Slide 1Isograds and Metamorphic ReactionsTypes of Metamorphic ReactionsTypes of Metamorphic Reactions Phase TransformationsSlide 5Types of Metamorphic Reactions ExsolutionTypes of Metamorphic Reactions Solid-Solid Net-transferSlide 8Types of Metamorphic Reactions DevolatilizationSlide 10Slide 11Slide 12Slide 13Slide 14Slide 15Types of Metamorphic Reactions Ion Exchange ReactionsTypes of Metamorphic Reactions Oxidation/Reduction (Redox)Types of Metamorphic Reactions Reactions involving Dissolved SpeciesGEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGYLecture 25Metamorphic ReactionsApril 6, 2009ISOGRADS AND METAMORPHIC REACTIONSThe basis of isograds are metamorphic reactionsUsually mineral-in reactions, but some isogrades can be mineral outIf we have good experimental and theoretical data on minerals and reactions, we can locate a reaction in P-T-X space and constrain the conditions under which a particular metamorphic rock formed.We will investigate the various types of metamorphic reactions, and discuss what controls themTYPES OF METAMORPHIC REACTIONS• Phase Transformations• Exsolution Reactions•Solid-Solid Net Transfer Reactions• Devolatilization Reactions• Continuous Reactions (NEXT LECTURE)• Ion Exchange Reactions• Oxidation/Reduction (redox) Reactions•Reactions involving Dissolved SpeciesTYPES OF METAMORPHIC REACTIONS PHASE TRANSFORMATIONSIsochemical phase transformations that depend on temperature and pressure onlyAl2SiO5CaCO3TYPES OF METAMORPHIC REACTIONS PHASE TRANSFORMATIONSBecause S for most polymorphic transformations is small, G between two alternative polymorphs is also small.Consequently...1) There is little driving force for the reaction to proceed commonly leading to metastable relics in the stability field of other, and 2) Coexisting polymorphs may actually represent non-equilibrium states (overstepped equilibrium curves or polymetamorphic overprints)Rock w/ Ky+Sil+And may indicate a fieldrather than aninvariant pointTEXTURE IS A GUIDEto discriminate incomplete reaction vs. equilibriumTYPES OF METAMORPHIC REACTIONS EXSOLUTIONAlbite-rich perthite exsolution (h)Orthoclase host (g)TYPES OF METAMORPHIC REACTIONSSOLID-SOLID NET-TRANSFER Differs from polymorphic transformations by involving solids of differing composition, and thus material must diffuse from one site to another for the reaction to proceedExamples:NaAlSi2O6 + SiO2 = NaAlSi3O8 Jd Qtz AbMgSiO3 + CaAl2Si2O8 = En AnCaMgSi2O6 + Al2SiO5 Di And4 (Mg,Fe)SiO3 + CaAl2Si2O8 = Opx Plag(Mg,Fe)3Al2Si3O12 + Ca(Mg,Fe)Si2O6 + SiO2 Gnt Cpx QtzTYPES OF METAMORPHIC REACTIONSSOLID-SOLID NET-TRANSFER If minerals contain volatiles, the volatiles must be conserved in the reaction so that no fluid phase is generated or consumedFor example, the reaction:Mg3Si4O10(OH)2 + 4 MgSiO3 = Mg7Si8O22(OH)2 Talc Enstatite Anthophyllite Igneous Deuteric Alteration  Regional MetamorphismTYPES OF METAMORPHIC REACTIONSDEVOLATILIZATIONAmong the most common metamorphic reactionsH2O-CO2 systems are most common, but the principles same for any reaction involving volatiles  Typically involve the devolution of water (dehydration) or CO2 (decarbonation)Reactions dependent not only upon temperature and pressure, but also upon the partial pressure of the volatile speciesFor example the location on a P-T phase diagram of the dehydration reaction: KAl2Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O Ms Qtz Kfs Sill W depends upon the partial pressure of H2O (pH2O)TYPES OF METAMORPHIC REACTIONSDEVOLATILIZATIONThe equilibrium curve represents equilibrium between the reactants and products under water-saturated conditions (pH2O = PLithostatic)Decrease in slope of the reaction curve at low P is due to large V of gas at low pressure and its rapid decrease (compression) with increasing P until reaching a maximum compressionTYPES OF METAMORPHIC REACTIONSDEVOLATILIZATIONRemoving water at equilibrium will be compensated by the reaction running to the right, thereby producing more water. This has the effect of stabilizing the right side of the reaction at the expense of the left sideSo as water is withdrawn the Kfs + Sill + H2O field expands slightly at the expense of the Mu + Qtz field, and the reaction curve shifts toward lower temperatureSo - the temperature of an isograd based on a devolatilization reaction is sensitive to the partial pressure of the volatile species involvedTYPES OF METAMORPHIC REACTIONSDEVOLATILIZATIONT-Xfluid phase diagrams - An alternative way to show the role of volatiles on the T of reactions at a specific pressureH2O and CO2 are by far the most common metamorphic volatiles, the X in T-X diagrams is usually the mole fraction of CO2 (or H2O) in H2O-CO2 mixtures= H2O/(H2O+CO2+....)Ms rapidly unstable as H2O  0TYPES OF METAMORPHIC REACTIONSDEVOLATILIZATIONFive types of devolatilization reactions, each with a unique general shape on a T-X diagram1) dehydration2) decarbonation3) dehydration + decarbonation4) hydration + decarbonation5) carbonation + dehydration Type 3: Tmax at XCO2 determined by the stoichiometric ratio of CO2/H2O producedCa2Mg5Si8O22(OH)2 + 3 CaCO3 + 2 SiO2 Tr CalQtz = 5 CaMgSi2O6 + 3 CO2 + H2O DiTYPES OF METAMORPHIC REACTIONSDEVOLATILIZATIONOpen vs. Closed (buffered) Fluid SystemsOPEN Volatiles escape with degassingXCO2 (ambient fluid) is constanta – Trem, Cal, or Qtz must be consumed before T can increased – Cal or Qtz must be consumed before T can increaseThe degassed fluid can be a metasomatic agent for shallower metamorphic reactions.TYPES OF METAMORPHIC REACTIONSDEVOLATILIZATIONOpen vs. Closed (buffered) Fluid SystemsCLOSED (buffered)Volatiles are trapped in the rock with degassing XCO2 must follow the reaction equibrium curve to the max T.a – degassing of CO2 & H2O drives system to b and increased Tb – Trem, Cal or Qtz must be consumed before T can increase c – degassing of CO2 causes system to move along reaction curve toward XCO2 = 1 Fluid composition is controlled (buffered) by the progress of the reactionTYPES OF METAMORPHIC REACTIONSION EXCHANGE REACTIONSReciprocal exchange of components between 2 or more mineralsMgSiO3 + CaFeSi2O6 = FeSiO3 + CaMgSi2O6Annite + Pyrope = Phlogopite + AlmandineExpressed as pure end-members, but really involves Mg-Fe (or other) exchange between intermediate solutionsBasis for many