies within the past four years that have significantly advanced our understanding of submarine hydrothermal systems, and to indicate a number of topics that need to be pursued to further the eventual development of a quantitative predictive model of hydrothermal circulation within the oceanic crust.

The recent studies on the East Pacific Rise and the Juan de Fuca Ridge have emphasized the rapid rates and magnitudes of changes observed in hydrothermal systems in response to volcanic and tectonic events. The initial processes accompanying crustal accretion events, their time scales, their variability, and their global importance are currently unknown, and will require time-series measurements beginning immediately following such an event and continuing over periods of years. A major breakthrough in our ability to conduct such studies occurred in 1993 when a real-time acoustic monitoring system for low-level seismicity along the Juan de Fuca Ridge using a network of permanent deep-ocean hydrophone arrays (SOSUS) owned by the U.S. Navy became operational. Within four days, a seismic event was detected north of Axial Seamount [Fox et al, 1994]. A response effort was launched and documented a lateral diking event associated with a small volcanic eruption similar to those observed on Hawaii and in Iceland [Embley et al, 1994]. This detection capability, combined with plans for in situ instrumentation over the long-term to investigate the coupling between volcanic, tectonic and hydrothermal events and the temporal variability of hydrothermal systems, will lead to a better understanding of crustal accretion processes.

In addition, we need to further our understanding of the spatial distribution of hydrothermal systems, particularly along slow-spreading ridges, and along mid-ocean ridges systems that are not well explored. Remote sensing technologies that detect anomalies in the physical and chemical characteristics of the water column are being used with some success to delineate ridge segments that appear to be hydrothermally active. However, pin-pointing the exact location of a vent field is problematic, and still somewhat serendipitous. This means that two approaches are necessary. The first is to conduct detailed multidisciplinary studies of known


hydrothermal fields in different settings in order to derive simple models that predict the combinations of tectonic and volcanic features that result in hydrothermal activity. The second approach is to combine water column surveys, which will define hydrothermally active segments, with high-resolution mapping and acoustic imaging surveys, which will provide information on tectonic and volcanic structure, of previously unexplored sections of the mid-ocean ridges.

Although alluded to only briefly in this review, another important type of axial hydrothermal system occurs in sedimented rifts — the setting of some of the largest deposits in the geologic record. At Middle Valley on the northern Juan de Fuca Ridge, active venting occurs through hundreds of meters of turbidites, and sulfides are also precipitated within the sediment column [Goodfellow and Franklin, 1993]. During ODP Leg 139 in 1991, a 94 m section of massive sulfides was drilled at one location, and downhole pressure and temperature sensors were installed in two holes to monitor the rebound of the system from the perturbations caused by drilling. Results from these studies, including models of the hydrothermal circulation system are expected soon; however, this drilling leg demonstrated not only the feasibility of drilling into sea floor sulfide deposits to investigate subsurface hydrothermal processes, but also the potential of using the perturbations caused by drilling to learn more about the physical characteristics of the hydrothermal system. This concept was expanded in the fall of 1994, when drilling at the TAG active hydrothermal mound on the Mid-Atlantic Ridge was combined with in situ instrumentation of the mound to document changes in the circulation of the hydrothermal system during and after drilling. This approach of sea floor instrumentation combined with drilling looks promising, and may represent a powerful way of investigating the physical properties of the oceanic crust.

One of the outstanding problems — and arguably one of the most difficult to solve — is quantifying the relative importance of thermal and chemical fluxes associated with focused high temperature fluid flow and diffuse lower temperature discharge. Black smoker fluids can be readily sampled, whereas methods for sampling and determining the areal extent of diffuse flow, which results from subsurface mixing of hydrothermal fluids with seawater, are just being developed. Consequently, flux calculations tend to overestimate the importance of high temperature fluid compositions and underestimate fluxes associated with diffuse flow [Alt, 1994]. This is further complicated by our lack of knowledge of the distribution of hydrothermal circulation cells both on-axis and off-axis. Off-axis, discharging fluids may circulate through, and react with, overlying sediments resulting in significantly different heat and chemical fluxes. In order to understand the role of hydrothermal processes in regulating the chemistry of seawater, the relative proportions of these different styles of venting, and the pervasiveness of the associated seawater-rock reactions within the crust must be determined. Since these cannot be determined by direct observations, flux estimates must rely on the development of models that relate permeability structure and its evolution to focused vs. diffuse flow while incorporating data on chemical exchanges during seawater-rock interactions obtained from studies of seafloor and ophiolitic rocks, and from experimental studies.

The recognition that seawater-rock reactions in the lower sheeted dikes and upper gabbros may determine vent fluid chemistry has been an important step forward. A next important

step is to investigate further the role played by phase separation of both magmatic and seawater-derived hydrothermal fluids in determining the chemistry of the fluids discharging at the sea floor. Vent fluid chemistry, analyses of fluid inclusions in altered rocks from the sea floor and from ophiolites, and experimental studies all suggest complex processes involving phase separation occur within the reaction zone [e.g. Berndt and Seyfried, 1990; Kelley et al, 1992, 1993; Von Damm et al, 1992; Butterfield and Massoth, 1994]. Additional studies and experiments are needed, in conjunction with the development of numerical models of two-phase flow of magmatic fluids and seawater, to critically examine this process and assess its importance.

The study of hydrothermal systems is still relatively young, and there are many fundamental questions that remain to be addressed. The upcoming years will provide a number of exciting opportunities to constrain the physical and chemical nature of hydrothermal circulation, and to refine our models of submarine hydrothermal processes.

Acknowledgments. I would like to express my gratitude to the many colleagues who provided their views on the most significant advances in their own fields of research, and offer my apologies to those whose work could not be included because of the brevity of this review. K. Becker and P. Saccocia reviewed and offered helpful comments on sections of this paper. Discussions with R. Detrick and K. Gillis were useful and are much appreciated. I gratefully acknowledge NSF grants OCE-9013150 and OCE-9314697 which in part supported this work. This is WHOI Contribution No. 8890.

References Alt, J. C , J. Honnorez, C. Laverne, and R. Emmermann, Hydrothermal

alteration of a 1 Km section through the upper oceanic crust, Deep Sea Drilling Project Hole 504B: mineralogy, chemistry, and evolution of seawater-basalt interactions, J. Geophys. Res., 91,10309-10335, 1986.

Alt, J. C , Subseafloor processes in mid-ocean ridge hydrothermal systems. In: S. Humphris. L. Mullineaux, R. Zierenberg and R. Thomson (eds): Physical, Chemical, Biological and Geological Interactions within Hydrothermal Systems, AGU Monograph, in press, 1994.

Alt, J. C , E. Zuleger, and J. Erzinger, Mineralogy and stable isotopic compositions of the hydrothermally altered lower sheeted dike complex, Hole 504B, Leg 140, In: J. Erzinger Becker, K., Dick, H.J.B., Stokking, L.B. et al. (Eds.), Proc. Ocean Drilling Program, Sci. Results, 137/140, College Station, TX, Ocean Drilling Program, in press, 1994.

Anderson, R. N., M. D. Zoback, S. H. Hickman, and R. L. Newmark, Permeability versus depth in the upper oceanic crust: in situ measurements in DSDP Hole 504B, eastern equatorial Pacific, / . Geophys. Res., 90, 3659-3669,1985.

Baker, E. T., A 6-year time series of hydrothermal plumes over the Cleft segment of the Juan de Fuca Ridge, / . Geophys. Res., 99, 4889-4904, 1994.

Baker, E. T., G. J. Massoth, and R. A. Feely, Cataclysmic venting on the Juan de Fuca Ridge, Nature, 329, 149-151,1987.

Baker, E. T., J. W. Lavelle, R. A. Feely et al., Episodic venting on the Juan de Fuca Ridge, / . Geophys. Res., 94, 9237-9250, 1989.

Becker, K., H. Sakai, C. Adamson et al., Drilling deep into young oceanic crust, hole 504B, Costa Rica Rift, Rev. Geophys., 27, 79-102, 1989.

Becker, K. Measurements of the permeability of the sheeted dikes in Hole 504B, ODP Leg 111. In: K Becker, H. Sakai and et al. (Eds), Proc. ODP, Sci. Results., I l l , College Station, TX, Ocean Drilling Program, 317-325, 1989.

Becker, K., In-situ bulk permeability of oceanic gabbros in Hole 735BB, ODP Leg 118, In: P.T. Robinson, R.P. Von Herzen and et al. (Eds), Proc. ODP, Sci. Results., 118, College Station, Texas, Ocean Drilling Program, 333-347, 1991.

Becker, K., R. H. Morin, and E. E. Davis, Permeabilities in the Middle Valley hydrothermal system measured with packer and flowmeter experiments, In: E.E. Davis, M.J. Mottl, A.T. Fisher and et al. (Eds.),


Proc. ODP, Sci. Results., 139, College Station, TX, Ocean Drilling Program, 613-626, 1994.

Berndt, M. E., and W. E. Sey fried Jr., Boron, bromine, and other trace elements as clues to the fate of chlorine in mid-ocean ridge vent fluids, Geochim. Cosmochim. Acta, 54, 2235-2245, 1990.

Berndt, M. E., and W. E. J. Seyfried, Calcium and sodium exchange during hydrothermal alteration of calcic plagioclase at 400°C and 400 bars, Geochim. Cosmochim. Acta, 57,4445-4451, 1993.

Bettison-Varga, L., R. J. Varga, and P. Schiffman, Relation between ore-. forming hydrothermal systems and extensional deformation in the

Solea graben spreading center, Troodos ophiolite, Geology, 20, 987-990, 1992.

Butterfield, D. A., and G. J. Massoth, Geochemistry of north Cleft segment vent fluids: temporal changes in chlorinity and their possible relation to recent volcanism, J. Geophys. Res., 99, B3, 4951-4968, 1994.

Cann, J. R., and M. R. Strens, Modeling periodic megaplume emission by black smoker systems, J. Geophys. Res., 94, 12227-12238, 1989.

Carlson, R. L., and C. N. Herrick, Densities and porosities in the oceanic crust and their variations with depth and age, J. Geophys. Res., 95, 9153-9170, 1990.

CASM, (Canadian American Seamount Expedition), Hydrothermal vents on an axial seamount on the Juan de Fuca Ridge, Nature, 313, 212-224, 1985.

Cathles, L. M., A capless 350°C flow zone model to explain megaplumes, salinity variations, and high-temperature veins in ridge axis hydrothermal systems, Econ. Geol., 88, 8, 1977-1988, 1993.

Chadwick, W. W., and R. W. Embley, Lava flows from a mid-1980s submarine eruption on the Cleft segment, Juan de Fuca Ridge, J. Geophys. Res., 99, B3, 4761-4776,1994.

Chadwick, W. W., Jr., R. W. Embley, and C. G. Fox, Evidence for volcanic eruption on the southern Juan de Fuca ridge between 1981 and 1987, Nature, 416-418, 1991.

Davis, E. E., D. S. Chapman, C. B. Forster, and H. Villinger, Heat-flow variations correlated with buried basement topography on the Juan de Fuca Ridge flank, Nature, 342, 533-537,1989.

Davis, E. E., D. S. Chapman, M. J. Mottl et al., FlankFlux: an experiment to study the nature of hydrothermal circulation in young oceanic crust, Can. J. Earth Sci., 29, 925-952,1992.

Delaney, J. R., V. Robigou, R. E. McDuff, and M. K. Tivey, Geology of a vigorous hydrothermal system on the Endeavour Segment, Juan de Fuca Ridge, J. Geophys. Res., 97, 19663-19682, 1992.

Detrick, R. S., P. Buhl, E. Vera et al., Multichannel seismic imaging of a crustal magma chamber along the East Pacific Rise, Nature, 326, 35-41, 1987.

Dick, H. J. B., J. Erzinger, L. B. Stokking, and et. al. Proc. ODP, Init. Repts., 140. College Station, TX: Ocean Drilling Program, 1992.

Embley, R. W., K. M. Murphy, and C. G. Fox, High-resolution studies of the summit of Axial Volcano, / . Geophys. Res., 95, B8, 12785-12812, 1990.

Embley, R. W., W. W. Chadwick, M. R. Perfit, and E. T. Baker, Geology of the northern Cleft segment, Juan de Fuca Ridge: Recent lava flows, sea-floor spreading, and the formation of megaplumes, Geology, 19, 771-775, 1991.

Embley, R. W., W. W. Chadwick, I. R. Jonasson et al., Initial results of the rapid response to the 1993 Co Axial event: Relationships between hydrothermal and volcanic processes, Geophys. Res. Lett., in press, 1994.

Embley, R. W. and W. W. Chadwick, Volcanic and hydrothermal processes associated with a recent phase of sea floor spreading at the northern Cleft segment: Juan de Fuca Ridge, J. Geophys. Res., 99, 4741-4760, 1994.

Fisher, A. T., K. Becker, T. N. Narasimhan et al., Passive, off-axis convection through the southern flank of the Costa Rica Rift, / . Geophys. Res., 95, 9343-9370, 1990.

Fisher, A. T., K. Becker, and T. N. Narasimhan, Off-axis hydrothermal circulation: Parametric tests of a refined model of processes at Deep Sea Drilling Project/Ocean Drilling Program Site 504, J. Geophys. Res., 99, 3097-3121, 1994.

Fouquet, Y., A. Wafik, P. Cambon et al., Tectonic setting and mineralogical and geochemical zonation in the Snake Pit sulfide deposit (Mid-Atlantic Ridge at 23°N), Econ. Geol., 88, 8, 2018-2036, 1993.

Fox, C. G., J. William W. Chadwick, and R. W. Embley, Detection of changes in ridge-crest morphology using repeated multibeam surveys, J. Geophys. Res., 97, 11149-11162, 1992.

Fox, C. G., W.E. Radford, R.P. Dziak, T.-K. Lau, H. Matsumoto and A.E. Schreiner, Acoustic detection of a seafloor spreading episode on the Juan de Fuca Ridge using military hydrophone arrays, Geophys. Res. Lett., in press, 1994.

Germanovich, L. N. , and R. P. Lowell , Percolation theory, thermoelasticity, and discrete hydrothermal venting in the Earth's crust, Science, 255, 1564-1567, 1992.

Gillis, K. M., and P. T. Robinson, Patterns and processes of alteration in the lavas and dykes of the Troodos Ophiolite, Cyprus, J. Geophys. to., 95, 21523-21548, 1990.

Gillis, K. M., and G. Thompson, Metabasalts from the Mid-Atlantic Ridge: new insights into hydrothermal systems in slow-spreading crust, Contrib. Mineral. Petrol, 113, 502-523, 1993.

Gillis, K. M., G. Thompson, and D. S. Kelley, A view of the lower crustal component of hydrothermal systems at the Mid-Atlantic Ridge, / . Geophys. Res., 98, 19597-19619, 1993.

Goodfellow, W. D., and J. M. Franklin, Geology, mineralogy, and chemistry of sediment-hosted clastic massive sulfides in shallow cores, Middle Valley, northern Juan de Fuca Ridge, Econ. Geol, 88, 8, 2037-2068, 1993.

Hammond, S. R., Relationships between lava types, seafloor morphology, and the occurrence of hydrothermal venting in the ASHES vent field of Axial Volcano, J. Geophys. Res., 95, B8, 12875-12893, 1990.

Hart, S. R., J. Blusztajn, H. J. B. Dick, and J. R. Lawrence, Fluid circulation in the oceanic crust: Contrast between volcanic and plutonic regimes, J. Geophys. Res., 99, B2, 3163-3173,1994.

Haymon, R. M., D. J. Fornari, M. H. Edwards et al., Hydrothermal vent distribution along the East Pacific Rise crest (9°09'-54'N) and its relationship to magmatic and tectonic processes on fast-spreading mid-ocean ridges, Earth Planet. Sci. Lett., 104, 513-534,1991.

Haymon, R. M., D. J. Fornari, R. A. Lutz et al., 1991 eruption site on the East Pacific Rise at 9°45'-52'N is evolving rapidly: results of AdVenture '92 dive series, RIDGE Events Newsletter, 3, (2), 1-2, 1992.

Haymon, R. M., D. J. Fornari, K. L. Von Damm et al., Volcanic eruption of the mid-ocean ridge along the East Pacific Rise crest at 9°45-52'N: Direct submersible observations of seafloor phenomena associated with an eruption event in April, 1991, Earth Planet. Sci. Lett., 119, 85-101, 1993.

Hekinian, R., D. Bideau, J. Francheteau et al., Petrology of the East Pacific Rise crust and upper mantle exposed in Hess Deep (eastern equatorial Pacific), J. Geophys. Res., 98, 8069-8094, 1993.

Jacobson, R. S., Impact of crustal evolution on changes of the seismic properties of the uppermost ocean crust, Reviews of Geophysics, 30, 23-42, 1992.

Johnson, H. P., and R. W. Embley, Axial Seamount: an active ridge axis volcano on the Central Juan de Fuca Ridge, J. Geophys. Res., 95, B8, 12689-12696, 1990.

Johnson, H. P., K. Becker, and R. Von Herzen, Near-axis heat flow measurements on the Northern Juan de Fuca Ridge: Implications for fluid circulation in oceanic crust, Geophys. Res. Lett., 20, 1875-1878, 1993.

Johnson, H. P., and S. W. Semyan, Age variation in the physical properties of oceanic basalts: implications for crustal formation and evolution, J. Geophys. Res., 99, B2, 3123-3134, 1994.

Karson, J. A., and P. A. Rona, Block-tilting, transfer faults, and structural control of magmatic and hydrothermal processes in the TAG area, Mid-Atlantic Ridge 26°N, Geol Soc. Am. Bull, 102, 1635-1645, 1990.

Kelley, D. S., P. T. Robinson, and J. G. Malpas, Processes of brine generation and circulation in the oceanic crust: Fluid inclusion evidence from the Troodos Ophiolite, Cyprus, J.Geophys. Res., 97, 9307-9322, 1992.

Kelley, D. S., K. M. Gillis, and G. Thompson, Fluid evolution in submarine magma-hydrothermal systems at the Mid-Atlantic Ridge, J. Geophys. Res., 98, 19579-19596, 1993.

Koski, R. A., I. R. Jonasson, D. C. Kadko et al., Composition, growth mechanisms, and temporal relations of hydrothermal sulfide-sulfate-silica chimneys at the northern Cleft segment, Juan de Fuca Ridge, / . Geophys. Res., 99, B3, 4813-4832, 1994.

Lalou, C , E. Brichet, and G. Thompson, Radionuclide gradients in two Mn oxide deposits from the Mid-Atlantic Ridge: possible influence of a hydrothermal plume, Can. Mineral, 26, 713-720,1988.

Lalou, C , G. Thompson, M. Arnold et al., Geochronology of TAG and Snake Pit hydrothermal fields, Mid-Atlantic Ridge: Witness to a long


and complex hydrothermal history, Earth Planet. Sci. Lett., 97, 113-128, 1990.

Lalou, C , J. L. Reyss, E. Brichet et al., New age data for Mid-Atlantic Ridge hydrothermal sites: TAG and Snake Pit geochronology revisited, J. Geophys. Res., 98, 9705-9713, 1993.

Langmuir, C. H., D. Fornari, D. Colodner et al., Geological setting and characteristics of the Lucky Strike vent field at 37°17'N on the Mid-Atlantic Ridge, EOS, Trans. Am. Geophys. Union, 74, 99, 1993.

Langseth, M. G., M. J. Mottl, M. A. Hobart, and A. T. Fisher, The distribution of geothermal and geochemical gradients near site 501/504: Implications for hydrothermal circulation in the oceanic crust. In: K. Becker, H. Sakai and et al. (Eds), Proc. ODP, Init. Rep. (Pt. A), 111, College Station, TX, Ocean Drilling Program, 23-34, 1988.

Larson, R. L., A. T. Fisher, R. D. Jarrard, K. Becker and ODP Leg 144 Shipboard Scientific Party, Highly permeable and layered Jurassic crust in the western Pacific. Earth Planet. Sci. Lett., 119, 71-83,1993.

Lawrence, M., and J. M. Edmond, Stability of hydrothermal systems: 21°N, TAG/MARK, Guaymas Basin, EOS, Trans. Amer. Geophys. Un.,73, 43, 525-526, 1992.

Lister, C. R. B., An explanation for the multivalent heat transport found experimentally for convection in a porous medium, J. Fluid Mech., 214, 287-320, 1990.

Lowell, R. P., P. Van Cappellen, and L. N. Germanovich, Silica precipitation in fractures and the evolution of permeability in hydrothermal upflow zones, Science, 260, 192-194, 1993.

Lowell, R. P. and L. N. Germanovich, On the temporal evolution of high-temperature systems at ocean ridge crests, J. Geophys. Res., 99, 565-575,1994.

Massoth, G. J., E. T. Baker, J. E. Lupton et al., Temporal and spatial variability of hydrothermal manganese and iron at Cleft segment, Juan de Fuca Ridge, J. Geophys. Res., 99, 4905-4923,1994.

M6vel, C , and M. Cannat, Lithospheric stretching and hydrothermal processes in oceanic gabbros from slow spreading ridges. In: T.J. Peters, A. Nicolas and R.G. Coleman.(Eds), Evolution of the Oceanic Lithosphere, Kluwer Academics, Hingham, MA, pp. 93-312, 1991.

Mottl, M. J. Hydrothermal convection, reaction, and diffusion in sediments on the Costa Rica Rift flank: Pore-water evidence from ODP Sites 677 and 678. In: K. Becker, K. Sakai and et al. (Eds), Proc. ODP., Sci. Results, 111, College Station, TX, 195-213,1989.

Murton, B. J., G. Klinkhammer, C. L. Van Dover et al., Direct measurements of the distribution and occurrence of hydrothermal activity between 27°N and 30°N on the Mid-Atlantic Ridge, EOS, Trans. Amer. Geophys. Un., 74, 99, 1993.

Nehlig, P., T. Juteau, V. Bendel, and J. Cotten, The root zones of oceanic hydrothermal systems: Constraints from the Samail ophiolite (Oman), J. Geophys. Res., 99, 4703-4713,1994.

Pezard, P. A., Electrical properties of mid-ocean ridge basalt and implications for the structure of the upper oceanic crust in Hole 504B, J. Geophys. Res., 95, 9237-9266, 1990.

Rona, P. A., Y. A. Bogdanov, E. G. Gurvich et al., Relict hydrothermal zones in the TAG hydrothermal field, Mid-Atlantic Ridge 26°N , 45°W, J. Geophys. Res., 98, 9715-9730,1993a.

Rona, P. A., M. D. Hannington, C. V. Raman et al., Active and relict seafloor hydrothermal mineralization at the TAG hydrothermal field, Mid-Atlantic Ridge, Econ. Geol., 88, 8, 1989-2017, 1993b.

Rona, P. A., and S. D. Scott, A special issue on sea-floor hydrothermal mineralization: new perspectives, Econ. Geol., 88, 8, 1935-1975, 1993.

Rosenberg, N. D., F. J. Spera, and R. M. Haymon, The relationship between flow and permeability field in sea floor hydrothermal systems, Earth Planet. Sci. Lett, 116,135-153,1993.

Saccocia, P. J., K. Ding, M. E. Berndt et al., Experimental and theoretical perspectives on crustal alteration at mid-ocean ridges, In: D. Lentz (Ed.) Alteration and Alteration Processes Associated with Ore-Forming Systems., 11, Geological Association of Canada, Short Course Notes, 403-431,1994.

Seewald, J. S., and W. E. Seyfried Jr., The effect of temperature on metal mobility in subseafloor hydrothermal systems: constraints from basalt alteration experiments, Earth Planet. Sci. Lett., 101, 388-403, 1990.

Seyfried Jr., W. E., K. Ding, and M. E. Berndt, Phase equilibria constraints on the chemistry of hot spring fluids at mid-ocean ridges, Geochim. Cosmochim. Acta, 55, 3559-3580, 1991.

Sinton, J. M., and R. S. Detrick, Mid-ocean ridge magma chambers, J. Geophys. Res., 91, 197-216, 1992.

Stakes, D., C. Mevel, M. Cannat, and T. Chaput, Metamorphic stratigraphy of Hole 735B, In: R.P. Von Herzen and P.T. Robinson et al. (Eds.), Proc. ODP Sci. Results., 118, College Station, TX, Ocean Drilling Program, 153-180, 1991.

Stein, C. A., and S. Stein, Constraints on hydrothermal heat flux through the oceanic lithosphere from global heat flow, J. Geophys. Res., 99, B2, 3081-3095, 1994.

Thompson, G., M. J. Mottl, and P. A. Rona, Morphology, mineralogy, and chemistry of hydrothermal deposits from the TAG area, 26°N Mid-Atlantic Ridge, Chem. Geol., 49, 243-257, 1985.

Thompson, G., S. E. Humphris, B. Schroeder et al., Active vents and massive sulfides at 26°N (TAG) and 23°N (Snake Pit) on the Mid-Atlantic Ridge, Can. Mineral., 26, 697-711,1988.

Tivey, M. K., S. E. Humphris, G. Thompson et al., Deducing patterns of fluid flow and mixing within the TAG active mound using mineralogical and geochemical data, J. Geophys. Res., in press, 1994.

Vanko, D. A., and D. S. Stakes, Fluids in oceanic layer 3: evidence from veined rocks, Hole 735B, Southwest Indian Ridge, In: R.P. Von Herzen and P.T. Robinson et al. (Eds.), Proc. ODP Sci. Results., 118, College Station, TX, Ocean Drilling Program, 181-215,1991.

Vanko, D. A., J. D. Griffith, and C. L. Erickson, Calcium-rich brines and other hydrothermal fluids in fluid inclusions from plutonic rocks, Oceanographer Transform, Mid-Atlantic Ridge, Geochim. Cosmochim. Acta, 56, 35-48, 1992.

Von Damm, K. L., Seafloor Hydrothermal Activity: Black Smoker Chemistry and Chimneys, Ann. Rev:Earth Planet. Sci., 18, 173-204, 1990.

Von Damm, K. L., D. C. Colodner, and H. N. Edmonds, Hydrothermal fluid chemistry at 9-10°N EPR '92: big changes and still changing, EOS, Trans. Amer. Geophys. Un., 73, 524, 1992.

Wheat, C. G., and M. J. Mottl, Hydrothermal circulation, Juan de Fuca Ridge eastern flank: Factors controlling basement water composition, J. Geophys. Res., 99, B2, 3067-3080, 1994.

Wilkens, R. H., G. J. Fryer, and J. Karsten, Evolution of porosity and seismic structure of upper oceanic crust: Importance of aspect ratios, J. Geophys. Res., 96, 17981-17995, 1991.

Zonenshain, L. P., M. I. Kuzmin, A. P. Lisitsin et al., Tectonics of the Mid-Atlantic rift valley between the TAG and MARK areas (26-24°N): evidence for vertical tectonism, Tectonophysics, 159, 1-23, 1989.

S. E. Humphris, Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543. (e-mail: susan @ copper, whoi .edu)

(Received June 20, 1994; revised December 22,1994: accepted January 5, 1995)

Documents

Hydrothermal processes at mid-ocean ridges