Metamorphism
The role of fluids
Bjørn Jamtveit and Haakon Austrheim
Physics of Geological Processes,University of Oslo, P.O.Box 1048 Blindern,
N-0316 Oslo, Norway
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The evolution of the Earth’s lithosphere is, in a major way, affected by
metamorphic processes. This applies to its chemical and mineralogical
composition as well as its physical properties on all scales from the
nanometer scale to the size of the tectonic plates. Studies of metamorphism
during the last couple of decades indicate that fluids may be as important
in a changing lithosphere, as water is in the biosphere. History dependent
observables of metamorphic rocks, such as their microstructures,
compositional variations, deformation features etc, directly reflect the
dynamics of fluid-rock interactions. The migration of the fluids produced
during
prograde
metamorphic
processes
or
consumed
during
retrogression, links metamorphism at depth to the evolution of the
hydrosphere, the atmosphere, and the biosphere. The goal of this issue is to
highlight the importance of fluids during metamorphism, and how the
fluids connect the metamorphic realm to other Earth systems, including life
itself.
THE EVOLVING LITHOSPHERE
Most of the Earth’s lithosphere evolves under conditions where metamorphic
processes are the dominant transformation mechanisms. Metamorphism
controls the mineralogical, the microstructural and to some extent the
compositional evolution of the lithosphere. Thereby, it also controls the physical
properties (density, seismic velocity, rheology etc) that determine how the
lithosphere and the various entities making up the lithosphere behave when
affected by the forces of plate tectonics.
From the early 80’s until quite recently, the metamorphic evolution of the Earth’s
crust was to a large extent regarded as a more or less passive recorder of it’s
pressure and temperature history as dictated by some plate tectonic processes
(cf. England and Thompson, 1984). Today, however, metamorphism is
recognized to be almost as dependent on the presence of fluids, as biological
systems are on the presence of water (Yardley, 2009). Fluid-rock interactions
3
during metamorphism may, in a major way, control large-scale geodynamic
processes by controlling: the mechanical strength and thus the support of
mountain belts (Jackson et al., 2004); the density of the lithosphere and thus the
subsidence of sedimentary basins (Kaus et al., 2005); and the volatile content of
the mantle wedge during subduction processes, thereby affecting arc magmatism
(Connolly and Kerrick, 2001a). Even our ability to interpret the larger scale
features of the lithospheric mantle through seismic waves may be affected by
metamorphic volatilization processes (Boudier et al., 2009).
The release or consumption of fluids by metamorphism are two fundamentally
different processes in the lithosphere. When magmatic or metamorphic rocks
formed at elevated temperatures are exposed to water or other fluids at
temperatures below the temperature of their formation, they usually react to
form rocks with a higher content of volatile components. This is retrograde
metamorphism, and it depends critically on the nature and abundance of
external fluids (Jamtveit et al., 2000; 2008). More than any other factor, including
pressure and temperature variations, the timing of retrogressive metamorphism
is controlled by the supply of external fluids.
Near or at the Earth’s surface, similar reactions of rocks with fluids play an
important role in weathering (Røyne et al., 2008), which may involve natural
CO2-sequestration by carbonate precipitation (Kelemen and Matter, 2008;
Oelkers et al., 2008).
A fundamentally different situation arises when relatively volatile-rich
sedimentary or metamorphic rocks experience a period of temperature increase.
For thermodynamic reasons, this will normally lead to rock decomposition with
the production of a fluid phase. This is prograde metamorphism, and a key
process in this case is the expulsion of fluid from the site of fluid generation
(Connolly, 2010). Fluid migration during prograde metamorphism has many
similarities to the generation and expulsion of melts during partial melting in the
crust or mantle (cf. Spiegelman et al., 2001).
Migration of fluids to and from the sites of retrograde and prograde
metamorphism plays a key role in the global H2O and carbon cycles and
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ultimately in the coupling between metamorphism and the near-surface
hydrosphere, atmosphere and biosphere realms (cf. Connolly and Kerrick,
2001b; Svensen et al., 2004; Rupke et al., 2004; Kelemen and Matter, 2008; Bach
and Frueh-Green, 2010).
METAMORPHIC RATE CONTROLS
Interpretations of metamorphic mineral assemblages and microstructures was
for a long time, and to some extent still is, carried out within the conceptual
framework of equilibrium thermodynamics (e.g. Spear, 1995). The equilibrium
assumption is based on the idea that metamorphic reactions are fast compared
to the rate of change in externally controlled variables, such as temperature and
pressure. Although this may be a good approximation in some situations, the
equilibrium approach has the very unfortunate aspect that all history-dependent
features of the rocks are ignored, thereby also excluding the most important
sources of direct information about the underlying metamorphic processes that
generated the rock of interest (figure 1). The central role of kinetics in
controlling both microstructures and mineral assemblages in metamorphic rocks
has been illustrated by numerous experimental, theoretical, and petrographic
studies over the last two decades (see for example Rubie, 1998; Lüttge et al.,
2004; Pattison and Tinkham, 2009, and references therein). Several of these
studies point to slow nucleation as a key factor affecting overstepping of
metamorphic reactions. Moreover, the realization that most metamorphic phase
transformations take place by dissolution-reprecipitation reactions (Putnis and
John, 2010) emphasizes the role of fluids as a central ingredient in
metamorphism, even when their role is merely catalytic. Limited, or transient,
availability of intergranular fluids have been put forward as the most plausible
explanation for slow kinetics during regional metamorphism (Baxter, 2003).
A generic rate law for metamorphic reactions may be expressed by three main
terms:
R ≈ k(T) Gn As
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Where R is the rate of reaction (for example in mol/m3s), k(T)is a temperature
dependent kinetic rate constant, G is the ‘overstepping’ of the equilibrium
condition (or chemical affinity), n is a constant, and As is the specific surface area.
In most models of metamorphic kinetics, the reaction rate, R, falls with time
because the reactions tends to push the system towards equilibrium and thus
reduces the affinity of the reaction. In addition, as the reaction progresses, the
area of reactive mineral surface will normally be reduced as the amounts of
reactants diminish, and the reactant grain may be shielded from the fluid by
product phases (cf. Martin and Fyfe, 1970). Such models do however, largely
ignore, the complexity of reactive fluid-mineral interfaces.
Recent studies have demonstrated that the processes taking place at the
interface across which fluids and minerals interact may be far more complex
than hitherto thought. The interface itself may evolve in a highly dynamic way
leading to a wide range of structures, including complex pore structures and
fracture patterns (cf. Putnis, 2009; Renard et al, 2009; Raufaste et al., 2010).
Most importantly, these surface processes may generate reactive surface areas
much larger than those predicted by existing reactive-transport models, and
keep generating new surface area by fracturing and porosity generation as the
reactions proceed. As discussed below, this is absolutely essential for the rate of
volatilization processes in the Earth crust. Mechanical effects in particular, may
cause an increase in reactive surface area by reaction driven fracturing
processes, so that the reaction rate may, for a while, accelerate with time rather
than slowing down continuously after the onset of reaction (Røyne et al., 2008;
Jamtveit et al., 2009).
In some cases, mechanical effects may slow down or even halt metamorphic
reactions by preventing the system from expanding – a phenomenon known as
mechanical closure (Schmid et al., 2008).
FLUID PRODUCTION
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Prograde metamorphism will in most
cases produce fluids through
devolatilization reactions. The porosity occupied by these fluids will have a
tendency to move upwards due to the effect of gravity acting on the difference
between their density and the density of the surrounding rock. Thus, it has long
been recognized that rocks undergoing prograde metamorphism may only have
a discrete fluid phase when devolatilization reactions actually take place
(Walther and Orville, 1982). If the rate of fluid production is high relative to the
rate at which fluid can escape through preexisting permeability, fluid escape will
be associated with local hydrofracturing (Flekkoy et al., 2001). Evidence for this
is seen in abundant veining during regional metamorphism of volatile-rich rocks
(e.g. Fisher and Brantley, 1992; Cesare, 1994; Widmer and Thompson, 2001).
Slow nucleation may cause overstepping even of devolatilization reactions
(Rubie, 1998; Pattison and Tinkham, 2009), and consequently reactions may
proceed relatively rapidly after they first start (more rapidly that predicted if it is
assumed that slow addition of heat by conduction is rate controlling). This would
further promote fracturing and vein formation. The intimate coupling between
devolatilization reactions and porosity (fluid) migration is emphasized by
numerical modeling (Connolly and Podladchikov, 1998; Connolly, 2010). These
models suggest that fluid loss, even on a crustal scale, may be focused by the
generation of porosity waves (focused pulses of fluids). The existence of such
‘waves’ is supported by observations of pipe-like fluid release structures
associated with sill intrusions in sedimentary basins where the heat from the
sills drives fluid production and fluid pressure increase (figure 2; Jamtveit, et al.
2004). Recent studies have provided strong evidence that metamorphic fluid
release of greenhouse gasses from carbon-rich sedimentary rock have triggered
some of the largest climate changes, and associated extinctions, in the history of
the Earth (Svensen et al., 2004; Svensen and Jamtveit, 2010).
FLUID CONSUMPTION
When metamorphic rocks experience a period of cooling, they will often not
back-react with fluids to form a lower temperature mineral assemblage. The
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main reason is that fluids produced during prograde metamorphism escape, and
are no longer available for retrograde reactions. Supply of external fluids is
therefore critical for the progress of retrograde metamorphic reactions.
Furthermore, if such external fluids are supplied, they are likely to be strongly
out of equilibrium with the rock in which they infiltrate. Consequently,
retrograde volatilization reactions are often fast and they may produce a variety
of complex patterns and features typical of far-from-equilibrium systems.
In a system that is essentially closed to all major components, except the
externally supplied volatile components (usually H2O or CO2), volatilization
reactions will normally result in a significant increase in the total solid volume.
Under ‘far-from-equilibrium’ conditions, this may produce local overpressures
far beyond what is required to fracture the rock (cf. Ostapenko, 1976; Wheeler,
1987). Since the fracturing generates new surface area where volatilization
reactions may start, this tends to speed up the overall rate of volatilization and
allows pervasive fluid-rock interaction at considerably larger scales than would
otherwise be possible. In the following, we examine important examples of such
fracturing assisted volatilization processes.
Serpentinization
Serpentinization is probably the most important metamorphic hydration
processes. The reaction of olivine-rich mantle derived peridotites to form
serpentinites is associated with a reduction in rock density from ca. 3.3 g/cm3 to
less than 2.7 g/cm3. Hydration of end-member forsterite (Mg-olivine) can be
described by the reaction:
(1) 2Mg2SiO4 + 3 H2O = Mg3Si2O5(OH) 4 + Mg(OH) 2
forsterite
+ water = serpentine
+ brucite
In a natural system the Fe-component of the original olivine will be at least
partly absorbed by a corresponding Fe-component in the brucite. This
component may subsequently be involved in a very important redox reaction
that oxidizes ferrous Fe to form magnetite (Fe3O4) and hydrogen in the water is
8
reduced to form H2 (cf. Bach et al., 2006). Through the Fischer-Tropsch reaction,
the H2 may reduce CO2 to CH4. Thus serpentinization may be responsible for the
CH4 seepage at midoceanic ridges (Charlou et al., 1998) and for the hydrogen
based microbial communities (Takai et al., 2004). This environment with
possible production of abiotic metane and other hydrocarbons is regarded as
the most likely location for the origin of life (Konn et al., 2009).
Complete serpentinization results in a solid volume increase of nearly 50%, and
causes a pronounced change in rheology, magnetic properties, and porosity.
Reaction (1) furthermore produces considerable heat (≈290 kJ/kg forsterite
consumed), and serpentinization has been suggested as an explanation of
observed heat flow anomalies in oceanic basins (Delescluse and Chamot-Rooke,
2008).
The rate of serpentinization is at a maximum at a temperature close to 300C
(Martin and Fyfe, 1985; Wegner and Ernst, 1983). At lower temperatures, the
kinetic constant for the serpentinization reaction (k(T)) limits the overall rate. At
higher temperatures, the approach to equilibrium reduces the affinity of the
hydration reactions (G) and thus its rate. The higher the reaction rate (and the
concomitant rate of rock volume increase), relative to the rates of ductile
deformation mechanisms, the more probable it is that serpentinization will
cause fracturing, because there is insufficient time for the rock to accommodate
the extra volume generated. Figure 3 shows fracture patterns developed in
serpentinized peridotes. The characteristic length-scales of the fractures vary
over more than 4 orders of magnitude from a few meters to less than 100
microns.
Natural CO2-sequestration
In the presence of C-bearing fluids, carbonation of peridotites (figure 4) is
produces even more heat than hydration reactions, and these reactions are
expected to be even faster than hydration at temperatures below 200C (cf.
Kelemen and Matter, 2008). Carbonation is a natural CO2-sequestration process,
9
and although the maximum rate of carbonation occurs at temperatures below
the ‘metamorphic regime’ (150-200C, depending on pressure), the physical and
chemical processes involved (as well as their coupling) are very similar.
Petrographic studies indicate that infiltration of C-bearing fluids into peridoties
often utilizes pre-existing fractures associated with sea-floor serpentinization.
However, it is also evident that the carbonate growth itself produces sufficient
‘swelling pressure’ (cf. MacDonal and Fyfe, 1985) to generate additional
fractures. There is little doubt that both serpentinization and carbonation of
peridotites it to a major extent influenced by mechanical processes, in particular
generation of reactive surface area by fracturing over a large range of scales.
Mechanical effects
Most experimental studies on the rates of metamorphic reactions have been
carried out using fine-grained rock powders. Because the stress build-up in such
systems is rarely enough to cause fracturing, the mechanical effects on reaction
rates are negligible. However, this does limit the relevance of such experiments,
and kinetic theories based on these, to fluid-rock reactions in natural systems.
Peridotites, in particular, are often coarse grained and almost invariably undergo
extensive fracturing during reaction with fluids (cf. MacDonald and Fyfe, 1985;
O’Hanley, 1992; Iyer et al., 2008). Some of this fracturing may occur as a
response to far field (tectonic-) stresses, however, there is ample evidence that
the volume change associated with the serpentinization process itself may
generate stresses sufficient to fracture rocks and mineral grains under a range of
conditions (cf. Iyer et al., 2008; Jamtveit et al., 2008).
To our knowledge, no experimental studies have so far examined the effect of
fracturing on metamorphic reactions rates. However, numerical models (cf.
Jamtveit et al., 2008; Røyne et al., 2008) have produced micro- and outcrop-scale
structures very similar to those observed in natural systems (figure 5). A key
feature of such models is that the reaction-driven fracturing accelerates the rate
of reaction by increasing the reactive surface area (Ar) at an early stage of the
reaction. Furthermore, because the length of the fractures may easily exceed the
10
typical diffusion distances, the overall volume of rocks affected by reaction with
fluid will be much larger than predicted by a pure reaction diffusion model. This
argument is further strengthened by the fact that reactions that consume fluids
normally produce an increase in the solid volume, and in the absence of
fracturing, pre-existing porosity would rapidly be clogged by the reaction and
the reaction would stop due to lack of fluids.
METASOMATISM: OPEN VERSUS CLOSED SYSTEMS
The issue of how to deal with thermodynamic equilibrium in open systems
relevant to metamorphic rocks was the subject of heated debate in the 1950s
and 1960s (cf. Weill and Fyfe, 1964, and references therein). In the context of the
preceding discussions on metamorphic kinetics, the question of how to deal with
open systems reappears as a central issue in petrology. The main challenge is the
coupling between mass transfer and stress generation. In a reacting rock volume,
where open system behavior is possible, two limiting scenarios are possible. The
system may change its volume and thus cause perturbations to the local stress
field, or it may maintain a constant volume by exchanging mass with its
environment. In practice, a combination of these scenarios will occur, but one or
the other is often dominant.
The limited amounts of fluids available during metamorphism often put severe
constraints on ‘open system behavior’. The early stages of sea-floor
serpentinization usually lead to very modest changes in rock composition (Bach
et al., 2006). This stage is invariably associated with extensive fracturing and
formation of characteristic mesh textures (cf. figure 5c). Later alteration under
more open-system conditions appears to be more of a volume for volume
replacement process. Volume for volume replacement also seems to be common
in other metamorphic processes characterized by high fluid fluxes, such as
albitization, rodingitization, scapolization etc. The settings in which these
processes occur are usually connected to regions with high thermal gradients,
such as around cooling intrusives, or localized zones of deformation that act as
fluid channels. Pervasive retrogressive metamorphism outside such zones is
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almost invariably associated with reaction-driven fracturing, highlighting the
general importance of mechanical processes for fluid-rock reactions in the
lithosphere.
FUTURE PERSPECTIVES
Earth science is no longer a narrative about the past. Increasing the relevance of our
science for resolving challenges connected to energy, environment, water, and other
natural resources etc., demands more focus on the future of our planet. This requires
more focus on the processes that shapes our planet, than on its equilibrium states. The
future of metamorphic petrology as a relevant field of research requires that we
increase the focus on those features of metamorphic rocks that act as a memory of the
underlying processes. Such features are not the equilibrium features. It is the opinion
of the authors of this article that more focus on the complex patterns of metamorphic
rocks, and in particular the microstructures and their coupling to both chemical and
physical processes, may save metamorphic petrology from ending up as a largely
anachronistic branch of geology. We furthermore believe that the role of fluids is
essential in controlling the evolution of the lithosphere, and also firmly connects
metamorphic processes to the evolution of the hydrosphere, biosphere and
atmosphere. This is hopefully demonstrated through the accompanying articles in this
volume of Elements.
ACKNOWLEDGEMENTS
This study was supported by a Center of Excellence grant to the PGP (Physics of
Geological Processes Center) from the Norwegian Research Council
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FIGURES AND FIGURE CAPTIONS
Figure 1. Example of a microstructure with a memory of the underlying processes.
Reaction induced fracturing of scapolite (Sca) around bent aggregates of prehnite
(Pre), albite (Ab) and titanite (bright inclusions). The fracturing and bending occur as
prehnite, albite and titanite replace phlogopite (not seen) and causes a volume
expansion generating compressive stresses. It is these stresses that crack the brittle
scapolite and bend the more ductile prehnite. The fractures allow more fluid into the
system, allowing the replacement process to proceed. Example from the Bamble
sector, SE-Norway.
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Figure 2. Satellite images from the Loriesfontein area in the western part of the Karoo
Basin, South Africa. The dry landscape is dominated by a relatively flat topography
with numerous circular hills. These hills, seen as bright "blobs" in the images, are
hydrothermal pipes filled with brecciated and metamorphosed shale. The pipes
formed as a result of methane degassing during contact metamorphism of black shale
some 182.5 Ma. The dolerite intrusions responsible for the contact metamorphism are
now located about 100 meters below the surface. The carbon gas made it to the
paleosurface and contributed to global warming in the Jurassic (cf. Svensen and
Jamtveit, 2010). The pipes are typically 100-150 meters in diameter (note the road in
the upper left area of the image).
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Figure 3. Serpentinites across scales. The field of view ranges over ca. 4 orders of
magnitude, from ca. 10 meters (a) to 1 mm (c). a) Serpentinized peridotites from the
Norwegian Caledonides at Røragen, near Røros (6233N, 1148’E). b) Serpentinized
peridotite from the mélange Cailifornia. c) photomicrograph of mesh texture in partly
serpentinized olivine crystal form the Leka ophiolite complex, Norway.
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Figure 4. Examples of natural CO2 sequestration. a) Ophicarbonate from Røragen, SE
Norway. Fractured clasts of serpentine (green) in a calcite-dominated matrix. The
fractures through the clasts are also filled with carbonate. Coin is 2 cm across. b)
Back scattered electron (BSE-) image showing fractured olivine (light grey) partly
replaced by calcite (Cc) surrounded by serpentine (dark grey). The fracturing is
related to stresses set up by the serpentinization reaction. It is these fractures that
allow carbon-bearing fluids to penetrate into fresh olivine grains. Sample from the
Oman ophiolite.
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Figure 5. Microstrutcures and mechanical models of fragmentation. a) Fracturing
around partly serpentinized olivine crystals in a plagioclase matrix from a troctolite
(Duluth gabbro). b) Discrete element model of fracturing around expanding polygons
in elastic matrix (2 phase system) (see Jamtveit et al., 2008 for details). Fractures
connect unreacted grains to regions in contact with a fluid and thus help propagating
the hydration process. c) Mesh texture generated during serpentinization of olivine
grain in partly serpentinized peridotites from the Leka ophiolite complex, Norway
(see Iyer et al., 2008 for details). d) Discrete element model of fragmentation of
rectangular domain (mineral grain) driven by reaction induced swelling during
reactions with a fluid that get access to domain boundaries and fracture walls (Røyne
et al., 2008).