PRESSURE, TEMPERATURE, FLUID PRESSURE CONDITIONS OF METAMORPHISM

Transcription

1 Complementi di Petrografia N.O Scienze Geologiche, Lezione n. 3 Definizione, limiti, tipi e fattori del metamorfismo Questa prima parte degli appunti al Corso di Complementi di Petrografia per il Nuovo Ordinamento del Corso di Studi in Scienze Geologiche è tratta dalla Enciclopedia of Life Supporting System (EOLSS). La versione integrale è pubblicata al sito Web PRESSURE, TEMPERATURE, FLUID PRESSURE CONDITIONS OF METAMORPHISM Marco Scambelluri, Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, Corso Europa 26, Genova, Italy msca@dipteris.unige.it Keywords: metamorphism, temperature, pressure, fluid Contents 1. Introduction 2. General Features of Metamorphism 2.1. Pressure-Temperature Conditions of Metamorphism 2.2. Types of Metamorphism 2.3. Kinetics of Metamorphic Reactions 3. Temperature 4. Pressure 5. Variations of Metamorphic Mineral Assemblages in Dependence of Pressure and Temperature 6. Role of the Fluid Phase During Metamorphism 6.1 The Catalysis of Mineral Reactions by Fluids 6.2 Incorporation of Fluids in Metamorphic Rocks and Associated Compositional Variations 6.3 Fluid Release in Metamorphic Rocks: Dehydration and Decarbonation Reactions 6.4. Influence of Fluids on the Behaviour of Metamorphic Rocks 7. Trends in Metamorphic Petrology Glossary Metamorphism: the mineralogical and structural changes of solid rock in response to environmental conditions. Temperature: it is the most important variable in metamorphic processes, since many metamorphic reactions are driven by changes in temperature. It is a measure of how hot the rocks are; it is ususally referred to in degree Celsius ( C) or in Kelvins (K; K = C + 273,15). Pressure: it is the second most important variable and many metamorphic reactions are pressuredependent. It is a measure of the force per unit area to which a rock is subjected, and depends on the density of the overlying rock column and on the depth. It is measured in bar ( 1 bar = atmospheres), kilobar (1 kbar = 1000 bar) or in gigapascal (1 Gpa = 10 kbar). Fluid: most metamorphic rocks contain chemical species such as H 2 O, CO 2, H 2, O 2, CH 4 and S either as components of metamorphic mineral (e.g. hydrated silicates, carbonates, sulfiddes), or as a fluid phase in the pores of rocks. Significant volumes of H 2 O and CO 2 can be evolved from rocks during metamorphic dehydration and/or decarbonation reactions, thereby producing a free fluid phase in metamorphic rock systems.

2 Summary This paper presents an insight on the main aspects related to the metamorphism of rocks. Metamorphism is a process of mineralogical and compositional change that has affected considerable volumes of rocks that are presently exposed on the Earth s surface, and that constitute large parts of old cratonic areas, mountain buildings, oceanic basins. These rocks still preserve at surface environments relict, metastable, associations of metamorphic minerals and/or metamorphic mineral inclusions that indicate their past crystallization at geological environments such as the base of the continental crust, the boundaries between colliding plates, the deep mantle. The study and analysis of these rocks has thus given important answers on the modes of accretion and consumption of the Earth s crust, on the evolution of mountain chains and continents and on a number processes which have driven the evolution our planet through geological timescales. Metamorphic transformations in rocks normally occur in the solidus state and in absence of significant production of partial melts and magmas. They are driven by changes, which normally result from tectonic processes, in some of the following variables: the pressure and temperature conditions of a given rock body, its bulk chemical composition, the availability and access of a C-O- H bearing fluid phases, the presence and/or absence of rock deformation. Pressure, temperature, bulk rock and fluid compositions are the principal factors controlling the type of the metamorphic mineral assemblages produced, which are indicative of the dominant thermal regime during the transformation and of the geodynamic environment. An increasing amount of research work and evidence has been brought in the last decade to recognize the fundamental role played in metamorphic processes by the fluid phases, particularly in the kinetics and enhancement of metamorphic reactions, in the diffusion and transport of elements at variable scales in metamorphic environments, in the development of large scale phenomena such as partial melting of crust and mantle. 1. Introduction Metamorphism of a given rock body occurs in response to significant changes of intensive variables, such as pressure, temperature and composition, which disturb the pre existing equilibrium conditions and force the rock to reach a new state of more stable equilibrium. The IUGS subcommission on the systematics of metamorphic rocks, has defined metamorphism as the process causing substantial changes in the mineralogy, structure and/or bulk chemical composition of a given rock volume. Changes are due to physico-chemical conditions different from the ones attained in sedimentary and diagenetic environments, and may include partial melting as long as most of the rock volume remains in a solid state. If significant change in the bulk rock composition is the dominant process due to open system behaviour of a given rock body, the term metasomatism is applied. In metamorphic terranes, preservation of large volumes of rocks recording various events of recrystallization and of reconstitutive phase changes, provide evidence on the evolution of the Earth s system through time. Metamorphism is generally associated with large-scale tectonic processes that cyclically occurred during the Earth s history. On a global scale, metamorphic rocks are dominant terrestrial materials and are among the oldest rock dated so far at 3.7 billion years: their study is therefore a key to deciphering the main events and large-scale mass movements which dominated the Earth s history from her infancy to present. This article aims at reviewing the main driving forces of metamorphism, grossly individuated with temperature, pressure and role of the fluid phases involved. It will not consider the important effects of deformation on rock recrystallization, on the catalysis of mineral reactions and on reaction kinetics. This article bases on several textbooks on the petrography and petrology of

3 metamorphic rocks (Powell, 1978; Best, 1982; Yardley, 1989; Bucher and Frey, 1994, Spear, 1995), these texts represent the most relevant sources of basic information on the process of metamorphism. 2. General Features of Metamorphism 2.1 Pressure-Temperature Conditions of Metamorphism The physico-chemical conditions at which metamorphic transformations begin depend on the type, texture and composition of the rock material involved. Organic matter for instance, records metamorphic changes at significantly lower temperatures than silicate and carbonate rocks. In many rock systems, the boundary between diagenesis and metamorphism is faint and arbitrary, and metamorphic phase transitions appear to develop at temperatures as low as 200 C (Figure 1A). Key minerals such as carpholite, stilpnomelane, paragonite and zeolites are indicators for the beginning of metamorphism. The high temperature limit of metamorphism corresponds to the onset of partial melting of a given rock system: metamorphic rocks recording high temperature recrystallization associated with partial melting, display granitic melt layers and/or pockets, closely associated with restitic rock volumes. The latter appear depleted in fusible components which were uptaken by the melt phase. The partial melting temperatures of rocks depend on pressure, rock composition, as well as on the amount and on the composition of fluid present. At a constant pressure and in presence of water-rich fluids, partial melting of granitic rocks starts at lower temperature (about 650 C) than melting of basaltic rocks (about 700 C). In absence of fluid, partial melting temperatures are as high as 1000 C for granitic systems and 1100 C for basaltic ones (Fig. 1A) Pressure Gpa Pressure Gpa Not realized on Earth 200 METAMORPHISM MAGMATISM METAMORPHISM MAGMATISM Temperature C Figure 1. A: pressure versus temperature diagram showing the main domain of metamorphism in crustal rocks, comprised between the field of diagenesis at low temperature and pressure and the wet solidus of granites and of basaltic systems (drawn

4 after Bucher and Frey, 1994). B: pressure - temperature diagram showing the field of metamorphism in mantle peridotites, delimitated at high temperatures by the dry peridotite solidus and, at lower temperatures, by the peridotite solidus in presence of water and of CO 2. The above temperature ranges thus represent the upper temperature limits for crustal metamorphism. At mantle conditions the temperature limit of metamorphism rises significantly, solid state changes in this environments being attained at much higher temperatures. Figure 1B shows the solidus curves of peridotites in absence of fluid (dry solidus) and in presence of carbonic (solidus + CO 2 ) and aqueous solutions (solidus + H 2 O): it appears that in absence of fluid phases, subsolidus (metamorphic) phase transitions in mantle rocks can be attained at temperatures as high as C and at relevant depths for a range of geothermal gradients. Great part of upper mantle peridotites thus record subsolidus changes and thereby behave as metamorphic rocks. Concerning the pressure limits of metamorphism, low pressure transformations occur in most contact aureoles formed during magma ascent and emplacement at shallow levels in the Earth s crust. The high pressure limit of metamorphism is yet unknown. In the recent past, it was thought that the maximum pressures attained by metamorphic rocks buried at convergent plate margins and recrystallized under high-pressure conditions (eclogite-facies rocks) did not exceed 10 kilobar pressure, roughly corresponding to 30 kilometers depth. The more recent discovery that coesite is the stable SiO 2 form in eclogite-facies rocks of several orogenic belts (as for instance the Italian Alps, the Caledonides of Norway, the Dabie-Shan mountains of China) has set the high pressure limit of metamorphism above 30 kilobars ( km; ultra-high pressure metamorphism) (Chopin, 1984; Smith, 1984; Wang et al., 1989). Finding of diamonds in some ultra-high pressure rocks has set the possible limit at much deeper levels (Sobolev and Shatsky, 1990; Dobrzhinetskaya et al., 1995). An extraodinary record of ultradeep provenance of mantle rocks has been recently proposed for the metamorphic garnet lherzolites of Alpe Arami in the Swiss Alps (Dobrzhinetskaya et al., 1996; Bozhilov et al., 1997) and of Western Norway (Van Roermund and Drury, 1998). Although a debate is going on to assess the exact origin and depth of provenance of these rocks, particularly the Arami garnet peridotite (Hacker et al., 1997; Green et al., 1997; Trommsdorff et al., 2000), they are thought to retain phase transformations of deep mantle olivine (wadsleyte) and of majoritic garnet (forming at depths close to the transition zone in the upper mantle,) into mineral assemblages stable at shallower depths of about km in the upper mantle. These recent discoveries derive from the application of advanced techniques to the current analysis of metamorphic rocks and shift the pressure boundary of metamorphism recorded by rocks presently exposed on surface to extreme depths into the upper mantle, thereby enabling to considerably deepen our knowledge on the behaviour of the Earth s interior. 2.2 Types of Metamorphism Metamorphism can be manifest over large regions such as orogenic chains at convergent plate margins, cratonic areas, oceanic basins, and extensional environments where deep crustal and/or mantle rocks are slowly exhumed to the surface. On the other hand, metamorphism can be the result of local-scale processes such as development of kilometer-large contact aureoles around plutons intruded at high crustal levels in cool country rocks, or frictional heating along major faults. Orogenic metamorphism dominates in mountain buildings, where considerable volumes of rocks with different paleogeographic and lithosheric provenance are tectonically stacked together as the result of large scale movements during plate convergence and collision. These orogenic cycles bring surficial crustal rocks to mantle depths and then return them to the Earth surface, thus causing superposition of several metamorphic events coupled with permanent ductile deformations. Main

5 features of metamorphic rocks exposed in orogenic belts are their strongly deformed structures, developed as the result of stress and deformations developed at variable temperatures and pressures during their orogenic pathways. However, metamorphism and deformation are extremely heterogeneous in these rocks, and the records of the starting materials are systematically preserved in several undeformed rock domains, thus enabling to reconstruct the whole history of rock materials involved in the orogenic cycle. Oceanic basins are diffusely floored by mafic and ultramafic rocks metamorphosed at variable temperatures and moderate pressures in the presence of seawater-derived solutions. A great amount of petrographic, petrologic and geochemical works have been performed on oceanic metamorphic rocks in the course of Ocean Drilling Projects aimed at defining the dynamics of present-day oceans. Diffuse features of oceanic gabbros, basalts and peridotites are hydration reactions at variable conditions which affect these rocks in the vicinity of oceanic ridges and during lateral spreading off the oceanic centres. Close to mid oceanic ridges, the lithosphere is cut by hydrothermal systems where deep seawater penetration occurs (down to about 3 km and more) accompanied by complex fluid/rock interactions which determine an exchange of components with the surrounding rocks. These processes locally bring to metasomatism of rocks, consisting of Mg-, Ca, Na-enrichments and diffuse Si-depletion. The above features significantly control the element and volatile budgets in oceans and enrich the oceanic lithosphere in exogenic and crustal components, which become recycled into the mantle once the oceanic lithosphere is deeply buried along subduction zones. Contact metamorphism is the most diffuse type of local metamorphism affecting rocks at the contact with intrusive and extrusive igneous bodies. Metamorphic and metasomatic changes in these country rocks are determined by heat flow and by infiltration of late-stage igneous fluids emanating from the magma chamber. As a consequence, aureoles of contact metamorphic rocks generally develop around plutons. The extension and width of aureoles depend on several factors such as volume and composition of intruded magmas, on the depth of emplacement as well as on the properties of country rocks. The volume of an intrusive body is important because large plutons bring more heat than smaller ones. Also, the compositions of the dominant intrusive rock type is another parameter controlling the overall temperature, since granitic melts form at much lower temperatures than basaltic ones. The intrusion depth determines the thermal gradient and heat flow between hot plutons and country rocks. The highest temperature differences are attained at surface crustal levels, since at deep crustal environments the temperature differences between country rocks and magmas are much lower. 2.3 Kinetics of Metamorphic Reactions The process of metamorphism does not affect homogeneously a given rock body; from kilometric to microscopic scales the records of earlier evolutionary stages can survive a metamorphic event. Metamorphic terrains therefore contain rock domains which equilibrate at certain (dominant) metamorphic conditions; these domains are spatially associated with minor volumes of rocks that still record previous geologic events, manifest as relict minerals and as preserved rock textures. The basement rocks of Western Norway represent a well known example of such an association of rock volumes recording metamorphic imprints acquired at different geologic ages. Here mafic rocks with Precambrian granulite - facies metamorphism outcrop as relict bodies inside major volumes of mafic rocks reequilibrated at eclogite - facies conditions during the Caledonian orogenesis. Granulitic and eclogitic domains were never separated during the whole time span from Precambrian to Caledonian (Griffin and Carswell, 1985), and their spatial association has been shown to result from unfavourable reaction kinetics during transformation of granulites into eclogites. Detailed structural and petrologic studies (Austrheim, 1987) demonstrated

6 that this transition mainly took place in the rock volumes more intensely affected by plastic deformation and by infiltration of aqueous fluid solutions. Similarly, widespread exposure of rocks formed in the deep crust and mantle, indicates unfavourable reaction kinetics during their exhumation pathways to the Earth s surface. Deep metamorphic mineral assemblages survive outside their stability field due to a combined effect of exhumation rates faster than the ones of metamorphic mineral reactions, and lack of fluid infiltration. Presence of fluids would cause hydration reactions and re-equilibration of high grade minerals into hydrous mineral assemblages stable at shallow crustal environments. Despite mineral reactions are expected on the base of thermodynamic predictions, high activation energies can be required for the transformation to develop. Until these energy barriers are not trespassed, metamorphic reactions and, most importantly, nucleation and growth of new metamorphic minerals, do not occur. Activation energies are lowered by plastic rock deformation, by significant temperature overstepping of a mineral reaction boundary, and by presence of fluids and deformation which control the diffusions of cations. Field and petrographic experience indicate that rock volumes less affected by fluid and deformation activity are the ones less intensely transformed and better preserving the pre-metamorphic features. The general preservation of deep rocks at the Earth s surface, as well as diffuse survival in metamorphic terrains of pre-metamorphic features, indicate that slow reactions kinetics occurred systematically, thereby preventing the full recrystallization of a given rock volume during a certain metamorphic event. When catalytic factors do not assist mineral reactions, rocks do not transform during a given metamorphic event and the nature of pre-metamorphic materials metastably survives the transformation. 3. Temperature Temperature is a driving force of metamorphism. Temperature increases with depth in the Earth and the rate at which it changes with depth defines a geothermal gradient. Geothermal gradients in the average continental crust are usually around 30 C/Km, but large variations occur depending on the thermal structure of the various terrestrial environments. The variability of geothermal gradients is reflected by variations of heat flow measured at the surface, and the amount of heat at a given geological setting depends on the following sources: 1) heat rising from the mantle; 2) heat produced by radiogenic decay, particularly effective in acid crustal rocks; 3) heat associated with flow and intrusions of large magmatic bodies; 4) heat brought to the surface during fast uprise of hot, deep seated rocks either in extensional settings, or in areas of rapid uplift of mountain chains. Variations of surface heat flow shown in Figure 2 reflect the geothermal gradients at several geodynamic environments (after Yardley, 1989). Surface heat flow W/m Mid-oceanic ridges oceanic crust of increasing age Subduction zone trenches Volcanic arcs Back-arc basins 500 km Figure 2. Variability of surface heat flux at several geodynamic settings (drawn after Yardley, 1989).

7 The highest surface heat flows are attained at mid-oceanic ridges, where uprise of asthenospheric mantle to superficial levels is accompanied by partial melting and widespread magmatic activity. Here the heat flux from the mantle and from large magmatic chambers are the major components of such high geothermal gradients. The surface heat flow progressively decreases away from mid-oceanic ridges, since lateral spreading off oceanic centres is accompanied by aging, hydration and cooling of the oceanic lithosphere. The lowest geothermal gradients are attained in Figure 2 at subduction zones, where burial to great depths of cool crustal material screens the heat flux from the mantle and strongly depresses the geotherms. In crustal and in supra-subduction environments the geothermal gradients rise, due to a combined effect of increased heat flux from the mantle, radiogenic decay and uprise of magmatic bodies. The latter activity is particularly significant at compressive margins. The different values of surface heat flow therefore reflect various thermal regimes attained in the different tectonic and geodynamic settings. The rates at which temperature changes with depth, i.e. the geothermal gradient, in the various tectonic and geodynamic environments are shown in the pressure-temperature diagram of Figure 3. The diagram shows the range of metamorphic conditions attained in various settings and also reports for reference the melting curves for dry and wet granite and basaltic systems. High-pressure low temperature metamorphism develops in subduction zones and in geologic regions were rapid overthrusting of large continental masses has occurred. Low pressure high temperature metamorphism characterizes the environments of high heat flow, such as the oceanic ridges, the island arcs and the contact aureoles. Finally, the pressure temperature region of intermediate gradients is generally achieved in orogenic belts were collision of continental slices has occurred Pressure GPa Not realized on Earth Subduction zones Continental orogenic belts wet granire solidus Island arcs, mid ocean ridges contact aureoles wet basalt solidus dry granite solidus dry basalt solidus Depth km Temperature C

8 Figure 3. Pressure versus temperature diagram reporting the ranges of geothermal gradients (shaded areas) attained at different geodynamic settings (drawn after Spear, 1995). A general process showing metamorphic and textural variations in rocks during an overall process of temperature increase during Burial of a metamorphic rock along a given geotherm is shown in Figure 4. Pressure P2 P1 1 A(OH) + B 2 C + D + H O 2 C E 1 2 C A(OH) E C B T1 T2 Temperature Figure 4. Schematic pressure - temperature diagram reporting a metamorphic path (bold line) crosscutting two mineral reaction boundaries (1 and 2) producing new mineral assemblages at the expense of reactant phases. The two drawings at the right side of the diagram show the reaction microstructures and the possible textural relations between reactant and product phases formed as the result of reactions 1 and 2 (after Bard, ). As a result of increasing temperature and pressure the rock will experience heating and will undergo prograde metamorphism. In Figure 4, a rock recording prograde metamorphism will trespass the reaction curve where minerals A + B will react to produce a new metamorphic assemblage made of B + C + a free aqueous fluid phase. Subsequently, at higher temperature and pressure a new mineral phase E will form at the expense of mineral C through the reaction C = E. The nucleation and growth of product phases C, D, E will take place at either at the grain boundaries, or above the reactant phases A, B (or C in case of reaction 2 in Fig. ). The newly formed minerals will surround the reacting materials and the latter will appear as corroded, relict precursor mineral phases. Along the prograde metamorphic path of Figure 4, a new paragenesis will be stable at the increased temperature conditions and a new rock fabric will be therefore produced. Conversely, if a volume of rock is involved in a cooling process, it may undergo retrograde metamorphism. The reactions shown in Figure 4 all involve formation nucleation and growth of new mineral assemblages during a heating process. Crystallization of the new mineral assemblages will lower the free energy of the whole system. However, even when formation of the new mineral assemblage is energetically favoured, minerals will not necessarily nucleate and grow. This largely depends on the activation energies required for the transformation to occur: these represent energetic barriers to nucleation and growth, that must be overcome by means of catalysts and through significant temperature increase beyond the reaction boundary. The excess heating required is called overstepping and the amount of overstepping is related to the nature of the mineral reactions and on

9 their temperature dependence (Fig. 5). Figure 5A shows a reaction with large temperature dependence, such as many dehydration and decarbonation reactions: this implies that the reaction boundary must be overstepped of a small amount to nucleate and grow the reaction products. The reaction shown in Figure 5B has a small temperature dependence which may result in a larger overstepping. The overstepping effect if minerals have an energy budget stored in the form of strain. Gibbs Free Energy Nucleii of B A + C B A Gibbs Free Energy Nucleii of E E B A + C T Overstepping Temperature T Overstepping Temperature Figure 5. Gibbs energy versus temperature diagram showing the temperature dependence of two mineral reactions (A + B = C and A + B = E) and the amount of overstepping required to actually achieve nucleation of the product phases B and E. 4. Pressure The changes in pressure are another dominant cause of metamorphism: they are related to variations in the depth reached by metamorphic rocks during their history. The prevailing pressure at a given depth is defined by the average density of the overlying rock column. It can be calculated from P = ρgh, where g is the acceleration due to gravity, ρ is the density, h is the depth. Pressure is normally measured either in kilobar (1000 bar; 1 bar = atmospheres) or in gigapascal (10 Kbar = 1GPa). The total pressure acting at a point in the crust or mantle is given by the weight of overlaying rocks and is called lithostatic pressure. This pressure is assumed to be isotropic, i.e. is equal in all directions. Lithostatic pressure does not cause rock deformation, and many rocks undergoing metamorphism at high and very high pressures may not display significant distortion and may still preserve pre-metamorphic textures. Non isotropic (oriented) pressures often occur in metamorphic environments as the result of tectonic forces causing permanent rock deformation. Non-isotropic pressure is one major factor that causes permanent ductile and brittle deformations and that controls the textural features of rocks (metamorphic foliations, compositional layerings, vein systems). Ductile deformation causes plastic flow of rock materials along high strain horizons (shear zones), diffusion and redistribution of chemical components in metamorphic rocks. In metamorphic terrains deformation is heterogeneous and rocks showing highly deformed fabrics (tectonites and mylonites) outcrop close to rocks with the same bulk composition and metamorphic mineral assemblages, but showing undeformed textures (massive and coronitic rocks). The similarity of metamorphic assemblages indicates the these texturally different rock types underwent metamorphic recrystallizations at the same pressure-temperature conditions, but under different deformation regimes. Heterogeneity in the intensity of deformation in closely associated rock volumes is a general feature of metamorphic rocks and depends on a process known as deformation partitioning, which causes the spatial coexistence of high-strain and of low-strain domains. The

10 latter still preserve textural and mineralogical records of evolutionary stages predating a given metamorphic and deformation event. In all geologic environments changes in pressure- and depth-related changes are induced by tectonic processes. Pressure changes recorded by rocks presently exposed in orogenic belts indicate that during subduction at convergent plate margins large portions of surface materials have been tectonically buried into the mantle to depths exceeding 100 kilometers, thereby developing assemblages made of high density metamorphic minerals (including garnet and pyroxene). These tetctonic units made of high and ultrahigh pressure metamorphic rocks were then returned to the surface during tectonic exhumation processes, causing the retrograde metamorphic imprint of the high pressure minerals by less dense, hydrated mineral assemblages. Variations in depth associated with pressure changes also occur in subsiding sedimentary basins during overall lithospere extension and thinning. This type of metamorphism experienced by deeply buried sedimentary rocks is called burial metamorphism. 5. Variations of Metamorphic Mineral Assemblages in Dependence of Pressure and Temperature Barrow (1893) firstly mapped in the crystalline basement rocks of the Scottish Highlands a sequence of zones characterized by the first appearance in pelitic schists of metamorphic index minerals (such as chlorite, biotite, garnet, staurolite, kyanite and sillimanite), which were related to an increasing grade of metamorphism, i.e. to a progressive increase of temperature and pressure conditions. The individual minerals are systematically distributed in distinct regional mineral zones (metamorphic zones), and adjacent zones are separated by isograds, and/or reaction isograds. These correspond to surfaces of equal metamorphic grade separating domains metamorphosed at different pressure and temperature conditions, and therefore represent paleogeotherms. Since Barrow s work, studies and maps have been performed on metamorphic crystalline basements in order to understand the distribution of the different mineral assemblages in and to relate them to a regional variability in the main parameters governing metamorphism, such as pressure, temperature and the bulk compositions of rocks. The concept of index mineral then evolved to mineral paragenesis: an assemblage of metamorphic minerals coexisting in textural and chemical equilibrium in a rock of a given bulk composition, and formed at a certain temperature and pressure. In a metamorphic terrain rocks metamorphosed at the same pressure-temperature conditions develop different parageneses in dependence of their different bulk composition. Therefore the concept of metamorphic facies (Eskola, 1915) was proposed to define the whole set of mineral parageneses occurring in spatially associated rock types of diverse bulk chemical compositions and metamorphosed under the same broad pressure temperature conditions. Figure 6 shows the ranges of pressure and temperature conditions at which the metamorphic facies are interpreted to form. The boundaries between the different facies are not sharp because most characterizing mineral assemblages form by continuous, rather than discrete (discontinuous), reactions. This depends on the fact that most minerals form complex solid solutions and that metamorphic fluids correspond to compositionally complex solutions: these features determine a shift of mineral reactions over pressure temperature regions and make the transition among different facies to along pressure temperature intervals. Figure 6 also reports the gradients typical of various geodynamic settings and shows that the transition from one facies to the other, as mapped in several metamorphic terrains, depends on the thermal gradients that affect rocks during their history. As a consequence Miyashiro (1973) emphasized that certain metamorphic facies are commonly associated to the exclusion of others in different orogenic belts; a metamorphic facies series in a given terrain is therefore diagnostic of the geothermal gradient attained and of the metamorphic paths followed by rocks during their evolution.

11 Pressure GPa Not realized on Earth Eclogite Blueschist High P/T Greenschist Prehnite- Pumpellyite Intermediate P/T Epidote amphibolite Amphibolite Granulite 0.2 Zeolite Low P/T Temperature C Figure 6. Pressure temperature diagram ilustrating the stability field of the various metamorphic facies and reporting the major types of metamorphic facies series attained at different tectonic envirronments (drawn after Spear, 1995). High P/T metamorphism is characteristic of subduction zones, intermediate P/T is characteristic of continental collision zones and low P/T characterizes ocean ridges and island arcs. The ranges of geothermal gradients of the above geodynamic environments are shown in Figure Role of the Fluid Phase During Metamorphism It has been stressed in the previous chapters that metamorphic processes are extremely heterogeneous and that development of metamorphic reactions and mineral growth strongly depends on the availability of catalysers. In this scenario, presence of fluid phases at the reaction sites and the composition of these fluids play an important role. As outlined in section 3.2, in the basement rocks of Western Norway fluid infiltration controlled the eclogitization of deep continental granulitic crust. The metamorphic recrystallization of these rocks was not simply a function of pressure, temperature and rock composition, but strongly depended on the presence of metamorphic fluids. Fluids also accomplish mass transfer and element mobility at various scales. Dehydration and/or dehydration-melting of crustal rocks during orogenic processes can be associated to large scale mass transfer and can produce significant compositional variations in the deep continental crust (Clemens and Droop, 1998). Upwards migration of metamorphic fluids released during subduction of oceanic plates causes hydration and metasomatism of the mantle wedges overlaying the subduction zones: this lowers the mantle solidus and favours its partial melting with consequent

12 formation of the arc and calcalkaline magmas typically produced at convergent plate margins (Tatsumi, 1989). These geologic examples illustrate that fluids play a major control on the catalysis of metamorphic reactions, on mass transfer and on the mechanical properties of rocks and minerals. 6.1 The Catalysis of Mineral Reactions by Fluids The efficiency of fluids as catalysers of mineral reactions is qualitatively well known and has led several authors to suggest that water availability at the reaction sites represents the most important factor for the development of relevant mineral reactions, such as for instance the gabbro to eclogite transition at subduction zones (Ahrens and Schubert, 1975; Rubie, 1990). The presence of water during transformation of aragonite to calcite lowers the activation energy required for the reaction to occur (about half the energy necessary at dry, water absent, conditions), and rises the reaction rates of several orders of magnitude (Fig. 7; Brown et al., 1962) T C ARAGONITE CALCITE 100 Ma 1 Ma DRY 3 10 a time WET 1a 1month 1 day 1 hr /T K Figure 7. Aragonite - calcite transformation at dry and wet conditions shown as a function of time and temperature (Brown et al., 1962). Fluids are generally incorporated in hydrous minerals and are released in the course of devolatilization reactions during heating and burial of rocks. The presence of metamorphic fluids can thus be limited to short periods of time coincident with mineral devolatilization episodes and with infiltration of fluids deriving from external sources. Fluids are heterogeneously distributed in rocks and preferentially locate along intergranular surfaces and within pore spaces. The interganular domains of a rock can be: i) dry; ii) hydrated, but undersaturated in fluid; iii) saturated in fluid. Below a given water/rock ratio, water molecules are incorporated in crystalline lattices, particularly in the regions close to grain boundaries. The water dissolved in such domains facilitates diffusion (grain boundary diffusion), probably substituting strong Si-O bonds with weaker Si-OH bonds, thereby effectively reducing the activation energy required for metamorphic transformations. Above a critical water/rock ratio, grain boundaries become water saturated and a free fluid phase is present in the system, and this fluid phase can be interconnected along grain edges. In presence of a free fluid phase the diffusion mechanism changes from diffusion in a solid crystalline net (grain boundary diffusion) to diffusion within a fluid medium along interfaces between mineral grains (interface diffusion).

13 6.2. Incorporation of Fluids in Metamorphic Rocks and Associated Compositional Variations. Sedimentary rocks contain large amounts of fluid fixed in the structure of hydrous minerals and present as a free phase in pore spaces; before diagenesis and prograde orogenic and burial metamorphism the rocks are at their maximum state of hydration. Differently, igneous rocks have low initial fluid and water contents and incorporate fluid as the result of secondary alteration driven either by hydrothermal, or by meteoric waters. In this respect, alteration and hydration of the oceanic crust and lithosphere is particularly relevant, since in oceanic basins rocks of igneous and/or metamorphic origin having very low initial water contents (basalts, gabbros, mantle peridotites) become significantly enriched in water, carbon dioxide and other volatile components as the result of interaction with seawater-derived solutions. Water/rock interaction during the alteration of oceanic basalts induce relevant compositional variations of several rock components (MgO, CaO, Na 2 O, K 2 O, Cl, H 2 O, CO 2, stable C-O-H isotopes) as a function of fluid composition and chemistry, of the alteration temperature and of water rock ratios during the alteration process (Mottl, 1983; Gregory and Taylor, 1981). The progressive enrichment in MgO and depletion in CaO documented in many oceanic basalts (Fig. 8) is the result of alteration by seawater at increasing water/rock ratios. Compared to fresh (unaltered basalts), the compositions of altered basalts sampled in the Mid Atlantic Ridge (MAR), in the East Pacific Rise (EPR) and in the Indian Ocean (Fig. 8) display a general decrease in CaO coupled with an increase in MgO. Comparable compositional trends have been obtained during laboratory experiments by reacting seawater with basalts at increasing water rock ratios (W/R 10 to 50, 62 and 150). CaO Fresh basalts W/R 10 MAR basalts EPR and Indian Ocean basalts W/R 10 W/R 50, 62, W/R 50, 62, 125 A MgO Figure 8. MgO versus CaO diagram showing the compositional variations in oceanic basalts as the result of interaction with seawater. The diagram reports the compositional field of unaltered basalts; the compositions of variably altered oceanic basalts from the Mid-Atlantic Ridge (MAR), from the East Pacific Rise (EPR) and from the Indian ocean. Also reported are the results of seawater-basalts experiments at increasing water/rock ratios (W/R 10, W/R 50, 62, 125) (Mottl, 1983).

14 The data indicate that alteration of oceanic rocks is a complex process that can imply, besides enrichment in the fluid and volatile contents of rocks, significant gains and losses of major and trace element components, and can even cause metasomatic compositional changes. The metamorphic reactions between fluids and rocks can produce significant variations of the starting rock compositions; they will control the type of mineral assemblages and the composition of fluids which are going to be produced if these rocks become involved in a new metamorphic cycle (for example prograde metamorphism during subduction and/or plate collision). 6.3 Fluid Release in Metamorphic Rocks: Dehydration and Decarbonation Reactions. Fluid release during tectonic and metamorphic processes is both related to compaction of rock material with consequent expulsion of free fluid phases present in pore spaces and fluid inclusions, and to devolatilization reactions. The latter occur in the general form hydrated mineral assemblage = less hydrated (or dry) assemblage + fluid at increasing temperature, and control the depths of fluid release and the amounts of fluids produced during a metamorphic cycle. The reactions involving breakdown of OH-bearing minerals and liberation of a water-rich phase are most common. Figure 9A shows as an example the breakdown reaction of talc to enstatite + quartz + fluid. A 3 2 X = 0.13 CO2 X = 0.5 CO2 X = 1 CO2 B Pressure tlc H2O a = 0.4 H2O a = 0.6 a = 1 H2O H2O a = 0.8 En + qtz+ H2O P kbar 1 Calcite + quartz + fluid Wollastonite + fluid Temperature T C Figure 9. A: pressure - temperature diagram showing the dehydration curve talc = enstatite + quartz + water for different fluid compositions (after Bucher and Frey, 1994). B: pressure - temperature diagram reporting the reaction calcite + quartz = wollastonite + CO 2 for different CO 2 concentrations in the fluid phase (after Best, 1982). In a pressure-temperature diagram, the dewatering reactions generally display a change in the slope of the reaction curve from positive at low pressure, to negative at high pressure. This fact is due to significant variations in the molar volume of the fluid produced (high molar volumes at low pressure and low at high pressures), which in turns determines the V of the reaction and, hence, the slope of the reaction curve. The shape of the reaction curve in Figure 9A indicates that talc dehydration (as well as dehydration of other hydrous mineral phases) can be caused by increasing pressures and temperatures (heating and burial), and by decrease in pressure. The temperature at which a dewatering reaction takes place depends on the fluid composition. Breakdown of talc, for instance, occurs at progressively lower temperatures if the associated fluid is not pure water and dissolves other components (such as CO 2, CH 4, N 2, Cl) and the water activity in the fluid becomes lower than 1 (Fig. 9A). Presence of complex fluids the compositions of which is significantly

15 different from pure water has been largely described in the geological and petrological literature (Andersen et al., 2001): one main consequence of the presence of such fluids is a significant restriction of the pressure-temperature the stability fields of reactant hydrous phases. Similarly, the stability fields of carbonatic phases reduce strongly if the associated fluid does not correspond to pure CO 2 and contains extra components, such as water, methane and nitrogen. This relationship is shown in Figure 9B, where the reaction curve calcite + quartz = wollastonite + CO 2 is shown for variable concentrations of CO 2 in the fluid. The CO 2 /H 2 O ratios in metamorphic fluids can significantly change from one rock domain to the other during a given metamorphic event, as a function of the amount of fluid and of its mobility, of the type of mineral assemblage buffering the fluid composition, and of the permeability of rocks involved. 6.4 Influence of Fluids on the Behaviour of Metamorphic Rocks. The distribution of water in the Earth s crust ad mantle plays a control on the rheological properties and on the ductility of rocks. The effects of water on the behaviour of rocks have been quantified by means of laboratory deformation experiments involving dry and wet rocks and minerals as starting materials. Water has a softening effect on the minerals and rocks undergoing ductile deformation: this effect is known since a long time as hydrolitic weakening and has been quantitatively well studied in quartzites, granites and dunites, i.e. the rock materials representative of crust and mantle (Ranalli, 1995). The stress versus strain diagrams of Figure 10 indicate that quartz and olivine crystals containing trace amounts of dissolved water are much more ductile than dry crystals. stress MPa dry natural A B wet syntetic strain Figure 10. Stress versus strain diagrams for dry and wet olivine and quartz. The weakening effect of water on olivine indifferently affects coarse and fine grained crystals (Figure 10B). Comparable relationships have been shown to occur in granitic rocks and in materials made of polycrystalline albite aggregates: in these materials the temperature at which occurs the transition between fragile and ductile regimes lowers of C for water contents of about 0.2 weight percent (Tullis and Yund, 1980). The distribution of metamorphic aqueous fluids in the crust and mantle therefore has relevant implications on their rheological properties and strongly enhances the localization of important deformation processes along given horizons corresponding to water-enriched domains. The latter therefore represent candidates to become

16 ductile shear zones, where significant plastic flow and mass transfer occur during main large scale tectonic events which accomplish the main changes ruling the geodynamic history of the Earth. The partial pressure of the free fluid phase in the pore spaces of rocks and its volume expansion during heating represent another important variable during metamorphism. The pressure of fluid (P fluid ) counteracts the lithostatic pressure (P litho ), and locally the fluid pressure can overcome the lithostatic one. In such cases rocks undergo hydraulic fracturing: fluid and rock pressures re-equilibrate and fluids flow throught the rock along fracture systems. Norris and Henley (1976) have shown that compaction of rocks during regional metamorphism and tectonic is accompanied with dehydration reactions and temperature increase, causing heating and volume expansion of the produced fluids. These processes cause an increase in fluid pressure, thereby leading to hydrofracturing of rocks. Diffuse development vein systems during the entire tectonic and metamorphic history of regional metamorphic rocks indicates that the one envisaged by Norris and Henley can be an ubiquitous process. Figure 11 illustrates the molar volumes of water as a function of pressure and temperature, together with a typical clockwise pressure temperature path of metamorphic rocks undergoing a prograde burial followed by retrograde and decompressional exhumation, as diffusely documented in most orogenic regional metamorphic terrains V H2O cm mole Pressure kbar Temperature C Figure 11. Pressure - temperature diagram showing the molar volumes of water and a typical clockwise metamorphic path (Norris and Henley, 1976). Once a free fluid phase is present in the rock pores, either as entrapped connate water, or as a metamorphic phase produced during dewatering reactions, its molar volume will continuously increase along the burial path, as well as along the exhumation path. The continuous volume expansion of fluids in the metamorphic pile episodically brings to fluid pressures higher than the lithostatic ones and hence to hydrofracturing of the metamorphic rocks. The approach of Norris and Henley (1976) explains the diffuse finding of multiple generations of fracture systems in

17 metamorphic rocks and the continuous spatial redistribution of fluids, mass transfer and fluid channelling along fracture systems. The above features indicate that ductile deformation appear extremely favoured in presence of small amounts of dissolved fluids, whereas high partial pressures of free pore fluids favour the brittle fracturing of rocks. Actually, little is known on the exact state and location of fluid during active deformation. By means of experiments and microstructural studies at the transmission electron microscope, Tullis and Yund (1996) have shown that free intergranular fluids become incorporated in minerals during active deformation, thus significantly favouring plastic deformation and mass and cation diffusion. During the post-deformation annealing of rocks materials investigated, the fluids previously incorporated in minerals during active deformation and wetting crystal defects, slip planes and microfractures, become re precipitated at the grain edges under the form of pore fluids. 6. Trends in Metamorphic Petrology In the recent past, research in metamorphic petrology has been devoted to understand processes controlling the development of mineral parageneses in the various rock types, to develop thermodynamic modelling of metamorphic processes and mineral reactions, and to the assessment of the pressure and temperature conditions of metamorphism through the application of geothermobarometers. All of these studies, joined with textural investigations to recognize the protoliths and the reaction steps of metamorphic rocks, and with radiogenic isotope geochemistry to define the actual ages of metamorphic events, have been largely used to reconstruct the thermal - tectonic regimes during metamorphism and the pressure - temperature - time paths followed by metamorphic rocks during their histories. More recently, the interest has been focused on the role of fluids during metamorphism, their control on reaction processes, on rock deformation, as well as on mass and heat transfer at several crustal and mantle environments. Modelling of fluid behaviour in rocks, coupled with experiments on element partitioning between fluids and the coexisting solids at variable pressures and temperatures, is a fertile branch of metamorphic petrology and geochemistry enabling to quantify the mass transfer via fluid phases. Another important aspect of metamorphic research concerns the continuous finding of ultrahigh pressure mineral records that witness the extraordinary depths reached by some metamorphic rocks. This research line has been favoured by the advancements in experimental petrology, and by the application of technologies enabling to investigate at nanoscales the structures of rock forming minerals. These research trends will deepen our knowledge on the processes acting at the deepest, inaccessible, levels of our planet and governing the dynamics of the Earth s litosphere and mantle. Bibliography Anderson B.D.O. and Moore J.B. (1979). Optimal Filtering, Englewood Cliffs: Prentice Hall. [This is a book that presents all the essential aspects of filtering in dynamical systems] Andersen T., Frezzotti M.L., Burke E.A.J. (2001). Fluid inclusions: phase relationships methods applications. Lithos 55, 320 pg. [This is a special volume that presents all the essential aspects of fluid phase petrology through the analysis of fluid inclusions in rocks] Ahrens T.J. & Schubert G. (1975). Gabbro-eclogite transition and its geophysical significance. Reviews in Geophisics and Space Physics 13( 2),