Metamorphism The IUGS has proposed the following definition of metamorphism Metamorphism is a subsolidus process leading to changes in mineralogy and or texture for example grain size and often in chemical composition in a rock These changes are due to physical and or chemical conditions that differ from those normally occurring at the surface of planets and in zones of cementation and diagenesis below this surface They may coexist with partial melting The Limits of Metamorphism Low temperature diagenesis limit grades into The boundary is somewhat arbitrary Diagenetic weathering processes are indistinguishable from metamorphic Metamorphism begins in the range of 100150oC for the more unstable types of protolith Some zeolites are considered diagenetic and others metamorphic pretty arbitrary The Limits of Metamorphism High temperature limit grades into melting Over the melting range solids and liquids coexist If we heat a metamorphic rock until it melts at what point in the melting process does it become igneous Xenoliths restites and other enclaves are considered part of the igneous realm because melt is dominant but the distinction is certainly vague and disputable Migmatites mixed rocks are gradational Metamorphic Agents and Changes Temperature typically the most important factor in metamorphism Figure 1 9 Estimated ranges of oceanic and continental steady state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface After Sclater et al 1980 Earth Rev Geophys Space Sci 18 269 311 Metamorphic Agents and Changes Increasing temperature has several effects 1 Promotes recrystallization increased grain size Larger surface volume ratio of a mineral lower stability Increasing temperature eventually overcomes kinetic barriers to recrystallization and fine aggregates coalesce to larger grains Metamorphic Agents and Changes Increasing temperature has several effects 2 Drive reactions that consume unstable mineral s and produces new minerals that are stable under the new conditions 3 Overcomes kinetic barriers that might otherwise preclude the attainment of equilibrium Metamorphic Agents and Changes Pressure Normal gradients may be perturbed in several ways typically High T P geotherms in areas of plutonic activity or rifting Low T P geotherms in subduction zones Figure 21 1 Metamorphic field gradients estimated P T conditions along surface traverses directly up metamorphic grade for several metamorphic areas After Turner 1981 Metamorphic Petrology Mineralogical Field and Tectonic Aspects McGrawHill Metamorphic Agents and Changes Stress is an applied force acting on a rock over a particular cross sectional area Strain is the response of the rock to an applied stress yielding or deformation Deviatoric stress affects the textures and structures but not the equilibrium mineral assemblage Strain energy may overcome kinetic barriers to reactions Fluids Metamorphic Agents and Changes Evidence for the existence of a metamorphic fluid Fluid inclusions Fluids are required for hydrous or carbonate phases Volatile involving reactions occur at temperatures and pressures that require finite fluid pressures The Types of Metamorphism Different approaches to classification 2 Based on setting Contact Metamorphism Pyrometamorphism Regional Metamorphism Orogenic Metamorphism Burial Metamorphism Ocean Floor Metamorphism Hydrothermal Metamorphism Fault Zone Metamorphism Impact or Shock Metamorphism The Progressive Nature of Metamorphism Prograde increase in metamorphic grade with time as a rock is subjected to gradually more severe conditions Prograde metamorphism changes in a rock that accompany increasing metamorphic grade Retrograde decreasing grade as rock cools and recovers from a metamorphic or igneous event Retrograde metamorphism any accompanying changes Types of Protolith Lump the common types of sedimentary and igneous rocks into six chemically basedgroups 1 Ultramafic very high Mg Fe Ni Cr 2 Mafic high Fe Mg and Ca 3 Shales pelitic high Al K Si 4 Carbonates high Ca Mg CO2 5 Quartz nearly pure SiO2 6 Quartzo feldspathic high Si Na K Al What happens to our PROTOLITH when acted on by AGENTS OF CHANGE Agents of Change T P fluids stress strain Metamorphic Reactions Solid solid phase transformation Solid solid net transfer Dehydration Hydration Decarbonation Carbonation Solid solid phase transformation Polymorphic reaction a mineral reacts to form a polymorph of that mineral No transfer of matter only a rearrangment of the mineral structure Example Andalusite Sillimanite Al2SiO5 Al2SiO5 Solid solid net transfer Involve solids only Differ from polymorphic transformations involve solids of differing composition and thus material must diffuse from one site to another for the reaction to proceed Examples NaAlSi2O6 SiO2 NaAlSi3O8 Jd Qtz Ab MgSiO3 CaAl2Si2O8 CaMgSi2O6 Al2SiO5 En An Di And Solid Solid Net Transfer II If minerals contain volatiles the volatiles must be conserved in the reaction so that no fluid phase is generated or consumed For example the reaction Mg3Si4O10 OH 2 4 MgSiO3 Mg7Si8O22 OH 2 Talc Enstatite Anthophyllite involves hydrous phases but conserves H2O It may therefore be treated as a solid solid net transfer reaction Hydration Dehydration Reactions Metamorphic reactions involving the expulsion or incorporation of water H 2O Example Al2Si4O10 OH 2 Al2SiO5 3SiO2 H2O Pyrophyllite And Ky Quartz water Carbonation Decarbonation Reactions Reactions that involve the evolution or consumption of CO2 CaCO3 SiO2 CaSiO3 CO2 calcite quartz wollastonite Reactions involving gas phases are also known as volatilization or devoltilization reactions These reactions can also occur with other gases such as CH4 methane H2 H2S O2 NH4 ammonia but they are not as common Systems Rock made of different minerals Metamorphic agents of change beat on it metamorphic reactions occur A closed system does not gain or lose material of any kind An open system can lose stuff liquids gases especially Outside world Hunk o rock Thermodynamics Primer Thermodynamics describes IF a reaction CAN occur at some condition T P composition typically Second Law of thermodynamics G H T S Where G Gibb s free energy determines IF the REACTION will go forward G spontaneous H is enthalpy has to do with heat S is entropy has to do with bonds and order Thermodynamics vs Kinetics Thermodynamics comparing the potential ENERGY of things what is more stable Will a reaction occur at some T P soln melt