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The IUGS has proposed the following definition of metamorphism:

Metamorphism. The IUGS has proposed the following definition of metamorphism:

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The IUGS has proposed the following definition of metamorphism:

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  1. 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.”

  2. The Limits of Metamorphism • Low-temperature limit grades into diagenesis • The boundary is somewhat arbitrary • Diagenetic/weathering processes are indistinguishable from metamorphic • Metamorphism begins in the range of 100-150oC for the more unstable types of protolith • Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary

  3. 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

  4. 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.

  5. 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

  6. 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

  7. Metamorphic Agents and Changes • “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 • Pressure

  8. 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. McGraw-Hill.

  9. 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

  10. Metamorphic Agents and Changes Fluids 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

  11. 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 ShockMetamorphism

  12. The Progressive Nature of Metamorphism • Prograde: increase in metamorphic grade with time as a rock is subjected to gradually more severe conditions • Progrademetamorphism: 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

  13. Types of Protolith Lump the common types of sedimentary and igneous rocks into six chemically based-groups 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. Hydration/ Dehydration Reactions • Metamorphic reactions involving the expulsion or incorporation of water (H2O) • Example: • Al2Si4O10(OH)2 <=> Al2SiO5 + 3SiO2 + H2O Pyrophyllite And/Ky Quartz water

  19. 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

  20. 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

  21. Thermodynamics Primer • Thermodynamics describes IF a reaction CAN occur at some condition (T, P, composition typically) • Second Law of thermodynamics: • DG=DH – TDS • Where G, Gibb’s free energy determines IF the REACTION will go forward (-DG=spontaneous) • H is enthalpy – has to do with heat… • S is entropy – has to do with bonds and order…

  22. Thermodynamics vs. Kinetics • Thermodynamics – comparing the potential ENERGY of things  what is more stable? Will a reaction occur at some T,P, soln, melt composition go or Not? • Kinetics  IF thermodynamics says YES, the reaction should occur (always toward lower energy!) kinetics determines how fast • Minerals out of equilibrium pass the thermodynamic test but the kinetics of their reaction is very slow…

  23. Phase diagrams • Tool for ‘seeing’ phase transitions • H2Oice H2Oliquid • Reaction (line) governed by DG=DH – TDS • Phase Rule: • P+F=C+2 • Phases coexisting + degrees of freedom = number of components + 2 • Degree of freedom  2= either axis can change and the phase stays the same  where??

  24. Phase diagrams • Let’s think about what happens to water as conditions change… • P+F=C+2 • Point A? • Point B? • Point C? A B C

  25. Mineral Assemblages in Metamorphic Rocks • Equilibrium Mineral Assemblages • At equilibrium, the mineralogy (and the composition of each mineral) is determined by T, P, and X • Relict minerals or later alteration products are thereby excluded from consideration unless specifically stated

  26. The Phase Rule in Metamorphic Systems • Phase rule, as applied to systems at equilibrium: F = C - P + 2 the phase rule P is the number of phases in the system C is the number of components: the minimum number of chemical constituents required to specify every phase in the system F is the number of degrees of freedom: the number of independently variable intensive parameters of state (such as temperature, pressure, the composition of each phase, etc.)

  27. The Phase Rule in Metamorphic Systems C = 1 (Al2SiO5) • F = 1 common • F = 2 rare • F = 3 only at the specific P-T conditions of the invariant point (~ 0.37 GPa and 500oC) Consider the following three scenarios: Figure 21-9. The P-T phase diagram for the system Al2SiO5 calculated using the program TWQ (Berman, 1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  28. Metamorphic facies • P-T conditions, presence of fluids induces different metamorphic mineral assemblages (governed by thermodynamics/ kinetics) • These assemblages are lumped into metamorphic facies (or grades)

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