Metamorphism. Bjørn Jamtveit

Transcription

1 Metamorphism Bjørn Jamtveit Physics of Geological Processes, University of Oslo, P.O.Box 1048 Blindern, N-0316 Oslo, Norway

2 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 4 th 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

3 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 m 3 /m 2 s 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.

4 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

5 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: Fisher KM (2002) Waning buoyancy in the crustal roots of old mountains. Nature, 417: 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: Jamtveit B, Malthe-Sørenssen A, and Kostenko O (2008) Reaction enhanced permeability during retrogressive metamorphism. Earth and Planetary Science Letters, 267, 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: 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.

6 FIGURES

7 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/cm 3 to about 3.5 g/cm 3 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% H 2 O 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).