Geol 2312 Igneous and Metamorphic Petrology Lecture 25 Metamorphic Reactions April 6, 2009

Isograds and Metamorphic Reactions The basis of isograds are metamorphic reactions Usually mineral-in reactions, but some isogrades can be mineral out If 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 them

Types 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 Species

Types of Metamorphic Reactions Phase Transformations Isochemical phase transformations that depend on temperature and pressure only Al2SiO5 CaCO3

Types of Metamorphic Reactions Phase Transformations Because DS for most polymorphic transformations is small, DG 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 field rather than an invariant point TEXTURE IS A GUIDE to discriminate incomplete reaction vs. equilibrium

Types of Metamorphic Reactions Exsolution Albite-rich perthite exsolution (h) Orthoclase host (g)

Types of Metamorphic Reactions Solid-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 proceed Examples: NaAlSi2O6 + SiO2 = NaAlSi3O8 Jd Qtz Ab MgSiO3 + CaAl2Si2O8 = En An CaMgSi2O6 + Al2SiO5 Di And 4 (Mg,Fe)SiO3 + CaAl2Si2O8 = Opx Plag (Mg,Fe)3Al2Si3O12 + Ca(Mg,Fe)Si2O6 + SiO2 Gnt Cpx Qtz

Types of Metamorphic Reactions Solid-Solid Net-transfer 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 Igneous Deuteric Alteration  Regional Metamorphism

Types of Metamorphic Reactions Devolatilization 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 species For 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 Reactions Devolatilization The 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 compression

Types of Metamorphic Reactions Devolatilization Removing 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 side So 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 temperature So - the temperature of an isograd based on a devolatilization reaction is sensitive to the partial pressure of the volatile species involved

Types of Metamorphic Reactions Devolatilization T-Xfluid phase diagrams - An alternative way to show the role of volatiles on the T of reactions at a specific pressure H2O 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 Ms rapidly unstable as H2O  0 = H2O/(H2O+CO2+....)

Types of Metamorphic Reactions Devolatilization Five types of devolatilization reactions, each with a unique general shape on a T-X diagram 1) dehydration 2) decarbonation 3) dehydration + decarbonation 4) hydration + decarbonation 5) carbonation + dehydration Type 3: Tmax at XCO2 determined by the stoichiometric ratio of CO2/H2O produced Ca2Mg5Si8O22(OH)2 + 3 CaCO3 + 2 SiO2 Tr Cal Qtz = 5 CaMgSi2O6 + 3 CO2 + H2O Di

Types of Metamorphic Reactions Devolatilization Open vs. Closed (buffered) Fluid Systems OPEN Volatiles escape with degassing XCO2 (ambient fluid) is constant a – Trem, Cal, or Qtz must be consumed before T can increase d – Cal or Qtz must be consumed before T can increase The degassed fluid can be a metasomatic agent for shallower metamorphic reactions.

Types of Metamorphic Reactions Devolatilization Open vs. Closed (buffered) Fluid Systems CLOSED (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 T b – 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 reaction

Types of Metamorphic Reactions Ion Exchange Reactions Reciprocal exchange of components between 2 or more minerals MgSiO3 + CaFeSi2O6 = FeSiO3 + CaMgSi2O6 Annite + Pyrope = Phlogopite + Almandine Expressed as pure end-members, but really involves Mg-Fe (or other) exchange between intermediate solutions Basis for many geothermobarometers Causes rotation of tie-lines on compatibility diagrams

Types of Metamorphic Reactions Oxidation/Reduction (Redox) Involves a change in oxidation state of an element MH - 6 Fe2O3 = 4 Fe3O4 + O2 FMQ - 2 Fe3O4 + 3 SiO2 = 3 Fe2SiO4 + O2 QIF - Fe2SiO4 = 2 Fe0 + SiO2 + O2 Fe+3 (Ferric) = oxidized Hematite(Fe2O3 ): 2Fe+3, Magnetite (Fe3O4): 2 Fe+3 + 1 Fe+2 Fayalite (Fe2SiO4): 2Fe+2 Fe+2 (Ferrous) = reduced These reactions are known as oxygen buffers. Provided these phases are present, the abundance (fugacity) of oxygen (fO2) at a particular temperature will be controlled by changing proportions of the Fe-bearing phases.

Types of Metamorphic Reactions Reactions involving Dissolved Species Minerals plus ions and neutral molecules dissolved in a fluid For example – hydrolysis reaction of feldspar altering to kaolinite (clay) 2 KAlSi3O8 + 2 H+ + H2O = Al2Si2O5 (OH)4 + SiO2 + 2 K+ Kfs aq. species kaolinite aq. species Hydrothermal Mineralization Model for Volcanic Massive Sulfide (VMS) Eeposits