Chapter 26: Metamorphic Reactions If we treat isograds as reactions, we can: Understand what physical variables might affect the location of a particular isograd We may also be able to estimate the P-T-X conditions that an isograd represents From Understanding Earth, Press and Siever. Freeman.

1. Phase Transformations Isochemical Polymorphs of SiO 2 or Al 2 SiO 5 or graphite-diamond or calcite-aragonite Simplest: depend on temperature and pressure only

1. Phase Transformations Figure A portion of the equilibrium boundary for the calcite- aragonite phase transformation in the CaCO 3 system. After Johannes and Puhan (1971), Contrib. Mineral. Petrol., 31, Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

1. Phase Transformations Figure The P-T phase diagram for the system Al 2 SiO 5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Calculated using the program TWQ (Berman, 1988, 1990, 1991). Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

1. Phase Transformations Small  S for most polymorphic transformations  small  G between two alternative polymorphs, even several tens of degrees from the equilibrium boundary  little driving force for the reaction to proceed  common metastable relics in the stability field of other Coexisting polymorphs may therefore represent non- equilibrium states (overstepped equilibrium curves or polymetamorphic overprints)

2. Exsolution Figure T-X phase diagram of the system albite- orthoclase at 0.2 GPa H 2 O pressure. After Bowen and Tuttle (1950). J. Geology, 58, Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

3. Solid-Solid Net-Transfer Reactions 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

3. Solid-Solid Net-Transfer Reactions Examples: NaAlSi 2 O 6 + SiO 2 = NaAlSi 3 O 8 JdQtzAb MgSiO 3 + CaAl 2 Si 2 O 8 = CaMgSi 2 O 6 + Al 2 SiO 5 EnAnDiAnd 4 (Mg,Fe)SiO 3 + CaAl 2 Si 2 O 8 = OpxPlag (Mg,Fe) 3 Al 2 Si 3 O 12 + Ca(Mg,Fe)Si 2 O 6 + SiO 2 GntCpx Qtz

Figure Temperature-pressure phase diagram for the reaction: Albite = Jadeite + Quartz calculated using the program TWQ of Berman (1988, 1990, 1991). Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

3. Solid-Solid Net-Transfer Reactions 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: Mg 3 Si 4 O 10 (OH) MgSiO 3 = Mg 7 Si 8 O 22 (OH) 2 TlcEn Ath involves hydrous phases, but conserves H 2 O It may therefore be treated as a solid-solid net- transfer reaction

3. Solid-Solid Net-Transfer Reactions When solid-solution is limited, solid-solid net- transfer reactions are discontinuous reactions Discontinuous reactions tend to run to completion at a single temperature (at a particular pressure) There is thus an abrupt (discontinuous) change from the reactant assemblage to the product assemblage at the reaction isograd Discontinuous reaction:  + 1 and X Liq fixed

4. Devolatilization Reactions Among the most common metamorphic reactions H 2 O-CO 2 systems are most common, but the principles same for any reaction involving volatiles Reactions dependent not only upon temperature and pressure, but also upon the partial pressure of the volatile species

4. Devolatilization Reactions For example the location on a P-T phase diagram of the dehydration reaction: KAl 2 Si 3 AlO 10 (OH) 2 + SiO 2 = KAlSi 3 O 8 + Al 2 SiO 5 + H 2 O Ms Qtz Kfs Sill W depends upon the partial pressure of H 2 O (p H2O ) This dependence is easily demonstrated by applying Le Châtelier’s principle to the reaction at equilibrium

4. Devolatilization Reactions The equilibrium curve represents equilibrium between the reactants and products under water- saturated conditions (p H2O = P Lithostatic ) Figure 26.2(a). P-T phase diagram for the reaction Ms + Qtz = Kfs + Al 2 SiO 5 + H 2 O showing the shift in equilibrium conditions as p H2O varies (assuming ideal H 2 O-CO 2 mixing). Calculated using the program TWQ by Berman (1988, 1990, 1991). After Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

KAl 2 Si 3 AlO 10 (OH) 2 + SiO 2 = KAlSi 3 O 8 + Al 2 SiO 5 + H 2 O MsQtzKfsSillW lSuppose H 2 O is withdrawn from the system at some point on the water-saturated equilibrium curve: p H2O < P lithostatic lAccording to Le Châtelier’s Principle, removing water at equilibrium will be compensated by the reaction running to the right, thereby producing more water lThis has the effect of stabilizing the right side of the reaction at the expense of the left side lSo as water is withdrawn the Kfs + Sill + H 2 O field expands slightly at the expense of the Mu + Qtz field, and the reaction curve shifts toward lower temperature

Figure P-T phase diagram for the reaction Ms + Qtz = Kfs + Al 2 SiO 5 + H 2 O showing the shift in equilibrium conditions as p H2O varies (assuming ideal H 2 O-CO 2 mixing). Calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

4. Devolatilization Reactions p H2O can become less than P Lith by either of two ways  P fluid < P Lith by drying out the rock and reducing the fluid content  P fluid = P Lith, but the water in the fluid can become diluted by adding another fluid component, such as CO 2 or some other volatile phase  In Fig I calculated the curves for the latter case on the basis of ideal H 2 O-CO 2 mixing

4. Devolatilization Reactions An important point arising from Fig is: The temperature of an isograd based on a devolatilization reaction is sensitive to the partial pressure of the volatile species involved An alternative: T-X fluid phase diagram  Because H 2 O and CO 2 are by far the most common metamorphic volatiles, the X in T-X diagrams is usually the mole fraction of CO 2 (or H 2 O) in H 2 O- CO 2 mixtures  Because pressure is also a common variable, a T ‑ X fluid diagram must be created for a specified pressure

4. Devolatilization Reactions Figure T-X H2O phase diagram for the reaction Ms + Qtz = Kfs + Sil + H 2 O at 0.5 GPa assuming ideal H 2 O-CO 2 mixing, calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

4. Devolatilization Reactions Figure Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Figure Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

4. Devolatilization Reactions Shape of ~ all dehydration curves on T-X fluid diagrams is similar to the curve in Fig  Maximum temperature at the pure H 2 O end, and slope gently at high X H2O, but steeper toward low X H2O, becoming near vertical at very low X H2O  Reaction temperature can thus be practically any temperature below the maximum at p H2O = P lith  Must constrain the fluid composition (if possible) before using a dehydration reaction to indicate metamorphic grade

Figure Calculated P-T equilibrium reaction curve for a dehydration reaction illustrating the full loop that is theoretically possible. From Winter (2010). An Introduction to Igneous and Metamorphic Petrology, Prentice Hall. A rare exception

4. Devolatilization Reactions Decarbonation reactions may be treated in an identical fashion For example, the reaction: CaCO 3 + SiO 2 = CaSiO 3 + CO 2 (26.6) Cal Qtz Wo Can also be shown on a T-X CO2 diagram Has the same form as reaction (26.5), only the maximum thermal stability of the carbonate mineral assemblage occurs at pure X CO2

4. Devolatilization Reactions P-T phase diagram for the reaction Cal + Qtz = Wo + CO 2 Figure T-X CO2 phase diagram for the reaction Cal + Qtz = Wo + CO 2 at 0.5 GPa assuming ideal H 2 O-CO 2 mixing, calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

5 types of devolatilization reactions, each with a unique general shape on a T-X diagram Type 3: T max at X CO2 determined by the stoichiometric ratio of CO 2 /H 2 O produced Ca 2 Mg 5 Si 8 O 22 (OH) CaCO SiO 2 Tr CalQtz = 5 CaMgSi 2 O CO 2 + H 2 O Di Figure Schematic T-X CO2 phase diagram illustrating the general shapes of the five types of reactions involving CO 2 and H 2 O fluids. After Greenwood (1967). In P. H. Abelson (ed.), Researches in Geochemistry. John Wiley. New York. V. 2, Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

5. Continuous Reactions Figure Geologic map of a hypothetical field area in which metamorphosed pelitic sediments strike directly up metamorphic grade. From Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

5. Continuous Reactions Two possible reasons: 1. Such contrasting composition that the garnet reaction is different Example: garnet in some pelites may be created by the (unbalanced) reaction: Chl + Ms + Qtz  Grt + Bt + H 2 O(26.11) Whereas in more Fe-rich and K-poor pelites, garnet might be generated by an (unbalanced) reaction involving chloritoid: Chl + Cld + Qtz  Grt + H 2 O(26.12)

5. Continuous Reactions 2. The reaction on which the isograd is based is the same in each unit, but it is a continuous reaction, and its location is sensitive to the composition of the solutions (either solid of fluid) involved The offsets this creates in an isograd are usually more subtle than for reason #1, but in some cases they can be substantial We will concentrate on this second reason here

5. Continuous Reactions Fig Isobaric T-X phase diagram at atmospheric pressure After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24, Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. “Melt-in” isograd?

5. Continuous Reactions Discontinuous reactions occur at a constant grade Chl + Ms + Qtz  Grt + Bt + H 2 O (26.11) in KFASHF = C –  + 2 = 5 – = 1

5. Continuous Reactions If Chl + Ms + Qtz  Grt + Bt + H 2 O (26.11) in KFMASH were a continuous reaction, then we would find chlorite, muscovite, quartz, biotite, and garnet all together in the same rock over an interval of metamorphic grade above the garnet-in isograd The composition of solid solution phases vary across the interval, and the proportions of the minerals changes until one of the reactants disappears with increasing grade

Continuous reactions occur when F  1, and the reactants and products coexist over a temperature (or grade) interval Fig. 26.9a. Schematic isobaric T-X Mg diagram representing the simplified metamorphic reaction Chl + Qtz  Grt + H 2 O. From Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Pseudosections address (only) the reactions (continuous or discontinuous) affecting a rock of specific X bulk Schematic P-T “pseudosection” for a specific bulk composition (in this case for Mg/(Mg + Fe) = 0.5 and quartz-excess) showing the stability fields of Chl + Qtz, Chl + Grt + Qtz, and Grt + Qtz. Note the continuous nature of the reaction when all solid phases are present (shaded area). Note also that one can contour the shaded divariant field in (b) for specific compositions of either garnet (as has been done) or chlorite. The boundaries and contours would change for a different X bulk (i.e. different X Mg ), and even the field assemblages might change: for example the higher temperature garnet + quartz field would be garnet + chlorite if the SiO 2 content were so low that quartz were consumed before chlorite by the reaction. Fig. 26.9b. Schematic P-T “pseudosection” for a specific bulk composition (in this case for Mg/(Mg + Fe) = 0.5 and quartz-excess) showing the stability fields of Chl + Qtz, Chl + Grt + Qtz, and Grt + Qtz. Note the continuous nature of the reaction when all solid phases are present (shaded area). Note also that one can contour the shaded divariant field in (b) for specific compositions of either garnet (as has been done) or chlorite. The boundaries and contours would change for a different X bulk (i.e. different X Mg ), and even the field assemblages might change: for example the higher temperature garnet + quartz field would be garnet + chlorite if the SiO 2 content were so low that quartz were consumed before chlorite by the reaction. From Winter (2010) An Introduction to Igneous and Metamorphic Petrology.

6. Ion Exchange Reactions Reciprocal exchange of components between 2 or more minerals – MgSiO 3 + CaFeSi 2 O 6 = FeSiO 3 + CaMgSi 2 O 6 – 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

Figure AFM projections showing the relative distribution of Fe and Mg in garnet vs. biotite at approximately 500 o C (a) and 800 o C (b). From Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1. MSA. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

6. Redox Reactions Involves a change in oxidation state of an element  6 Fe 2 O 3 = 4 Fe 3 O 4 + O 2  2 Fe 3 O SiO 2 = 3 Fe 2 SiO 4 + O 2 At any particular pressure these become oxygen buffers Fig Isobaric T-f O2 diagram showing the location of reactions (26.13) - (26.15) used to buffer oxygen in experimental systems. After Frost (1991), Rev. in Mineralogy, 25, MSA, pp Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

7. Reactions Involving Dissolved Species Minerals plus ions neutral molecules dissolved in a fluid One example is hydrolysis: – 2 KAlSi 3 O H + + H 2 O = Al 2 Si 2 O 5 (OH) 4 + SiO K + Kfsaq. specieskaolinite aq. species Can treat such reactions in terms of the phase rule and the intensive variables: P, T, and concentrations of the reactant species   T-P diagrams for fixed or contoured C i   Isobaric T-C i diagrams   Isobaric and isothermal C i - C j diagrams   Reaction above might be handled by a T vs. C K + / C H + diagram

Reactions and Chemographics We can use chemographics to infer reactions Any two phases in a binary system can react to from a phase between them Fo + Qtz = En Mg 2 SiO 4 + SiO 2 = Mg 2 Si 2 O 6 En + Per = Fo Mg 2 Si 2 O MgO = 2 Mg 2 SiO 4 Per + Qtz = Fo or En If we know the chemographics we can determine that a reaction is possible (and can dispense with balancing it)

Reactions and Chemographics What reaction does this ternary system allow? Fig From Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Reactions and Chemographics A + B + C = X below x-in isograd above x-in isograd

Reactions and Chemographics What reaction does this system allow? Fig From Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Reactions and Chemographics What reaction is possible between A-B-C-D? Fig a. From Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. A compatibility diagram for some metamorphic zone

A + B = C + D This is called a tie-line flip, and results in new groupings in the next metamorphic zone At the isograd Above the isograd Below the isograd Fig From Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Petrogenetic Grids P-T diagrams for multicomponent systems that show a set of reactions, generally for a specific rock type Petrogenetic grid for mafic rocks Fig Simplified petrogenetic grid for metamorphosed mafic rocks showing the location of several determined univariant reactions in the CaO-MgO-Al 2 O 3 -SiO 2 -H 2 O-(Na 2 O) system (“C(N)MASH”). Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Text figures that I don’t have time to cover in my 1-semester class Fig T-X CO2 phase diagram fro 2 reactions in the CaO-MgO-SiO 2 - H 2 O-CO 2 system at 0.5 GPa, assuming ideal mixing of non-ideal gases, calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Text figures that I don’t have time to cover in my 1-semester class Figure The Al 2 SiO 5 T-P phase diagram from Figure 21-9 (without H 2 O). Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Text figures that I don’t have time to cover in my 1-semester class Figure Schematic one-component T-P phase diagram showing the topology of a four- phase multisystem in which all invariant points are stable. Because only three phases (C+2) coexist at an invariant point, a complete system should have four invariant points, each with one phase absent. Phases absent at invariant points are in square brackets, phases absent for univariant reactions are in parentheses. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Text figures that I don’t have time to cover in my 1-semester class Figure A portion of the P-T phase diagram for SiO 2 (Figure 6-6) showing two stable invariant points [Trd] and [Liq] and two metastable ones. [b- Qtz] occurs at negative pressure, and [Crs] is truly metastable in that it is the intersection of metastable extensions. From Spear (1993) Metamorphic Phase Equilibria and Pressure- Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1. MSA.

Text figures that I don’t have time to cover in my 1- semester class Figure a. Hypothetical reaction D + E = F in a two- component phase diagram. Note that the D-absent and E- absent curves must both lie on the side of the initial univariant curve opposite to the field in which D + E is stable. Likewise the F-absent curve must lie on the side opposite to the field in which F is stable. b. A second hypothetical univariant curve (D-absent) is added. c. The complete topology of the invariant point can then be derived from the two initial reactions in (b). The chemographics may then be added to each divariant field. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure a. Sketch from a photomicrograph showing small crystals of kyanite (purple) and quartz (blue) in a larger muscovite grain (green). Small crystals of fibrolitic sillimanite also occur in the muscovite. Glen Cova, Scotland. b. Sillimanite needles in quartz (blue) embaying muscovite (green). Pink crystals are biotite. Donegal, Ireland. After Carmichael (1969). Contrib. Mineral. Petrol., 20,

Figure A possible mechanism by which the Ky  Sil reaction can be accomplished while producing the textures illustrated in Figure 26.20a and b. The exchange of ions shown between the two local zones is required if the reactions are to occur. After Carmichael (1969). Contrib. Mineral. Petrol., 20, Text figures that I don’t have time to cover in my 1-semester class

Figure An alternative mechanism by which the reaction Ky  Sil reaction can be accomplished while producing sillimanite needles associated with biotite with plagioclase occupying embayments in the biotite. The exchange of ions shown between the two local zones is required if the reactions are to occur. After Carmichael (1969). Contrib. Mineral. Petrol., 20,