Download Metamorphism

 Metamorphism
Bjørn Jamtveit
Physics of Geological Processes, University of Oslo, P.O.Box 1048 Blindern,
N-0316 Oslo, Norway
E-mail: [email protected]
CHANGE
According to Winkler (1979): “Metamorphism is the process of mineralogical and
structural changes of rocks in their solid state in response to physical and chemical
conditions which differ from the conditions prevailing during the formation of the
rocks; however, the changes occurring within the domains of weathering and
diagenesis are commonly excluded”. In terms of the processes involved, there is
however no sharp distinction between diagenesis, weathering and metamorphism.
Neither is there any sharp transition between metamorphism and the onset of
magmatic processes during partial melting of metamorphic rocks at high
temperatures.
Although the very concept of metamorphism implies change, the study of
metamorphic rocks was until recently focused on states rather than change. Time was
mainly thought of as the age of a rock, the number of million years that had past since
the minerals comprising the rock was last in thermodynamic equilibrium. Today, time
is also the 4th dimension in which the observable patterns of metamorphic rocks
evolve according to coupled irreversible reaction-, transport-, and deformation
processes. Accordingly, over the last couple of decades, there has been a gradual
change in focus during studies of metamorphic rocks. Metamorphic petrologists have
become increasingly interested in metamorphism, and thus in inferring the underlying
processes from an observed pattern. Increasing efforts are thus spent on careful
observations of the often very complex patterns of metamorphic rocks. The art of
petrography, that by many was considered obsolete in the wake of modern computer
technology, is about to become fashionable again when the focus change from being
to becoming.
Figure 1, illustrates some of the most important changes taking place during
metamorphism. These include changes in mineralogy and mineral composition,
microstructures, and rock composition (Figure 1). Such changes are associated with
sometimes even dramatic changes in physical properties, such as density, porosity,
strength, modes of deformation etc (cf. Escartin et al., 2001). Through its effects on
rock properties, metamorphism may significantly influence the way the Earth’s crust
responds when subjected to the forces of plate tectonics. Metamorphism affects the
way mountains form and evolve (Fisher, 2002), and thus also the evolution of
landscapes at the Earth’s surface. It may affect the way oceanic plates bend and get
subducted in a collision with a continent (Escartin et al., 2001; Ranero et al., 2003),
and through its effects on fluid migration, it also influences the chemical
differentiation of the Earth’s crust, including the formation of major ore-deposits (eg.
Phillips and Powell, 2009).
CAUSES AND RATES OF METAMORPHISM
Metamorphism may occur whenever a given rock is subject to conditions under which
its mineral assemblage is no longer thermodynamically stable (again ignoring the
regimes of weathering and melting). Under fluid absent conditions however, the rate
at which metamorphism takes place will in most cases be too slow for the
metamorphic changes to have significant effects on the rock properties, and for the
external world in general (see Fig.1 by Putnis and John, this volume). In this volume,
we will mainly be interested in metamorphism, to the extent that it has direct or
indirect effects on the evolution of the Earth crust on a scale that is observable in the
field, and therefore in situations where metamorphism occurs in the presence of
fluids.
As pointed out by Connolly (this volume), metamorphism during a rise in temperature
(prograde metamorphism) is normally associated with fluid production through
metamorphic devolatilization reactions. In such a case, the rate of heating is expected
to control the rate of metamorphism, and thus the rate of fluid production. Large sale
heating associated with plate tectonic processes is a slow process. Temperature rises
of a few degrees per million years will produce average fluid fluxes on the order of
10-10 m3/m2s when the fluid producing reactions actually take place. Although this
may seem like a small number, the real fluid migration rate in the pores or fractures of
the rocks is ≈ flux/porosity. Even for a relatively high porosity of 1%, the actual fluid
migration rates would be on the order of 0.3 m/year, and focusing of the fluid flow
into channelways would speed up the flow rate even further. Thus, even during
prograde regional metamorphism, fluid flow rates and associated flow-related
transport processes may at least locally be significant on ‘human’ time scales.
Fluid production driven by local heat sources such as magmatic intrusions (contact
metamorphism) may be even faster. During emplacement of large igneous provinces
in sedimentary basins, metamorphic fluids may be released at such rates and in such
quantities that it may even affect global climates (Svensen and Jamtveit, this volume)
and cause major perturbations to the biosphere.
In contrast to prograde metamorphism that produces fluids at a rate controlled by heat
transport, retrograde metamorphism is normally associated with consumption of
fluids when a metamorphic rock formed at elevated temperatures is exposed to fluids
at lower temperature. The rate of this process may obviously be controlled by the rate
of fluid supply. In some cases, in particular where fluid supply is related to seismic
activity and the generation of fracture networks, the actual fluid migration rates may
be much faster than the rates associated with prograde metamorphism. Fast fluid
migration increases the chances that fluids get in contact with rocks with which they
are far from equilibrium. In such situations, volume changes associated with rapid
reaction rates may lead to considerable perturbations of the local stress field.
Retrogressive metamorphism may therefore be a very dynamic process whereby
reaction, deformation and transport processes are intimately coupled, often resulting
in striking patterns such as metasomatic fronts (figure 1g), complex replacement
structures (Putnis and John, this volume), and reaction-driven fracture patterns (figure
2; Jamtveit and Austrheim, this volume). These non-equilbirum patterns, which are
observable at all scales from the nanometer scales to outcrop scales, contain key
information about the mechanisms of retrogressive metamorphism and thus about the
way the Earth crust gets hydrated (and in some cases carbonated).
Perhaps the most important example of retrogressive metamorphism occurs below the
sea floor. Also in this case, metamorphism is directly connected to the biosphere.
Along the spreading ridges, the chemical ingredients provided by the expulsion of
fluids involved in hydrothermal alteration (retrogressive metamorphism) of mafic and
ultramafic magmatic rocks is critical in sustaining the local biosphere (Bach and
Frueh-Green, this volume).
Hence, both prograde and retrograde metamorphism are key players in the dynamic
evolution of the substratum to which life itself in anchored, and the metamorphic fluid
is often the medium through which these realms (the biosphere and the geosphere) are
connected.
REFERENCES
Bach W, and Früh-Green G (2010) Hydration of the oceanic lithosphere and its
implications for sea-floor processes. Elements, 6, this volume
Connolly, JAD (2010) Metamorphic devolatilization and fluid flow: Time and spatial
scales. Elements, 6, this volume
Escartin, J, Hirth G, and Evans B (2001) Strength of slightly serpentinized peridotites:
Implications for the tectonics of oceanic lithosphere, Geology, 29: 1023-1026.
Fisher KM (2002) Waning buoyancy in the crustal roots of old mountains. Nature,
417: 933-936.
Jamtveit, B, Bucher-Nurminen K, and Stijfhoorn DE (1992) Contact metamorphism
of layered shale-carbonate sequences in the Oslo rift: I. Buffering, infiltration and the
mechanisms of mass-transport. Journal of Petrology, 33: 377-422.
Jamtveit B, Malthe-Sørenssen A, and Kostenko O (2008) Reaction enhanced
permeability during retrogressive metamorphism. Earth and Planetary Science
Letters, 267, 620-627
Phillips GN, and Powell R (2009) Formation of gold deposits: Review and evaluation
of the continuum model. Earth Science Reviews, 94: 1-21
Putnis A and John T (2010) Replacement processes in the Earth's crust. Elements, 6,
this volume
Ranero CR, Morgan JP, McIntosh K, Reichert C (2003) Bending-related faulting and
mantle serpentinization at the Middle America trench. Nature, 425: 367-373
Svensen H and Jamtveit B (2010) Global climate change driven by metamorphic
devolatilization. Elements, 6, this volume
Winkler HGF (1979) Petrogenesis of metamorphic rocks, 5th ed. 348 p.
FIGURES
Figure 1. Examples of metamorphism. a) to b) The dark fine grained magmatic basalt
in a) is transformed into a spectacular coarse grained green and red eclogite (b) during
metamorphism at high pressures and temperatures. During the metamorphic
transition, the augite (pyroxene), plagioclase and olivine in the basalt is transformed
into garnet (red), omphacite (green) and clinozoisite (white). A densification of the
rock from a density of about 2.9 g/cm3 to about 3.5 g/cm3 makes this transition
potentially important for large-scale geodynamic processes, including basin
subsidence and subduction. c) to d) A dark fine grained sedimentary shale is
transformed into bright an shiny mica-schist with large garnet crystals at intermediate
metamorphic pressure and temperature conditions. During this transition the rock
looses several weight% H2O and thus such a transition is an important source of
metamorphic fluids (cf. Connolly, this volume). Scales similar to figures a) and b). e)
to f) Microphotographs. Oolite-bearing limestone rich in fossils (e) will transforms
into an equigranular marble (f) during metamorphism. In this case, no major changes
in mineralogy nor composition occur, yet the rock’s microstructure is transformed
completely during coupled growth- dissolution and grain boundary migration
processes. Scale bar in e) also applies to f). g) Metamorphic zones of different
mineralogy and color around a fracture in contact metamorphic shale from the Oslo
rift (see Jamtveit et al., 1992 for details). Ca-rich fluids from neighboring limestones
entered the fractures and diffusional mass-transport generated zones of decreasing Cacontent away from the fracture. This is an example of metasomatism. g) Microphotograph of troctolite texture from the Duluth Igneous Complex, showing partly serpentinized olivine grains in a plagioclase matrix. A dense network of microfractures connect individual olivine crystals and allow hydrous fluids to move through the rock. The microfracture network is most extensively developed where the distance between neighboring olivines is smallest and the plagioclase matrix has been squeezed between the olivine grains during hydration and expansion. Small olivine grains in unfractured regions are virtually unaltered. This image illustrates the strong coupling beween metamorphic reactions, fluid migration and deformation (cf. Jamtveit et al., 2008).