Alteration of the oceanic lithosphere and its implications for seafloor processes
Wolfgang Bach1 and Gretchen L. Früh-Green2
Three-quarters of the global magmatism and 1/4th of 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
1
Geoscience Department, University of Bremen, 28359 Bremen, Germany, Email: wbach@uni-bemen.de
2
Institute of Geochemistry and Petrology, ETH Zürich, CH-8092 Zürich,
Switzerland, E-mail: frueh-green@erdw.ethz.ch
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, with large 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 MidAtlantic 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) (Figure 2).
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 Mohorovičić discontinuity
(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 talcamphibole-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 Crbearing 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 talctremolite-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 longlived at slow spreading ridges. These differences are expressed in the style of seawaterrock 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 430450˚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.
TAG
One of the largest and best-studied seafloor hydrothermal systems is the TAG
hydrothermal mound at the MAR 26°N (Humphris at al., 1995). In 1994, the Ocean
Drilling Program drilled several holes into the 200 m diameter and 50 m high sulfide
mound, which hosts active black and white smoker complexes. Drilling revealed a
complex internal stratigraphy, a dominance of breccias and a distinct mineralogical
zonation, all of which are very similar to those observed in the feeder zones of the fossil
hydrothermal system on the Galapagos Ridge and in massive sulfide deposits in
ophiolites, such as the Troodos massif on Cyprus (Humphris at al., 1995; German and
Von Damm, 2003). The upflow zone of hydrothermal fluids is zoned vertically and
horizontally, with extensive massive brecciated pyrite ores at the top of the deposits,
progressively grading into silica-anhydrite breccias that overlie an inner silica-pyritechlorite stockwork zone (Figure 4). The chlorite zone develops where upwelling
hydrothermal solution mix with seawater that is entrained into the upflow zone. In the
shallow and permeable parts of the system, mixing between hot hydrothermal fluids and
seawater lead to precipitation of anhydrite and pyrite. Anhydrite plays a major role in the
construction of the deposits and is precipitated through conductive heating of small
amounts of entrained seawater, forming a structural framework during periods of hightemperature activity, followed by periods of inactivity in which anhydrite dissolves and
causes the structure to collapse (Humphris et al., 1995). Fluids created by mixing of
seawater and hydrothermal fluids have low pH and upon further migration through the
sulfide mound, can remobilize elements like Zn, leading to zone refinement processes
within the sulfide deposit and white-smoker type fluids venting on the seafloor. TAG is
one of the largest sulfide deposits know in the oceans (4 Mio tons of sulfide with 30,00060,000 tons of Cu) and is similar in size to ancient sulfide deposits in ophiolites.
Serpentinite-hosted systems
To date, five known active hydrothermal systems along the northern MAR are hosted in
serpentinized peridotites with interdispersed gabbroic rocks and are located close to ridge
discontinuities and segment ends: the Rainbow (36°N), Logatchev (15°N) and Ashadze
(13°N), the Saldahna field (36°N), and the Lost City field (30°N). These hydrothermal
systems vary in venting temperature, fluid compositions and type of hydrothermal
deposits, but are characterized by high concentrations of H2, CH4, and other
hydrocarbons, attributed to seawater interaction and serpentinization processes (Charlou
et al., 2002; Kelley et al., 2005). The Rainbow, Logatchev and Ashadze fields are hightemperature (black smoker) systems with sulfide deposits deposited from low pH and Ferich fluids. Heat output of Rainbow has been estimated to be 1-5 GW (Thurnherr and
Richard, 2001), making it the largest vent field yet discovered in terms of heat flux. High
heat and fluid output, as well as the formation of sulfide deposits and high CO2
concentrations in the fluids, suggest that hydrothermal circulation in these high
temperature systems is likely driven by yet-to-be-detected crystallizing magma bodies
within or close to serpentinizing mantle peridotites.
The Lost City hydrothermal field is located on 1-1.5 my old serpentinized mantle rocks,
15 km west of the MAR axis and is distinctly different than all other known marine
hydrothermal systems (Figure 5). At Lost City up to 60m high carbonate-brucite
structures have formed where clear, high pH (from 9 to 11) fluids emanating from faults
in serpentinite-dominated basement rocks exit on the seafloor. The fluids are up to 91°C
and are depleted in metals and CO2, but have high concentrations of H2 and CH4 (and
other low molecular weight hydrocarbons) that serve as important energy sources for
anaerobic microorganisms within the porous chimney walls (Kelley et al., 2005;
Proskurowski et al., 2008). Stable isotope and radiocarbon measurements on methane
venting at Lost City have demonstrated that it is ultimately derived abiotically from
mantle CO2 as opposed to seawater bicarbonate. This implies that seawater bicarbonate
carried with recharge fluids is largely removed, presumably in carbonate minerals, before
the abiotic reactions that form methane occur (Proskurowski et al., 2008).
Low to moderate temperature serpentinization reactions plays a key role in the production
of high pH fluids and thus has important consequences for the sequestration of CO2 from
seawater – an area of research that has stimulated considerable interest in industry and
science communities. Investigations of modern-day serpentinization processes and the
potential for CO2 sequestration through carbonate precipitation in peridotites have
particularly gained importance as a possible technology for the reduction of
anthropogenic CO2 input to the atmosphere (Kelemen and Matter, 2008).
OCEAN-CRUST EXCHANGE
From heat and geochemical budgets, one can derive that on the order of 1-4 TW of heat is
extracted from the ridges to the oceans (Sleep, 1991; Elderfield and Schulz, 1996). This
axial heat flux is only a small fraction of the total oceanic hydrothermal heat flux of 11
TW. The remaining heat is transported off-axis in ridge flank crust up to roughly 65
million years in age. The hydrothermal systems associated with these hydrologically
active ridge flanks are low-temperature (<65°C), producing lower zeolite-facies alteration
in the rocks.
The budget of Mg – one of the major cations in seawater – can only be explained when
both types of hydrothermal circulation are considered (Mottl and Wheat, 1994). Mg is
lost from seawater during interaction with ocean crust at all temperatures, but removal
essentially becomes complete at temperatures above 100°C. On the order of several 1013
kg of seawater are fluxed through on-axis hydrothermal systems per year. The fluids
become enriched in Ca, SiO2, alkalis, base metals, and sulfide but depleted in Mg, U, P,
and sulfate. In general, the ocean crust is either a source or a sink for the elements
dissolved in the oceans. Carbon dioxide, for example, is added to the oceans by
magmatic degassing along mid-ocean ridges, but ridge flank alteration constitutes a sink
of CO2 that is of about the same order of magnitude (1-2 x1012 moles/yr; Alt and Teagle,
1999). Likewise, alkali elements are leached from the rocks by seawater-derived fluids
in high-temperature axial hydrothermal processes, while in the low-temperature ridge
flank systems, they are lost from the circulating seawater to the ocean crust. The net
effect here is that the crust is a prominent sink for alkali elements, and other elements like
boron and uranium. Hydrous minerals (smectites, zeolites) and carbonates form in these
ridge flanks systems and slowly seal the crust, which also becomes increasingly insulated
from the oceans by accumulation of sediments. Both sediments and alteration minerals
provide an important source of water and CO2 in subduction zones.
SEAFLOOR VENTS AND LIFE
The discovery of hydrothermal vents in the deep sea more than 30 years ago led to the
now well-established recognition that geological processes along the 60,000 km long
mid-ocean ridges support rich and diverse chemosynthetic ecosystems at the seafloor. In
contrast to the ocean surface, water column, and marine sediments where essentially all
life depends directly or indirectly on photosynthetic energy, vent communities are fuelled
by chemical energy from the deep Earth (McCollom and Shock, 1997). There is an
increasing awareness that magmatic processes and reactions between seawater and
lithosphere support chemosynthetic ecosystems in many submarine geotectonic settings,
such as mid-ocean ridges, intraplate volcanoes, forearcs, backarc basins, submarine arc
volcanoes, continental margins and ridge flanks. What all these environments have in
common are high energy potentials in form of redox disequilibria (Figure 6).
The basis of the food web in the deep oceans is chemosynthetic microorganisms that
derive metabolic energy from the enzymatic catalysis of redox reactions. These
organisms can thrive wherever reduced compounds, released by magma degassing or
water–rock interactions, and oxidants mix within the ocean crust, at the seafloor, and in
particle plumes originating from vent sites in the deep sea. Microorganisms can also mine
energy from solids such as rocks, volcanic glass and sulfide minerals deposited at
hydrothermal vents. Even at sites remote from spreading axes and thermal vents,
chemosynthesis is now recognized to play a role in bare-rock systems of the deep ocean
(Edwards et al., 2005).
In addition to stripping or leaching reduced components from magmas and rocks,
geochemical energy is released where hydrogen is generated by hydrolysis during
reaction of water with reduced Fe (or other metals) in basaltic glass or in Fe-rich minerals
such as olivine. Dissolved H2 generated this way can be combined with CO2 or SO42- by
methanogens or sulfate reducers, respectively. In serpentinizing environments, such as at
Lost City, rare microbial communities of archaea and bacteria are supported by such
hydrogen-rich vent fluids (Kelley et al. 2005; Brazelton et al., 2006). Many of these
systems may host microbial communities and mineral-microbe interactions representing
early metabolic pathways on Earth (Martin et al., 2008). There are many examples of
how geochemical energy can be brought to the seafloor in fundamentally different ways
as fluid flow regimes, magma supply, and rock composition vary within the oceanic
lithosphere. Vent microorganisms can use this geochemical energy in a wide range of
aerobic and anaerobic processes, many of which have not been fully explored to date.
ACKNOWLEDGMENTS
This contribution summarizes decades of research by the scientific community dedicated
to studies of mid-ocean ridge processes, which to a large extent was conducted under the
auspices of InterRidge and its individual country programs. The authors thank the
numerous scientists who have contributed their knowledge to this subject, many of which
could not be directly cited here. We also acknowledge the ocean drilling programs ODP
and IODP and the participating scientists who have contributed greatly to a shift in
paradigms and a better understanding of the formation, alteration, and aging of the
oceanic lithosphere. We thank Mathilde Cannat and Håkon Austrheim for helpful
comments. WB acknowledges support by the DFG-Research Center/Excellence in the
cluster ‘The Ocean in the Earth System’, GFG acknowledges years of support by the SNF
to conduct studies of serpentinization and the Lost City hydrothermal system.
FIGURE CAPTIONS
Figure 1: Two end-member models of crustal architecture at slow-spreading ridges.
Although the seismic velocity structure is similar in both end-members, only one fits the
common interpretation of such seismic profiles in terms of a layered magmatic crust.
Note that in heterogenous crust, the Moho is a serpentinization front and not the crustmantle boundary. Modified from Cannat (1993) in Mevel (2003).
Figure 2: Conceptual models of tectono-magmatic processes and their effects on
alteration of the oceanic lithosphere associated with differences in spreading rates and
based on geophysical and petrological constraints. (a) Robust magmatism at fast
spreading ridges produces a relatively homogeneous, layered crust. Melts extracted from
rising asthenosphere feed a narrow, sill-like melt lens 1-2 km beneath the ridge axis that
grades downward into a partially crystallized mush zone, which is surrounded by a
transitional zone of solidified by still hot gabbroic rock. Fluid circulation is limited to the
crystallized upper crust and focused fluid circulation (dark red arrows) produce
hydrothermal vent fields along the ridge axis. The relative volumes of melt and crystal
mush vary along axis (b), especially near axis discontinuities (modified from Sinton and
Detrick, 1992). In contrast, slow spreading ridges are characterized by variations in
magmatic, tectonic and alteration processes along ridge segments. (c) Layered crustal
sections form where magma supply is high, whereas detachment faults develop during
extensive tectonic extension, where magma budgets are lower, and result in exposing
mantle-dominated lithosphere to the seafloor (modified from Boschi et al., 2006). Much
of the magma rising through this mantle section crystallizes at depth and forms variably
sized gabbroic bodies. Detachment faults may intersect the crystallizing magma bodies
and their feeder dikes and can channel high temperature fluids to feed large black smoker
vent fields (dark red arrows). Serpentinization progresses as the mantle sections are
uplifted and oceanic core complexes develop. Late normal faults and mass wasting can
focus low temperature, alkaline hydrothermal fluids derived from serpentinization
reactions (blue arrows), as seen at the Lost City vent field at the crest of the Atlantis
Massif. (d) Along-strike variations in magma supply at slow spreading ridges are
reflected by segment centers with relatively continuous, layered magmatic crust that
becomes thinner and discontinuous toward magma-poor segment ends. Ultramafic rocks
and serpentinization, with arrays of short strike slip and oblique faults, are common at
segments ends and contribute to a thick lithosphere, as inferred from seismic data
(modified from Cannat et al., 1995).
Figure 3: Phase relations in the H2O-NaCl system explaining the condensation of brines
during supercritical phase separation (modified from Bischoff and Rosenbauer, 1984).
The field for black smoker fluids encompassed many hundred observations (cf. German
and Von Damm, 2003).
Figure 4: A schematic cross section through the TAG hydrothermal mound and the
underlying stockwork zone. The basement underneath TAG reveals a vertical and
concentric zoning typical of hydrothermal upflow zones, with a silicified and sulfidized
central part and chloritized peripheral zones. Modified from Humphris et al. (1995).
Figure 5: Examples of (a) hydrothermal sulfide deposits and 360°C black smoker vents
driven by magmatic activity (Logatchev hydrothermal field)(©marum) and (b) carbonate-
brucite structures forming at moderate temperature (40-90°C) from high pH, metal-poor
fluids emanating from serpentinized mantle peridotites at Lost City.
Figure 6. Cartoon depicting different geotectonic settings in which hydrothermal systems
are developed and characterizing some of their principal differences. On axis systems
comprise basalt- and peridotite-hosted vent fields, the latter of which are only a few km
off-axis (figure is not to scale). Off-axis ridge flank systems are often related to
seamounts that penetrate the impermeable sediment layer. Arc systems are influenced by
CO2 and SO2 degassing of volatile-rich magma that reflect recycling of subducted
components. The small inset cartoons illustrate some of the pronounced differences in
substrate and vent fluid chemical compositions encountered in submarine vents around
the globe.
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