Metamorphism  

 

 

 

 

 

 

Bjørn Jamtveit

Physics of Geological Processes, University of Oslo, P.O.Box 1048 Blindern,

N-0316 Oslo, Norway

E-mail: bjorn.jamtveit@geo.uio.no

 

 

 

 

 

 

 

 

 

 

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 Ca-content 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).