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1Introduction to MetamorphismIntroduction to MetamorphismReading:Winter Chapter 21Chemical SystemsChemical Systems! An assemblage of coexisting phases (thermodynamic equilibrium and the phase rule)• A basaltic composition can be either:–Melt– Cpx + plag (± olivine, ilmenite…)– Or any combination of melt + minerals along the liquid line of descent– If uplifted and eroded → surface, will weather → a combinations of clays, oxides…“Metamorphism is a subsolidus process leading to “Metamorphism is a subsolidus 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.”Definition of MetamorphismDefinition of MetamorphismLower Limit of MetamorphismLower Limit 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 arbitraryUpper Limit of MetamorphismUpper Limit 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 • Migmatites (“mixed rocks”) are gradationalMetamorphic Agents and ChangesMetamorphic Agents and ChangesTemperature: typically the most important factor in metamorphismEstimated 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. SpaceSci., 18, 269-311.2Increased TemperatureIncreased Temperature• Promotes recrystallization which increases grain size• Larger surface/volume ratio of a mineral has lower stability• Increasing temperature eventually overcomes kinetic barriers to recrystallization, and fine aggregates coalesce to larger grainsHigh Temperature Effects High Temperature Effects • Reactions occur that consume unstable mineral(s) and produces new minerals that are stable under the new conditions!!3) Overcomes kinetic barriers that might 3) Overcomes kinetic barriers that might otherwise preclude the attainment of otherwise preclude the attainment of equilibriumequilibriumEffect of PressureEffect of 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 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 GradeMetamorphic GradeA general increase in degree of metamorphism without specifying the exact relationship between temperature and pressureDeviatoricDeviatoricStressStress• Lithostatic pressure is uniform stress (hydrostatic)• Deviatoric stress = unequal pressure in different directions• Deviatoric stress can be resolved into three mutually perpendicular stress (σ) components:σ1is the maximum principal stressσ2is an intermediate principal stressσ3is the minimum principal stress• In hydrostatic situations all three are equal3Stress and StrainStress and Strain• 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 reactionsTypes of Types of DeviatoricDeviatoricStresses: Stresses: •Tension• Compression• ShearIn tension: σ3is negative, and the resulting strain is extension, or pulling apartoriginal shapeoriginal shapestrain strain ellipsoidellipsoidThe three main types of deviatoric stress with an example of possible resulting structures. a. Tension, in which one stress in negative. “Tension fractures” may open normal to the extension direction and become filled with mineral precipitates. Winter (2001)σ1σ3In compression σ1is dominant: folding produces more homogenous flatteningThe three main types of deviatoric stress with an example of possible resulting structures. b. Compression, causing flattening or folding. Winter (2001)σ1σ3Foliation Allows Estimation of the Foliation Allows Estimation of the Orientation of Orientation of σσσσσσσσ11••σσ11> > σσ22= = σσ3 3 →→foliation and no lineationfoliation and no lineation••σσ11= = σσ22> > σσ3 3 →→lineation and no foliation lineation and no foliation ••σσ11> > σσ22> > σσ3 3 →→both foliation and lineationboth foliation and lineationFlattening of a ductile homogeneous sphere (a) containing randomly oriented flat disks or flakes. In (b), the matrix flows with progressive flattening, and the flakes are rotated toward parallelism normal to the predominant stress. Winter (2001)σ1Metamorphic Agents and ChangesMetamorphic Agents and ChangesShear motion occurs along planes at an angle to σ1The three main types of deviatoric stress with an example of possible resulting structures. b. Shear, causing slip along parallel planes and rotation. Winter (2001)σ14Metamorphic Fluids Metamorphic 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 pressuresFluid PressureFluid Pressure•Pfluidindicates the total fluid pressure, which


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UB GLY 206 - Intro to Metamorph

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