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EAS 2200 The Earth System Spring 2011 Lecture 10 Metamorphism What and Why Metamorphism refers to mineralogical and textural changes that occur in rocks as a consequence of changing conditions. Many rocks at the surface of the Earth are out of chemical equilibrium (that is to say, their minerals do not represent the most stable possible physiochemical assemblage). When subjected to increased temperature (and pressure), the chemical components in the rock (including both minerals and pore fluid) react in an attempt to achieve chemical and textural equilibrium. Metamorphism can also occur when “dry” rocks come in contact with water at elevated temperature. (Reactions also occur between water and rock at low temperature, but this is called weathering or diagenesis.) Metamorphism, Temperature, and Reaction Rates Metamorphism generally occurs in association with changes in temperature, particularly increases in temperature. This reflects the exponential increase in both reaction rates and diffusion rate with temperature. The relationship is given by the Arrhenius Equation: Rate ∝ e-E/RT Where E is a barrier energy, R is the gas constant, and T is temp. Minerals in a rock can often coexist in a metastable, nonequilibrium state at the surface of the Earth almost indefinitely. For example, the minerals that crystallized from magma at 1000˚C. Reaction rates increase exponentially with increasing temperature. Consequently, once a rock experiences elevated temperature, it begins to react and undergo chemical and textural changes. Textural Changes The simplest kind of metamorphism is where existing minerals simply recrystallize and there is no change in mineralogy Most common in monomineralic rocks. Examples: Limestone -> Marble, Sandstone -> Quartzite Almost always results in a coarsening of grain size. 1 EAS 2200 The Earth System Spring 2011 Lecture 10 Why does this happen? Because atoms at the surface of a substance are in a different environment than those in the interior, there is an excess energy at the surface, called the interfacial free energy. (Surface tension is a manifestation of this energy.) Ratio of surface area to volume decreases with increasing grain size. Consequently, increasing the size of crystals reduces the interfacial energy. Where more than one mineral is present, interfacial energy can sometimes be minimized if only specific minerals or specific mineral faces are in contact. When pressure is uniform, this is called static annealing Stress and Pressure Stress is force/area – same as pressure Normal stress results from force applied perpendicular (normal) to a surface Normal stress equal in all directions is pressure. Pressure resulting from the weight of rocks above is called lithostatic pressure. Shear stress results from force applied parallel to a surface. Recrystallization under Deformation When the pressure is not the same in all directions (i.e., non-lithostatic stress), some grain boundaries will experience more pressure than others. This favors dissolution of some crystal faces and growth of others. This results in the preferred orientation of crystals and development of a type of fabric known as foliation. Under normal stress, minerals orient long axes perpendicular to the stress. Under shear stress, minerals orient long axes parallel to stress. Schists and Schistocity Schistocity is an example of the foliation that develops as a result of preferred orientation of minerals - primarily micas (and other sheet silicates). Gneisses and Banding Gneisses are characterized by bands of light (quartz and feldspar) and dark (micas and amphiboles) minerals that were not originally present. This also results from a minimization of free energy. Nucleation requires less energy when the new crystal nucleates on an existing crystal of the same kind. Hence new micas grow on old micas, new quartz on old quartz, etc. Mineralogical Changes Most often, metamorphism involves replacement of pre-existing minerals with new ones. This reflects a drive toward the equilibrium mineral assemblage for prevailing conditions. As we have seen, individual minerals are generally stable over a limited range of temperature, pressure, and system composition. The equilibrium assemblage is the one that minimizes the energy of the system. Many metamorphic reactions involve hydration or dehydration reactions. Sedimentary rocks rich in clay minerals formed by weathering at the surface will progressively dehydrate as metamorphic temperatures increase. Igneous rocks, forming at high temperature in the virtual absence of water, will become hydrated during metamorphism at lower temperature. 2 EAS 2200 The Earth System Spring 2011 Lecture 10 Hydration Reactions Some examples of hydration reactions that occur during metamorphism of igneous rocks 2 Plagioclase + Diopside + Hematite + Water = Epidote + Enstatite 2CaAl2Si2O8 + 2CaMgSi2O6 + Fe2O3 + H2O = 2Ca2(Al,Fe)3Si3O12(OH) + 2MgSiO3 3 Enstatite + 1 Quartz + Water = Talc 3MgSiO3 + SiO2 + H2O = Mg3Si4O10(OH)2 Many minerals formed by metamorphism of igneous rocks have a green color; hence the term greenschist. Metamorphism at Mid-Ocean Ridges Hydrothermal systems at mid-ocean ridges hydrate the initially anhydrous (i.e., dry) basalt, replacing original minerals with ones such as chlorite, epidote, and albite to form greenschist. At higher temperature, these minerals are replaced by hornblende and plagioclase to form amphibolite. Dehydration Reactions Hydrous minerals break down at elevated temperature. Some examples of dehydration reactions that occur during metamorphism of sedimentary rocks: Pyrophyllite ↔ Quartz + Kyanite 2AlSi2O5(OH) ↔ 3SiO2 + Al2SiO5 + H2O Muscovite + Quartz ↔ K-Feldspar + Sillimanite KAl3Si3(OH)2 + SiO2 ↔ KAlSi3O8 + Al2SiO5 + H2O Metamorphic Facies As we have seen, individual minerals and assemblages of minerals are stable only over a limited range of temperature and pressure (and composition). This leads to the concept of facies. A metamorphic facies is a set of metamorphic mineral assemblages (parageneses), each for a specific rock composition, that form over a specific range of P and T. By reproducing the mineral assemblages in the laboratory, it is possible to relate specific facies to specific ranges of temperature and pressure. 3 EAS 2200 The Earth System Spring 2011 Lecture 10 Facies and Reactions Metamorphic Grade Metamorphic grade refers to the intensity of metamorphism. Grade is strongly related to, but not identical to, temperature. Prograde metamorphism occurs as temperature (and pressure) increase. Retrograde metamorphism occurs as temperature decreases. Metamorphic Zones Index minerals are ones that are stable over only limited range of conditions and hence useful in determining grade (as opposed to, say, quartz and feldspars). In the field, we can map out regions where these index minerals are present or absent. These regions correspond to zones. Zones for pelitic* metamorphic rocks: chlorite zone ((Fe,Mg)6(Si,Al)4O10(OH)8): chlorite + muscovite + qtz biotite zone: chlorite + biotite + muscovite + qtz garnet zone chlorite + biotite + garnet + muscovite staurolite ((Mg,Fe)2Al9Si4O22(OH)2) zone: staurolite + muscovite + biotite + qtz kyanite zone kyanite + biotite + muscovite + qtz 4 EAS 2200 The Earth System Spring 2011 Lecture 10 sillimanite zone garnet + biotite + sillimanite + muscovite + qtz orthoclase zone sillimanite + orthoclase + qtz and no muscovite (± melt ) Isograds Isograds are lines of equal metamorphic intensity. In practice, they correspond to lines where a specific mineral appears (often at the expense of another). Geothermometry & Geobarometry Many minerals have variable compositions, which is to say they are solid solutions. The exact composition of pairs these minerals will depend on temperature and pressure. By analyzing individual mineral compositions and using thermodynamic models based on laboratory experiments, it is often possible to determine the exact temperature and pressure at which a mineral assemblage (i.e., a rock) equilibrated. P-T metamorphic paths Sometimes, rocks do not reach complete chemical and textural equilibrium. In these cases, study of mineral chemistry and and textural relations between minerals can reveal the pressure-temperature path that a rock has followed. This can be quite useful in tectonic reconstructions. Why does metamorphism occur? There are two broad classes of metamorphism: Contact metamorphism This occurs when igneous intrusions heat the surrounding crustal rocks metamorphosing them. Usually restricted to a few km’s at most from intrusive body. Regional metamorphism This occurs when large area are carried to depth where the rocks experience higher temperature and pressure. Usually a result of compressive tectonic forces rather than mere burial by sedimentation. Contact Metamorphism Heat released as an intrusive magma body crystallizes and cools will diffuse into the surrounding country rock. The result is a metamorphic aureole. Width can vary from cm’s to km’s, depending on the size and depth of the intrusion. Contact metamorphism is often “dry”, but intrusions can set up hydrothermal circulation. Mid-ocean ridges and Yellowstone are examples. At shallow depth, contact metamorphism produces hornfels metamorphic facies. Regional metamorphism Regional metamorphism occurs when broad regions are buried to considerable depth (km’s). This usually happens as a result of compressive forces in continental collisions or subduction zones. Consequently, deformation often accompanies metamorphism. Metamorphism is closely related to tectonics and mountain-building, our next topics. A particular kind of metamorphism occurring at low temperature but high pressure produces blueschist and eclogite metamorphic facies and is associated with subduction. The Franciscan complex of the central and northern California coast consists of a chaotic mix that includes rocks metamorphosed under blueschist conditions. It formed 5 EAS 2200 The Earth System Spring 2011 Lecture 10 as oceanic crust and sediment accreted to North America from the subducting Farallon plate during the Mesozoic. Ultra-high Pressure Metamorphism Over the last decade or two, many examples have been found of rocks metamorphosed at pressures corresponding to depths of 100 to 200 km. These are known as ultra-high pressure (UHP) metamorphic belts. Very high pressure minerals such as coesite (SiO2) and diamond are diagnostic. Dabie Shan in China is among the best examples of UHP metamorphism. It is though to represent a peninsula-like piece of continental crust (e.g., Florida) carried into a subduction zone. It eventually “popped” back to the surface under its own buoyancy, like a submerged cork. 6