The presence of serpentinites on the seafloor has been documented for a very long time. On the basis of this occurrence and of the seismic structure, Hess proposed [73] that oceanic layer 3 consists of a hydrated mantle, i.e. serpentinites, and that the uniform thickness of the ocean crust corresponded to the depth of the 500 °C isotherm at mid-ocean ridges. The increasing knowledge of the oceanic lithosphere has demonstrated that this picture of the ocean crust was certainly extreme, but that serpentinites do represent a significant portion of the crust.

All the abyssal peridotites that crop out on the seafloor are more or less serpentinized. They commonly occur in slow to ultraslow spreading ridge environments. In the early days, when the topography of ridges was not well known, peridotites were dredged mostly from major scarps, which were essentially transform fault walls [22,23,27,28,100], although some also came from the axial valley [7]. It was also recognized that these ultramafic occurred in the Atlantic and Indian oceans, i.e. at slow spreading ridges, but this could be attributed to the higher density of fracture zones in this environment. It was argued that hydration of the underlying mantle peridotites was made possible by the presence of large faults, driving seawater at depth and the outcropping of serpentinites was commonly ascribed to diapiric emplacement, due to the low density of serpentine (2.5 g/cm³) with respect to the average density of the ocean crust (2.8 g/cm³) [7,27,28]. It is only during the last 15 years, when the knowledge of the mid-ocean ridge structure improved, that a general understanding for the location of peridotites outcrops has emerged, and it has been recognized that at slow to ultraslow spreading ridges, serpentinized peridotites are an important component of the crust.

Serpentinites also occur in other tectonic settings at the seafloor. They have been recorded at passive margins. Serpentinized peridotites were first documented at the Galicia margin where they form a few km wide and a hundred km long ridge, at the continent-ocean boundary [18]. Further work showed that this ridge is a major feature, which continues south to the Iberian margin [12]. This serpentinite ridge is interpreted as resulting from mantle exhumation due to crustal thinning during the initial stage of rifting. At the continent-ocean transition, the tectonic activity is inferred to favour seawater penetration and serpentinization [20,21]. The serpentinites exposed at the Galicia and Iberia margins have been drilled by the Ocean Drilling Program during Leg 103 [19] and 149 [134] respectively. A similar mechanism has been invoked to explain the occurrence of serpentinized peridotites at the Tyrrhenian margin, drilled during ODP leg 107 [31]. A similar occurrence of serpentinized peridotites [102] at the southwest Australian margin is interpreted as resulting from the rifting between Australia and Antarctica [13]. However, serpentinites also occur at active margins. They have been dredged at the Tonga trench [61] and the Mariana trench [60]. Subsequent work at the Mariana trench showed that sheared serpentinites crop out all along the trench in a chain of seamounts. The exposure of these serpentinites is clearly associated with subduction processes and is thought to result from diapiric emplacement, either from material from the subducting plate [16] or from the mantle wedge [67]. One of these seamounts was drilling during ODP leg 125 [68].

This review paper concentrates on serpentinites collected at, and in the vicinity of mid-ocean ridges, i.e. where their formation is related to accretionary processes. The mechanisms responsible for the exposure of serpentinites at the ocean floor in this context are discussed. A general description of typical ocean floor serpentinites is presented. The conditions of serpentinization (temperature, composition of the fluid phase) are evaluated. Our present knowledge of the consequences of serpentinization on the chemical composition of the crust, and on the physical properties of the ocean lithosphere is presented. Finally, the two major types of serpentinized peridotite hosted hydrothermal fields are described and their relation with serpentinization processes are discussed.

It is now well established that, in the global ridge system, serpentinites crop out essentially in slow to ultraslow spreading ridges [34]. In this environment, not only the amount of melting is low, but the lithosphere tends to be thicker. Melt delivery to the surface is episodic and tends to be focused at segment centres [49]. Moreover, the thick lithosphere allows tectonic activity and the development of deep rooted faults. Serpentinized peridotites are clearly not restricted to transform fault scarps, but also occur along the axial valley walls and even at the axial valley floor. A number of field studies suggests that, at least at segment ends, oceanic layer 3 may consist of more or less serpentinized peridotites intruded by gabbro pockets and directly overlain by basalts. This has been documented at the slow spreading Mid-Atlantic Ridge (MAR) [36,71,81,88], at the ultraslow spreading South West Indian Ridge (SWIR) [8,49,97] and more recently at the slowest ridge of the global system, the Gakkel ridge in the Arctic [98]. A thick gabbro layer as well as a dike complex are often missing. At a Penrose conference in 1972 [110], combining seismic studies, results from seafloor dredging and observations in ophiolite complexes, a layered structure was proposed for the ocean lithosphere (Fig. 1). According to the Penrose model, beneath a sediment cover, the oceanic crust is composed of magmatic rocks. Layer 2 consists of basaltic rocks, either extruded on the seafloor as pillow-lavas or intruded as dikes. Layer 3 consists of gabbroic rocks crystallized at depth. The seismically defined MOHO, located at about 6 km beneath the seafloor, separates the crust from the underlying mantle (Layer 4) consisting in peridotites, residues of partial melting. The crust formed at a slow and ultraslow spreading ridge clearly does not fit the Penrose model. It does not have a layered structure but is heterogeneous and its architecture varies along its axis [36] (Fig. 2). The outcropping of residual peridotites at segment ends is likely to result from the melt focusing at segment centres. In the absence or paucity of basaltic melt, plate divergence is accommodated by advection of asthenospheric mantle to the lithosphere and by tectonic processes that result in the exposure of rocks from the mantle. The tectonic activity also favours seawater penetration and alteration of the mantle peridotites. The layered and homogeneous seismic structure that is observed everywhere can be explained by the fact that gabbros and partially serpentinized peridotites have the same seismic velocities [34] (Fig. 1, see Section 6). This suggests that when serpentinized peridotites crop out at the surface, the MOHO does not correspond to the magmatic crust/residual peridotite boundary, but could be a hydration boundary, i.e. the boundary between serpentinized peridotites and fresh peridotites [73].

More recently, very low angle faults, i.e. megamullions or detachment faults, that accommodate spreading for long periods of time (i.e. over a million years), have been discovered at slow spreading ridges [33,91,123,127]. These long lived faults expose large portions of the oceanic lithosphere, and serpentinites as well as talcschists commonly crop out on their surfaces.

This new perspective on accretionary processes at slow spreading ridges suggests that serpentinized peridotites make up a significant portion of the so-called oceanic ‘crust’ at slow to ultraslow spreading ridges, although at this stage, it is still difficult to quantify. On the basis of geological observations, Cannat et al. [36] tentatively suggest that exposure of serpentinites could represent 20% of the seafloor, but in volume they would not exceed 5–15% of the crust. On the basis of seismic data, Carlson [37] comes to the estimate of 5% maximum. Further work is undoubtedly needed for a better understanding.

By contrast, in fast spreading ridges, most evidence suggests that the crust is layered and consists of extrusive basalts, a dike complex and a massive gabbro layer overlying mantle peridotites (Fig. 1). In this case, the MOHO is interpreted as the transition between a magmatic crust and the residual mantle peridotites [117]. Peridotites occur only below the MOHO, i.e. at depths $>6$ km. Given this general structure, major tectonic movements are necessary to expose them at the seafloor. Outcrops of serpentinized peridotites have been documented only in very particular areas: major transform faults (Garrett [14]), trenches opening at the boundary of rotating microplates, such as Easter and Juan Fernandez [46], and the tip of propagators such as Hess Deep [63]. In this case, as opposed to slow spreading environments, serpentinized peridotites do not represent a significant portion of the crust. The presence of serpentinized ultramafics beneath the gabbro layer cannot be completely ruled out, but serpentinization must then occur off axis, when the temperature has dropped below 500 °C.

Sample suites of serpentinized peridotites from all these environments have led to various types of studies (mineralogy, petrology, geochemistry, physical properties). The most comprehensive data sets come from sites drilled by the Ocean Drilling Program (ODP). Two adjacent sites (670 and 920) are located at the Mid-Atlantic Ridge, in the western wall of the axial valley of the MARK area (23°N) and represent serpentinites from a slow spreading environment [35,48]. Site 895 is located on the intrarift ridge of Hess Deep, which exposes rocks generated at the fast spreading East Pacific Rise [70]. The other samples which are referred to in this paper have been either dredged or collected with submersibles.

Abyssal peridotites collected from the seafloor consist primarily of harzbugites, with minor dunites and lherzolites. They are always between partly and completely altered. Altered peridotites display various appearances in colour: black, dark or light green, red, yellow. Serpentinites are generally black or green, while yellowish to reddish rock contain abundant clay minerals. This section concentrates on serpentinites sensu stricto, and synthesizes observations made on different sample sets [7,14,47,50,65,74,84,87,96,100,101,111].

3.1 Textures

3.2 Mineralogy

The serpentinization of abyssal peridotites results from their interaction with large volumes of a hydrous fluid, at temperatures below 500 °C, maximum stability of serpentine minerals [32], although a number of studies suggests that seawater penetration in the peridotites may start at a higher temperature [14,84].

Although the serpentinites are presently exposed on the seafloor, a number of factors indicate that at least part of the serpentinization occurred at depth, when the peridotites were cooling down. In this section, the conditions of serpentinization, i.e. the pressure, temperature, oxygen fugacity as well as the nature and composition of the fluid phase will be discussed. It is important to keep in mind that serpentinites are polyphased rocks that record successive stages of crystallization. This section will essentially consider processes associated with the peak of crystallization, i.e. the replacement of olivine by the mesh textured serpentine and replacement of pyroxenes by bastite. Determining the temperature and pressure of serpentinization is not an easy task, because serpentine minerals are stable over a wide range of temperatures (between 500 °C and room temperature) and because reaction producing serpentines are relatively insensitive to pressure.

4.1 Pressure of serpentinization

4.2 Temperature of serpentinization

4.3 Nature and composition of the altering fluid phase

The first and major consequence of serpentinization is hydration. Serpentine minerals contain over 12% water. In serpentinites, they represent the major phase and the amount of water has often been used as a proxy for the degree of serpentinization. The second consequence is oxidation of iron due to the formation of magnetite by reaction (3). However, there has been a number of debates on whether, besides these two effects, serpentinization has a chemical consequence on the bulk chemistry of the rocks. A good summary of these debates is given by O'Hanley [104]. The essence of the problem is the question of volume, because serpentinite has a much lower density than a fresh peridotite (2.5 g/cm³ versus 3.3 g/cm³). If serpentinization occurs at constant volume, the decrease in density must be accompanied by a loss of chemical elements. By contrast, if the chemical elements remain constant, then there must be a volume increase to account for the decrease in density.

The problem of volume increase due to hydration has not yet been completely solved. Most ocean floor serpentinites have been formed under static conditions; serpentine pseudomorphing primary minerals preserve the details of the original texture, and does not favour a volume increase at the crystal scale. At the sample scale, however, vein networks are commonly reported and can account for a volume expansion. From observations in continental serpentinites, O'Hanley [103] proposes that progressive serpentinization produces a kernel pattern than can account for volume expansion. This model could apply to oceanic serpentinites, but, unfortunately, no such detailed outcrop observations are available.

The problem in evaluating chemical fluxes is that the fresh protolith is never available for comparison with the altered rock. Although peridotites are relatively homogeneous in terms of mineral assemblages, the mineral compositions may vary widely, reflecting the amount of melting as well as possible reaction with percolating melts. Therefore there is no ‘standard composition’ that could be used as a reference.

5.1 Major elements

5.2 Trace elements

5.3 Discussion

Due to massive hydration and crystallization of secondary phases, the physical properties of abyssal peridotites are strongly modified by serpentinization. A summary of the major consequences are given in this section.

The average density of a fresh peridotite is approximately 3.3 g/cm³. Serpentine minerals have a density of approximately 2.5 g/cm³. Serpentinization is therefore responsible for a strong decrease in density. A compilation of density measurements of serpentinites [99] shows that, as expected, there is an inverse correlation between density and the extent of serpentinization (Fig. 8A). As discussed in Section 5, serpentinization does not seem to be accompanied by major chemical fluxes, and the consequence is an increase of volume. In the extensional environment of a mid-ocean ridge, this volume increase is likely accommodated by tectonic activity, as evidenced by vein networks.

The decrease in density affects the seismic velocities. Following the pioneer work of Christensen [43,44], all the seismic velocities measured on serpentinites have shown a strong dependence with the degree of serpentinization. A compilation [99] shows an inverse relation between the amount of serpentinization and seismic velocities (Fig. 8B). A partially serpentinized peridotite may therefore have the same seismic velocities as a gabbro [37,77], and a seismically layered oceanic crust may be compositionally heterogeneous (Fig. 1) [34]. Because observations and sampling on the seafloor are necessarily localized, evaluating the proposition of serpentinites in the crust is an arduous ask [37]. If serpentinized peridotites, which are residual rocks from the mantle, crop out at the seafloor, this raises the question of the significance of the MOHO, classically interpreted as the transition between magmatic, mafic rocks and residual, ultramafic rocks (see Section 2) [34]. This sharp seismic boundary could correspond to a hydration front, i.e. the transition between partially serpentinized peridotites and fresh peridotites, corresponding to an increase in density. Because serpentinization occurs at temperatures below 500 °C, this hydration front may also correspond to a thermal boundary, i.e. the 500 °C isotherm [73].

The magnetic properties of ocean floor peridotites are also strongly affected by serpentinization because it produces secondary magnetite [52]. Therefore, compared to a fresh peridotite, a serpentinite has a high magnetic susceptibility and a ferromagnetic behaviour [126]. Because of the crystallization of secondary magnetite, it also acquires a natural remanent magnetization (NRM) which can contribute to the magnetic anomalies of the seafloor [52]. At slow spreading ridges, it has been recognized that the off-axis traces of inside corner highs – massifs located at the intersection between a fracture zone and the ridge axis – are commonly marked by more positive magnetization, attributed to the occurrence of serpentinites [109,127]. Moreover, the skewness of magnetic anomalies on the seafloor has been ascribed to the contribution of serpentinized peridotites [53,54]. To interpret magnetization maps of the seafloor it is therefore essential to consider the possible presence of serpentinites.

A detailed study of the magnetic properties of oceanic serpentinites [108] has revealed the complexity of magnetite formation and how it affects the magnetic properties. The magnetic susceptibility is directly correlated to the amount of magnetite. However, this amount does not increase linearly with the rate of serpentinization as initially proposed [15] because the partitioning of iron between serpentine minerals and magnetite varies with the extent of serpentinization. The authors have shown that it remains modest at serpentinization rates below 75% because the iron partly enters serpentine minerals (up to 6% FeO). At higher serpentinization rates, the iron content of serpentine minerals decreases (2–3%) and more magnetite forms. They also show that the size of magnetite grains influences the NRM. The small sized magnetite grains associated with serpentine in mesh texture produce NRM as high as basalts. By contrast, in the case of large magnetite crystals, their occurrence as irregular aggregates likely promotes strong magnetostatic interactions between the grains and as a result reduces their coercitivity. Finally, low temperature oxidation of magnetite produces maghemite, and drastically decreases the magnetic susceptibility and NRM of serpentinites. In conclusion, this study confirms that serpentinites have generally high magnetic susceptibility and NRM and can contribute to magnetic anomalies. It demonstrates, however, that the modification in the magnetic properties of serpentinites is not linearly related to the rate of serpentinization. It is still necessary to better understand what controls the formation of magnetite to be able to predict the magnetic properties of the serpentinites peridotites in the ocean crust.

A last major effect of serpentinization concerns rheology which in turn affects the tectonic activity of the ocean lithosphere. Deformation experiments show that serpentinites, particularly when made of lizardite, are weaker than the other components of the oceanic lithosphere and display brittle deformation without dilatancy [55,56,112]. A small degree of serpentinization (<15%) reduces the strength of the peridotite to that of serpentine, while displaying the characteristic behaviour of pure serpentinites (e.g. non dilatant brittle deformation). The presence of lizardite-rich serpentinites in the crust may therefore strongly influence its strength and tectonics [55,57].

The fact that serpentinization could affect the chemistry of the water column was first documented in the 1990s at the Mid-Atlantic Ridge. Specific anomalies, characterized by high methane concentrations together with low TDM (total dissolved manganese) contents and turbidity were identified and shown to be associated with serpentinite outcrops [39,40,42]. Serpentinization produces H₂ because of the oxidation of iron during the formation of magnetite using the oxygen from the aqueous fluid phase [64,101]. Calculated ultramafic hosted vent fluid compositions predict abundant H₂ concentrations, as opposed to basalt hosted vent fluids [131], in agreement with the observations. The production of methane is attributed to Fischer–Tropsch type reactions such as:

The formation of methane as well as other hydrocarbon components during serpentinization have been reproduced experimentally at 300 °C, 500 bars [10].

Further evidence for the existence of hydrothermal activity associated with serpentinites is the discovery of black smokers hosted on serpentinized peridotites. The Logatchev field was first discovered at 14°45′N latitude [17], followed by the Rainbow field at 36°14′N latitude [62] on the Mid-Atlantic Ridge. Although they are built on serpentinites, these two hydrothermal fields share a number of characteristics with hydrothermal fields hosted in basalts (Table 2), and support biological communities. The fluids vent at high temperatures (>300 °C) and they build up sulfide chimneys. They have low pH, high H₂S and metal contents. However, they are characterized by high H⁺ contents, consistent with the production of hydrogen during serpentinization [51]. Moreover, the presence of complex hydrocarbon molecules [75] – similar to those produced experimentally [10] – has been documented at Rainbow. This could be of great biological importance. Not only can methane support microbial communities [79,92], but also complex hydrocarbons are considered as prebiotic molecules which could have played a role in the appearance of life on Earth. There are some differences between the two fields, however. Hydrothermal fluids from the Rainbow field yield very high chlorinities, metal and REE concentrations, which could reflect phase separation [51]. Another field, the Saldanha hydrothermal field has also been discovered at 36°30N latitude on the Mid-Atlantic ridge, at the southern tip of the FAMOUS segment. Active vents discharge clear warm fluids through sediments, but no data on fluid compositions are yet available [11]. It should be pointed out that the black smoker type of vents generate turbidity in the water column. Therefore, they cannot account for the methane anomalies not associated with TDM and turbidity documented in the water column [39] in the vicinity of serpentinite exposures.

The most recent discovery was the Lost City hydrothermal field [82]. This field is located off axis (1.5 Myear old crust) at 30°N, at the corner intersection between Atlantis fracture zone and the M.A.R. The fluids vent at low temperature (40–75 °C) and build spectacular hydrothermal chimneys (up to 60 m high) made up essentially of carbonates and brucite. The fluid compositions contrast with those from the Rainbow and Logachev sites (Table 2). In particular, they are characterized by high MgO and high pH, and low H₂S contents. The associated serpentinites indicate crystallization temperature of around 200 °C [66]. This field definitely differs from the two others both in temperature and chemistry of the fluids and associated deposits. Abundant microbial activity is associated with fluid venting and mineral precipitation [83]. In this case, the venting fluids could generate the methane anomalies not associated with turbidity that are commonly in the water column, in association with serpentinite outcrops.

Another evidence for hydrothermal activity was first documented by Bonatti et al. [29]. It consists of massive sepiolite deposits which are ascribed to the precipitation from a fluid phase resulting from serpentinization. The same type of deposits occurs at the western portion of the SWIR [8]. The presence of sepiolite on the ocean floor could therefore be used as an indicator of serpentinization at depth.

Hydrothermal activity requires heat and different possible heat sources can be invoked [8]: (i) latent heat released by crystallization of basaltic magma intrusions in the lithospheric mantle; (ii) cooling heat mined from the lithospheric mantle; (iii) heat released by exothermic serpentinization reactions [69]. The question is whether the only heat released by serpentinization reactions can activate hydrothermal circulation and can account for fluid temperatures as high as 350 °C. Lowell and Rona [89] show that the heat released depends both on the serpentinization rate and the mass flow rate. They calculate that, in the absence of a regional background heat flux, serpentinization rates need to be very high (10³ kg/s) and flow rates low (10 kg/s) to generate hydrothermal temperatures above 100 °C. Based on experimental diffusion rates, Mac Donald and Fyfe [90] evaluated that serpentinization rates are likely in the order of 0.1 kg/s. Even if this number can be increased if permeability generated by tectonic activity is considered, it remains extremely low compared to the value required. If a regional background heat flux is considered, the necessary rate of serpentinization decreases and becomes more realistic. But to generate hydrothermal temperatures of 300 °C, a high temperature regime, typical of magmatic systems at mid-ocean ridges is necessary. Lowell and Rona conclude that high temperature hydrothermal fields such as Rainbow require magmatic heat to drive the system. By contrast, the Lost City field can be driven by the heat released by serpentinization reactions. Provided that seawater can penetrate at depth, hydrothermal systems generated by serpentinization reactions are likely to be a significant component of hydrothermal activity at slow spreading ridges and generate anomalies in the water column. Bach et al. [8] came to the same conclusion. Assuming a heat capacity of seawater of 4 J g⁻¹ K⁻¹ and a water/rock ratio of 1, they calculate that serpentinization can heat up the circulating water by 25 to 150 °C. This can account for the range of temperatures measured at the Lost City type of hydrothermal field as well as for the formation of sepiolite deposits, but not for the Rainbow type of field.

These results suggest that black smoker vent fields (Rainbow type) can occur only at the ridge axis, where magmatic instrusions can provide heat to activate the hydrothermal circulation. Because the heat generated by serpentinization reactions can activate the Lost City type of vent field, such a type of hydrothermal activity can theoretically occur everywhere, either beneath the axial valley or off-axis. It only requires that seawater penetrates into unaltered peridotites. This can happen off-axis if tectonic activity generates permeability though fractures and faults. At this stage, we know that serpentinization is active at the axis since all the peridotites collected in the axial valley are serpentinized. However, we lack appropriate data to evaluate the intensity of off-axis serpentinization. Searching for methane and/or hydrogen anomalies in the water column off-axis would probably be the best strategy.

Recent developments on the understanding of the structure of mid-ocean ridges have demonstrated that serpentinites make up a significant portion of the seismically-defined crust. Therefore, the MOHO does not systematically correspond to the boundary between a magmatic crust and a residual mantle. In specific areas, particularly at the end of ridge segments generated in slow to ultraslow spreading environments, it may instead correspond to a hydration boundary, matching the 500 °C isotherm under the ridge axis. This has a major implication when evaluating the magmatic fluxes at mid-ocean ridges. The latter are clearly overestimated when the whole crustal thickness is considered. It is therefore of major importance to be able to quantitatively evaluate the proportion of serpentinized peridotites in the crust. Besides direct geological observation and sampling through dredging, diving and ocean drilling, it is necessary to develop tools, using indirect measurements such as seismic velocity profiles and/or magnetic properties.

Serpentinite fabrics show that most of the serpentinization occurs under static conditions. Determinations of serpentine species show the predominance of lizardite. By contrast, mineral associations as well as oxygen isotope fractionation demonstrate that serpentinization reactions start at high temperature, in the range of 300–500 °C, even if lower temperatures are also documented. This suggests that the nature of the serpentine species may be controlled by factors other than temperature. High temperature serpentinization likely starts at depth, at the ridge axis, by interaction of seawater or seawater derived fluids with cooling peridotites. The lack of evidence for a massive leaching of major elements favours a compensation of the decrease in density by a volume increase. Because serpentinization requires a major flux of water, this raises the question of fluid penetration. The extensional environment must generate permeability through normal faulting, fissuring and cracking. At temperatures below 500 °C, the availability of water must be the limiting factor for extensive serpentinization.

The serpentinization of essentially anhydrous peridotites results in a massive hydration and oxidation. Major elements do not seem to be strongly affected by serpentinization, although the local rodingitization of associated gabbroic dikes and formation and talcshists require the mobilization of calcium and silicium. The global chemical fluxes between peridotite and seawater, however, remain poorly constrained. They seem to be dependent on the temperature and the mineral association. In particular, the presence and degree of alteration of pyroxene is likely of major importance, because this mineral concentrates a large part of the trace and REE of the fresh peridotite. In the present state of knowledge, it is safe to state that serpentinites are enriched in water, chlorine, fluorine, boron, sulfur, δ³⁴S, ⁸⁷Sr compared with fresh peridotites. The subduction of heterogeneous oceanic lithosphere does not only drive water but other elements, in particular halogens, into the mantle. These can affect arc magmatism through the dewatering of the suducting slab or be recycled in the mantle. Serpentinites must therefore be considered as one of the components of the subduction factory.

The presence of serpentinites strongly modifies the physical properties of the crust (density, seismic velocities, magnetic properties) and its rheology. To interpret geophysical data and tectonic fabrics in slow-spreading mid-ocean ridge environments, it is essential to keep in mind that serpentinites may be an important component of the oceanic crust.

Finally, hydrothermal circulation cells are associated with serpentinization. These can be identified by anomalies in the water column: methane anomalies associated with serpentinite outcrops in the axial valley likely document active serpentinization. The Rainbow type hydrothermal vent field with high temperature black smokers, however, seems to require a magmatic heat source. In contrast, the Lost City-type hydrothermal vent field could be generated by serpentinization reaction heat only. Biological activity, supported by the production of hydrogen and methane, is associated with active venting. The recent discovery of these hydrothermal fields stresses that serpentinization at mid-ocean ridges must be considered in thermal and geochemical budgets.