Slide 1Slide 2Slide 3Metamorphic Agents and ChangesSlide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13What 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 ReactionsSystemsThermodynamics PrimerThermodynamics vs. KineticsPhase diagramsSlide 24Mineral Assemblages in Metamorphic RocksThe Phase Rule in Metamorphic SystemsSlide 27Metamorphic faciesSlide 29•The IUGS has proposed the following definition of The IUGS has proposed the following definition of metamorphism:metamorphism:““Metamorphism is a Metamorphism is a subsolidussubsolidus process leading to process leading to changes in mineralogy and/or texture (for example grain changes in mineralogy and/or texture (for example grain size) and often in chemical composition in a rock. These size) and often in chemical composition in a rock. These changes are due to physical and/or chemical conditions changes are due to physical and/or chemical conditions that differ from those normally occurring at the surface that differ from those normally occurring at the surface of planets and in zones of cementation and diagenesis of planets and in zones of cementation and diagenesis below this surface. They may coexist with partial below this surface. They may coexist with partial melting.”melting.”MetamorphismThe 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 zonesFigure 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.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 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 changesTypes of ProtolithLump the common types of sedimentary and igneous rocks into six chemically based-groups1. Ultramafic - very high Mg, Fe, Ni, Cr2. Mafic - high Fe, Mg, and Ca3. Shales (pelitic) - high Al, K, Si4. Carbonates- high Ca, Mg, CO25. Quartz - nearly pure SiO2.6. Quartzo-feldspathic - high Si, Na, K, AlWhat 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