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UVM GEOL 135 - Metamorphic Processes

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Slide 1Slide 2Slide 3Metamorphic Agents and ChangesSlide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11What happens to our PROTOLITH when acted on by AGENTS OF CHANGE??Solid-solid phase transformationSolid-solid net-transferSolid-Solid Net-Transfer IIHydration/ Dehydration ReactionsCarbonation / Decarbonation Reactions•Isograds for a single shale unit in southern Vermont•Which side reflects a higher grade, or higher P/T environment?The Limits of MetamorphismThe 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 arbitraryThe Limits of MetamorphismThe 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 gradationalMetamorphic Agents and Changes•Temperature: typically the most important factor in metamorphismFigure 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 ChangesIncreasing temperature has several effects1) 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 grainsMetamorphic Agents and ChangesIncreasing temperature has several effects2) Drive reactions that consume unstable mineral(s) and produces new minerals that are stable under the new conditions3) Overcomes kinetic barriers that might otherwise preclude the attainment of equilibriumMetamorphic 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 zonesMetamorphic 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 reactionsMetamorphic Agents and ChangesFluidsEvidence 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 pressuresThe Types of MetamorphismDifferent approaches to classification2. Based on setting–Contact Metamorphism•Pyrometamorphism–Regional Metamorphism•Orogenic Metamorphism•Burial Metamorphism•Ocean Floor Metamorphism–Hydrothermal Metamorphism–Fault-Zone Metamorphism –Impact or Shock MetamorphismThe 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 changesWhat 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–CarbonationSolid-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  SillimaniteAl2SiO5Al2SiO5Solid-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 AndSolid-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 Anthophylliteinvolves hydrous phases, but conserves H2OIt may therefore be treated as a solid-solid net-transfer reactionHydration/ Dehydration Reactions•Metamorphic reactions involving the expulsion or incorporation of water (H2O)•Example:–Al2Si4O10(OH)2 <=> Al2SiO5 + 3SiO2 + H2O Pyrophyllite And/Ky Quartz waterCarbonation / Decarbonation Reactions•Reactions that involve the evolution or consumption of CO2•CaCO3 + SiO2 = CaSiO3 + CO2 calcite quartz wollastoniteReactions involving gas phases are also known as volatilization or devoltilization reactionsThese reactions can also occur with other gases such as CH4 (methane), H2, H2S, O2, NH4+ (ammonia) – but they are not as


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UVM GEOL 135 - Metamorphic Processes

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