Hydration of the Oceanic Lithosphere and Its Implications for Seafloor Processes

Alteration of the oceanic lithosphere and its implications for seafloor processes

Wolfgang Bach1and Gretchen L. Früh-Green2

Three-quarters of the global magmatism and 1/4thof the global heat loss are associated with tectono-magmatic and hydrothermal processes governing ocean lithosphere accretion and its aging from ridge to trench.Hydrothermal reactions between seawater and oceanic lithosphere under zeolite- to granulite-facies conditions are linked with magmatic and deformation processes but differ in nature relative to spreading rates.Fast spreading ridges with frequent eruptions have telescoped metamorphic gradients and short-lived hydrothermal systems. The less magmatically-robust slow-spreading ridges are commonly cut by normal faults that expose ultramafic rocks on the seafloor and sustain long-lived hydrothermal systems with distinct vent fluid and fauna compositions.

Key words: ocean crust, water-rock interactions, geochemical fluxes, hydrothermal systems

1Geoscience Department, University of Bremen, 28359 Bremen, Germany, E-mail:

2Institute of Geochemistry and Petrology, ETH Zürich, CH-8092 Zürich, Switzerland, E-mail:

INTRODUCTION

On average approximately 3 km2 of new ocean crust is created annually along the >60,000 km long global network of mid-ocean ridges and back-arc spreading centers. This crust interacts with seawater throughout much of the lifetime of the seafloor (the average age of ocean crust is 60 million years). Fluid circulation and interaction with the crust is largely thermally driven and accounts for ¼ of the total global heat loss, or 11 TW (=Terawatts). Metamorphic conditions range from zeolite- to granulite-facies, with the degree of mass transfer being minimal to extensive. Hydrothermal circulation and interaction between seawater and the oceanic lithosphere are fundamental processes in controlling the composition of both seawater and the subducting oceanic plate. These processes play a critical role in regulating global heat and mass fluxes (German and Von Damm, 2003). They also determine geophysical and mineralogical properties of the oceanic lithosphere, with profound consequences for the mechanical behavior during aging and subduction. This is particularly relevant for volcanically starved mid-ocean ridge segments, where serpentinized mantle peridotite are denuded at the seafloor. The stability of serpentine and other hydrous phases determines the release of water in subduction zones. In submarine arc and back-arc hydrothermal settings located above subduction zones, silica- and volatile-rich magmas form as a consequence of the water flux into the mantle wedge. This, in turn, affects the composition of hydrothermal solutions, making arc-associated vent systems chemically and biologically distinct from their mid-ocean ridge counterparts. We discuss some of the principal processes controlling water-rock interaction in submarine hydrothermal systems, while highlighting some of the key differences in crustal architecture, magmatism and deformation between various geotectonic settings.

STRUCTURE OF THE OCEANIC LITHOSPHERE

Results of numerous oceanographic investigations and the ocean drilling programs (ODP and IODP) over the past three decades have changed the long-held view that the entire ocean crust is uniform in architecture and thickness, and have led to the recognition of fundamental differences in crustal accretion and alteration processes related to spreading rates (Figure 1). The principle of a uniform crust, characterized by a “layer-cake” structure of basalt, diabase (sheeted dike complex) and gabbro, is based on early geophysical data and comparisons with ophiolites, and is commonly referred to as the “Penrose ophiolite model” or “Penrose-type crust”, defined during a Penrose conference and published in Geotimes in 1972. This model is generally applicable for crust formed at fast spreading ridges, such as the East Pacific Rise, where melt supply is able to keep up with extension at the divergent plate boundary. However, it cannot be applied to much of the crust formed at oceanic ridges spreading at less than 40 mm yr–1 and which comprise nearly 50% of the global mid-ocean ridge system, extending from the Arctic Ocean, along the entire Mid-Atlantic Ridge (MAR) and into the SW Indian Ocean (SWIR). At these slow spreading ridges, melt supply does not always keep up with plate separation, which is accommodated by significant tectonic extension.

Recent investigations have shown that ultraslow spreading ridges, with spreading rates of <20 mm yr–1, have unique morphologies consisting of linked avolcanic and volcanic accretionary segments, withlarge variations in crustal thickness along ridge segments. Differences in melt volumes and focusing of melts are reflected by discontinuous volcanic centers separated by long stretches of avolcanic or melt-starved segments, in which the principal unit of accretion appears to be emplacement of mantle blocks onto the seafloor (Dick et al., 2003). Similarly, slow-spreading ridges, such as the Mid-Atlantic Ridge, are characterized by rugged topographies, a high degree of segmentation and considerable heterogeneities in crustal thickness, rock types, deformation and degree of alteration that reflect strong temporal and spatial variations in tectonic and magmatic processes (Figure 2). In these environments, serpentinized mantle peridotite and lower crustal plutonic rocks may represent 20-25% of the seafloor exposed through faulting (Cannat et al., 1997).

Using high-resolution multibeam bathymetry and side-scan sonar, recent studies have imaged broad, dome-like massifs with exceptional relief (>2.5 km) and with arched upper surfaces marked by distinctive spreading-direction-parallel corrugations and finer scale striations. These dome-shaped massifs are referred to as oceanic core complexes, in analogue to metamorphic core complexes on the continents, and form along long-lived detachment (normal) faults during phases of extensive tectonic extension, commonly associated with asymmetric spreading. Uplift and up to 40° rotation along these normal faults close to the ridge axis produce large individual domes or distinctive linear ridges with nearly symmetrical slopes. Detachment faults remove the basaltic upper crust and expose plutonic and upper mantle sequences from depths >6 km onto the seafloor in the footwalls (see Blackman et al., 2009, for a recent review) (Figure2).

Core complexes have been found everywhere along segments at the SWIR and MAR (Cannat et al., 2006; Smith et al., 2008). For example, between the Fifteen-Twenty and Marathon fracture zones of the MAR, 45 potential core complexes, some less than 25 km apart, are scattered along two segments (13° and 15°N segments) and extinct core complexes are identified on the seafloor off-axis (Smith et al., 2006). The modern view of ocean crust at slow spreading ridges is that some portions of segments may consist of more uniform, Penrose-like crust, with characteristic abyssal hill morphologies, while other portions of the segments are highly heterogeneous with attenuated or missing volcanic crust and with gabbroic intrusions trapped in partially serpentinized lithospheric mantle (Figures 1 and 2). The depth and spatial extent of serpentinization of oceanic peridotites has direct implications for the interpretation of seismic velocities and may influence where geophysicists estimate the position of the Mohorovičićdiscontinuity (Moho). In these environments, the Moho may not represent the crust-mantle boundary but may be a serpentinization front.

Detachment faulting and channeled fluid flow

Oceanic core complexes and their exposed detachment faults represent tectonic windows that provide access to deep-seated rocks and allow studies of mantle flow, melt generation and migration, strain localization, and alteration. In these settings, variable amounts of gabbroic melts are trapped in the upper mantle, and detachment fault zones provide fluid pathways that may be important for cooling the lithosphere as well as for long-lived fluid circulation that can sustain hydrothermal systems (Figure 2c). In heterogeneous sections of the oceanic lithosphere, where gabbroic rocks are volumetrically variable and interdispersed with serpentinized peridotites, extensive talc-amphibole-chlorite metasomatism as well as heterogeneous, crystal-plastic to cataclastic deformation mark the zones of detachment faulting. Geochemical and microstructural studies indicate a complex mutual interaction between the gabbroic and ultramafic rocks at temperatures <500°C and suggest that these zones represent localized circulation of oxidizing, Si-Al-Ca-rich fluids and mass transfer during high strain deformation (Escartin et al., 2003; Boschi et al., 2006).

The presence of mechanically weak minerals, such as talc, serpentine and chlorite, may be critical to the development of detachment faults and may enhance unroofing during seafloor spreading (Escartin et al., 2003). Talc in particular may be influential in lubricating and softening mylonitic shear zones and can lead to strain localization and focused hydrothermal circulation for up to a few million years. Because of the long-lived nature of oceanic detachment faults, core complex development has been associated with the formation of the largest high-temperature, mineral-rich hydrothermal deposits found to date on the seafloor (McCaig et al., 2007).

Serpentinization processes

Mantle-dominated lithosphere is a highly reactive chemical and thermal system, in which interaction with seawater has significant geophysical, geochemical and biological importance for the global marine system and for subduction zone processes (Mével, 2003). Serpentinization involves hydration of olivine, the dominant mineral in the oceanic mantle, and typically incorporates >10 weight % H2O into the rocks. Hydration reactions are accompanied by a 20-40% increase in volume, a decrease in bulk density, and changes in mineralogy and rheology that directly affect the strength and physical properties of the mantle, the magnetic and gravity signatures, and the seismic velocities. Serpentinization processes also have major consequences for long-term, global geochemical fluxes by acting as a sink for H2O, Cl, B, U, S, C and a source of Ca, Ni and possibly Cr to hydrothermal fluids, and by producing extremely reduced fluids (Janecky and Seyfried, 1986; Allen and Seyfried, 2003; Früh-Green et al., 2004). In addition, seafloor weathering of serpentinized abyssal peridotites may result in Mg loss (Snow and Dick, 1995).

Since the earliest experimental studies of serpentines and studies of Alpine peridotites and ultramafic rocks in ophiolites, it has been known that serpentinization produces heat through exothermic reactions and conditions of low oxygen fugacity (fO2), resulting in the generation of H2-rich fluids and native metals. The formation of H2 during serpentinization is attributed to the production of magnetite during olivine hydration and is generally described by simplified model reactions with end-member phases. In reality, serpentinization involves solid solutions and metastable reactions governed by local variations in the activities of elements such as Si, Mg, Fe, Ca, and C (e.g., Janecky and Seyfried, 1986; Früh-Green et al., 2004). Hydrogen concentrations in the fluid may be limited by the formation of brucite (MgOH)2 and carbonates that can incorporate Fe2+(Klein et al., 2009). Reduction accompanies the formation of FeNi alloys found in serpentinites, and a number of studies indicate that these alloys, magnetite, and/or Cr-bearing oxides may act as catalysts for the reduction of CO or CO2 to form abiotic methane and other hydrocarbons – a process referred to as Fischer-Tropsch-type reactions. The composition of any carbon-bearing fluid will thus be influenced by migration of the reduction front, and any CO2 or dissolved carbonate phases in the fluid may undergo reduction to hydrocarbons and graphite. The production of H2 and CH4 during serpentinization is also important in sustaining diverse microbial communities in subsurface and near-vent environments and may be significant for the existence of a deep biosphere (Früh-Green et al., 2004; Kelley et al., 2005).

The mineral assemblages and textures of oceanic serpentinites typically record progressive, static hydration reactions that take place under a wide range of temperatures, lithospheric depths, fluid compositions and redox conditions. The bulk volume of alteration is recorded by pseudomorphic mesh textures replacing olivine and bastite textures replacing pyroxene. Serpentinization is also accompanied by multiple generations of veins with a variety of vein-filling minerals, morphologies and textures that document local differences in formation mechanisms, stress regimes and fluid infiltration (Früh-Green et al., 2004; Andreani et al., 2007). Hydration expansion during serpentinization may also be a significant process in creating successive episodes of microfractures and in propagating cracks.

The products and sequence of serpentinization reactions depend on the depth, temperature and degree of seawater penetration into the lithosphere and will be influenced by bulk protolith compositions, the presence or absence of magmatic intrusions and/or trapped gabbroic melts, and structure (e.g., detachment faults, cataclastic fault zones). Thermodynamic constraints on phase equilibrium predict that high temperature serpentinization is sensitive to Si content and modal percentage of Opx (Allen and Seyfried, 2003; Klein et al., 2009). Depending on rock composition, serpentinization will commence once the shallow mantle sequences have cooled below 400-425°C, when olivine breakdown reactions to produce serpentine + magnetite are thermodynamically stable or below ~350° where serpentine + brucite are stable. Recent studies of serpentinites associated with detachment faults and core complexes indicate that serpentinization reactions in the footwall may initiate at depths of 4-6 km, which can be below the seismic Moho (Andreani et al., 2007). Early stages of high temperature seawater-mantle-gabbro interaction can also result in replacement of serpentine by talc-tremolite-chlorite assemblages to form fault rocks characteristic of detachment faults (Boschi et al., 2006). Progressive serpentinization reactions and veining continues until exposure on the seafloor. Pervasive serpentinization is likely to be restricted to the shallow lithosphere at depths <2 km, where brittle fractures and greater degrees of advective mass transport result in a transition to an open system of seawater infiltration and higher fluid/ratios. A progressive history of seawater-rock interaction in oceanic serpentinites and high integrated fluid/rock ratios are commonly recorded in shifts in oxygen, boron, strontium and neodymium isotope compositions (Boschi et al., 2006; Delacour et al., 2008).

MAGMATIC HYDROTHERMAL TRANSITION

The thermal and rheological structure of the oceanic crust is ultimately controlled by the balance between magmatic heat input and hydrothermal cooling (Phipps Morgan and Chen, 1993). During crustal accretion processes, this balance is determined by the complex interplay between magmatic, tectonic and hydrothermal processes. For example, although magmatic heat is the driving force for hydrothermal circulation of seawater in the oceanic crust, convective transport of heat influences the depths of crystallization and the cooling rates of magmas. Circulation is controlled by magmatic diking at fast spreading ridges, but may be tied more to extensional faults at slow spreading ridges (Wilcock and Delaney, 1996). Hydrothermal systems on the seafloor are small in size and short-lived at fast spreading ridges, while they are large and long-lived at slow spreading ridges. These differences are expressed in the style of seawater-rock interaction that can be observed in deep drill holes within the ocean crust and in ophiolites. Fast spread crust commonly shows progressive, static alteration controlled by fractures and microcracks, while in slow spread crust, more heterogeneous and extensive alteration is tied to ductile or brittle deformation in normal faults or to serpentinization processes in peridotite-dominated domains (Alt, 1995). In addition, the rheological behavior of the oceanic lithosphere, which likely plays a major role in the tectonic evolution of the ridge system, is determined by the hydration state of the solid crust and the presence of fluids.

The nature of the magmatic-hydrothermal interface in the oceanic crust is very poorly known, and we do not yet understand the mechanism by which heat is mined to drive mid-ocean ridge hydrothermal systems. Models of this interface fall into two major categories. One model assumes the presence of a conductive boundary layer between the hydrothermal cell and the heat source. Efficient heat transport in that conductive boundary layer is difficult to sustain, unless it is only a few meters thick (Cann et al., 1986). A second model, invoking a double-diffusive layer, assumes convective heat transport by brines that accumulate in the transition zone between hydrothermal and magmatic systems (Bischoff and Rosenbauer, 1989). These brines form by supercritical phase separation in the deep parts of hydrothermal systems at temperatures around 430-450˚C (Figure 3). It is commonly assumed that these temperatures are the maximum temperatures in the axial hydrothermal convection cells because of the rapid increase in the specific volume of the fluid above 400˚C. Large volumes of buoyant, vapor-rich fluids forming upon phase separation are expected to separate from the small volumes of brines. Yet other convection models do not invoke a brine layer and infer maximum fluid temperatures of 700˚C at the base of the circulation cell. Such high fluid temperatures are consistent with temperatures obtained from plagioclase-amphibole thermometry for the uppermost gabbros from fast spread crust (Manning et al., 1996).

AXIAL HYDROTHERMAL SYSTEMS

From a rock perspective, hydrothermal alteration and mineralization patterns in ocean crust and ophiolite crust share both similarities and significant differences. In both settings, a conceptual model that divides a hydrothermal system into a recharge zone, a reaction (or root) zone, and a discharge (or upflow) zone can be applicable (Alt, 1995). Another commonality is the development of large sulfide deposits that form within the upper crust and at the seafloor due to heating of circulating seawater by crystallizing magmas and leaching of metals from the basement (e.g., Humphris and Cann, 2001; German and Von Damm, 2003). In ophiolites, however, the leaching zones near the sheeted dike-gabbro transition and in the deep upflow zones show abundant epidosites (base-metal depleted and Ca-metasomatized rocks), a rock type that has as yet not been recovered from in situ mid-ocean ridge sections (e.g., Alt, 1995). It also appears that greenschist-facies metamorphism is much more pervasive in the sheeted dikes and upper gabbros in ophiolites. The presence of epidosites and Sr and O isotopes provide evidence that fluid fluxes were greater in ophiolite sections, and it has been suggested that alteration patterns and styles in most ophiolite sections resemble forearc crust more than mid-ocean ridge crust (Gillis and Banerjee 2000). The upper greenschist-facies to lower amphibolite-facies mineral assemblages found in the lower sheeted dike complex in the deepest drill hole in ocean crust (ODP Hole 504B in the eastern equatorial Pacific) are consistent with a thermodynamic assessment of mineral-fluid equilibrium on the basis of vent fluid geochemistry (Bowers et al., 1985). The chemical composition of hydrothermal vent fluids implies temperatures of fluid-rock interaction around 420 to 440˚C (Seyfried et al., 1991). The conceptual model therefore is that black smoker vent fluids rise up from the root zones to the seafloor fairly rapidly and undergo cooling (adiabatic and conductively) by a few tens of degrees.