Chapter 26: Metamorphic Reactions

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 Some workers have advocated that we distinguish field- based isograds in the classical sense from reaction-based isograds Reactions are always responsible for introducing or consuming mineral phases during metamorphism The classic notion of an isograd as the first appearance of a new mineral phase as one progresses up metamorphic grade is quite useful in the field, because a worker need only be able to recognize new minerals in a hand specimen If we understand the reactions that produce minerals, the physical conditions under which reactions occur, and what controls them, we can use this knowledge to better understand metamorphic processes 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 review the various types of metamorphic reactions, and discuss what controls them

1. Phase Transformations
Isochemical phase transformations (the polymorphs of SiO2 or Al2SiO5 or graphite-diamond or calcite-aragonite are in many ways the simplest to deal with The transformations depend on temperature and pressure only

Figure A portion of the equilibrium boundary for the calcite-aragonite phase transformation in the CaCO3 system. After Johannes and Puhan (1971), Contrib. Mineral. Petrol., 31, Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. For example, if CaCO3 is stable in a rock system, it should occur as calcite at temperatures and pressures below the equilibrium curve, and as aragonite at temperatures and pressures above the curve This explains why aragonite is the stable CaCO3 polymorph commonly found in blueschist facies terranes Providing that the boundary curves have been experimentally located accurately and that the mineralogy reflects equilibrium conditions, the presence of any polymorph may conveniently be used to set limits on the temperature and pressure conditions under which a rock formed

Figure The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Calculated using the program TWQ (Berman, 1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Independent of other minerals present, fluids, etc. Andalusite + Ms + Qtz + Bt +… will -> Sill at the transition grade regardless of other phases We used the presence of andalusite to indicate low-pressure metamorphic conditions, and by referring to Fig we can effectively limit the pressure of andalusite-bearing rocks to values below about 0.38 GPa The presence of two coexisting polymorphs in a single rock has often been taken to indicate that the metamorphic peak corresponded to equilibrium conditions along the univariant boundary curve separating the pair If an independent estimate of either pressure or temperature is available, the other parameter may then be estimated from the location of the equilibrium curve Thus if kyanite and sillimanite were to be observed together, for example, and the pressure were estimated via geobarometry to be 0.5 GPa, then the temperature of equilibration could be determined from Fig to be approximately 560oC If all three Al2SiO5 polymorphs were to be found in stable coexistence, the assemblage would indicate conditions at the invariant point (ca. 500oC and 3.8 GPa)

Small DS for most polymorphic transformations  small DG 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) Often by carefully observing the textures one can distinguish partial replacement and metastable coexistence from true stable equilibrium grain boundaries Hietanen (1956) reported all three Al2SiO5 polymorphs in northern Idaho, and proposed that metamorphic events in the area occurred near the invariant point More likely that kyanite is partially replaced by sillimanite during a prograde event near the kyanite-sillimanite boundary, and that andalusite replaces kyanite during a later event at lower pressure

2. Exsolution Figure T-X phase diagram of the system albite-orthoclase at 0.2 GPa H2O pressure. After Bowen and Tuttle (1950). J. Geology, 58, Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Covered in Mineralogy and previously in Petrology

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

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

Reaction curves typically pretty straight
DS and DV change little S changes more than DS(reaction) Figure Temperature-pressure phase diagram for the reaction: Albite = Jadeite + Quartz calculated using the program TWQ of Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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 Tlc En Ath involves hydrous phases, but conserves H2O It may therefore be treated as a solid-solid net-transfer reaction

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 Polymorphic transformations, exsolution, and solid-solid net-transfer reactions with little solid-solution are relatively straightforward metamorphic reactions, and are subject to variations in pressure and temperature, without complications due to variations in rock or fluid compositions The presence of reactants vs. products has often been used, in conjunction with experimental work that constrains the location of the reaction in P-T-X space, to set limits on the temperature and pressure conditions of a metamorphic event Discontinuous reaction: f + 1 and XLiq fixed

4. Devolatilization Reactions
Among the most common metamorphic reactions H2O-CO2 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

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) This dependence is easily demonstrated by applying Le Châtelier’s principle to the reaction at equilibrium

The equilibrium curve represents equilibrium between the reactants and products under water- saturated conditions (pH2O = PLithostatic) Fig is a P-T phase diagram showing the equilibrium reaction curve for reaction (26-6) The hydrous assemblage is always on the low-temperature side of the curve, and the evolved fluid phase is liberated as temperature increases The concave upward shape is characteristic of all devolatilization equilibrium curves at low pressure because the slope, as determined by the Clapyron equation is low at low pressures due to the high volume of the gas phase, but steepens quickly at higher pressures because the gas is most easily compressed P-T phase diagram for the reaction Ms + Qtz = Kfs + Al2SiO5 + H2O showing the shift in equilibrium conditions as pH2O varies (assuming ideal H2O-CO2 mixing). Calculated using the program TWQ by Berman (1988, 1990, 1991). After Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

KAl2Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O Ms Qtz Kfs Sill W
Suppose H2O is withdrawn from the system at some point on the water-saturated equilibrium curve: pH2O < Plithostatic According to Le Châtelier’s Principle, 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

Figure P-T phase diagram for the reaction Ms + Qtz = Kfs + Al2SiO5 + H2O showing the shift in equilibrium conditions as pH2O varies (assuming ideal H2O-CO2 mixing). Calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Fig with the equilibrium curve contoured for various values of pH2O

pH2O can become less than PLith by either of two ways Pfluid < PLith by drying out the rock and reducing the fluid content Pfluid = PLith, but the water in the fluid can become diluted by adding another fluid component, such as CO2 or some other volatile phase In Fig I calculated the curves for the latter case on the basis of ideal H2O-CO2 mixing

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-Xfluid phase diagram Because 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 Because pressure is also a common variable, a T‑Xfluid diagram must be created for a specified pressure

Figure T-XH2O phase diagram for the reaction Ms + Qtz = Kfs + Sil + H2O at 0.5 GPa assuming ideal H2O-CO2 mixing, calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Fig is a T-XH2O diagram for reaction (26-5) in which PLith = 0.5 GPa

The dots correspond to the dots in Fig. 26-3, since pH2O + pCO2 = PLith and ideal mixing assumes pH2O = XH2O PLith Figure Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Figure Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Shape of ~ all dehydration curves on T-Xfluid diagrams is similar to the curve in Fig. 26-2 Maximum temperature at the pure H2O end, and slope gently at high XH2O, but steeper toward low XH2O, becoming near vertical at very low XH2O Reaction temperature can thus be practically any temperature below the maximum at pH2O = Plith Must constrain the fluid composition (if possible) before using a dehydration reaction to indicate metamorphic grade

A rare exception Figure Calculated P-T equilibrium reaction curve for a dehydration reaction illustrating the full loop that is theoretically possible. From Winter (2001). An Introduction to Igneous and Metamorphic Petrology, Prentice Hall. A rare exception: the ideal curve for a complete devolatilization reaction curves back at very high pressures The top and low-T side is typically metastable, but in some low-T high-P reactions the retrograde devolatilization portion is stable

Decarbonation reactions may be treated in an identical fashion For example, the reaction: CaCO3 + SiO2 = CaSiO3 + CO2 (26-6) Cal Qtz Wo Can also be shown on a T-XCO2 diagram Has the same form as reaction (26-5), only the maximum thermal stability of the carbonate mineral assemblage occurs at pure XCO2

The temperature of a wollastonite isograd based on this reaction is obviously dependent upon pCO2 Figure T-XCO2 phase diagram for the reaction Cal + Qtz = Wo + CO2 at 0.5 GPa assuming ideal H2O-CO2 mixing, calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Figure A portion of the equilibrium boundary for the calcite-aragonite phase transformation in the CaCO3 system. After Johannes and Puhan (1971), Contrib. Mineral. Petrol., 31, Winter (2001) 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: Tmax at XCO2 determined by the stoichiometric ratio of CO2/H2O produced Ca2Mg5Si8O22(OH)2 + 3 CaCO SiO Tr Cal Qtz = 5 CaMgSi2O6 + 3 CO2 + H2O Di CO2 : H2O = 3 : 1 or ¾ to ¼ so Tmax at 0.75 CO2 Figure Schematic T-XCO2 phase diagram illustrating the general shapes of the five types of reactions involving CO2 and H2O fluids. After Greenwood (1967). In P. H. Abelson (ed.), Researches in Geochemistry. John Wiley. New York. V. 2, Winter (2001) 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 (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Imagine an idealized field area of steeply dipping metamorphosed pelitic sediments that strike directly up metamorphic grade The bulk chemistry of each unit is homogeneous, but differs somewhat from the other units in the area The garnet-in field isograd varies from unit to unit, occurring at different grades This may occur for one of two reasons (assuming that the rocks represent equilibrium mineral assemblages)

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 + H2O (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 + H2O (26-12) Offsets in a particular field isograd, when based on different reaction isograds, are often relatively large In the blank unit in the center of the map, garnet isn’t created at all. This could be a Mg-rich sandy pelite, or even a quartzite or marble

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 (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. A familiar example as illustration Suppose that we want to record the “melt-in” isograd, or the first appearance of a melt The temperature at which melt occurs depends on the Mg/Fe ratio of the bulk composition that we plan to melt 30% vs. 70% Fo would cause a shift in the “melt-in” field isograd of about 265oC!

5. Continuous Reactions Discontinuous reactions occur at a constant grade They are actually univariant (F = 1) on P-T phase diagrams, but pressure and temperature are not really independent, but constrained to follow a geothermal gradient or P-T-t path The P-T path crosses the reaction at a single grade If Chl + Ms + Qtz  Grt + Bt + H2O (26-11) were a discontinuous reaction (let’s say it occurred for pure Mg end-members), and is responsible for the formation of garnet in the map area above, the reaction should run to completion (when one of the reactants was consumed) at a single grade

5. Continuous Reactions Continuous reactions occur when F  1, and the reactants and products coexist over a temperature (or grade) interval If Chl + Ms + Qtz  Grt + Bt + H2O (26-11) 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 Schematic isobaric T-XMg diagram representing the simplified metamorphic reaction Chl + Qtz  Grt + H2O. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

6. 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

Figure AFM projections showing the relative distribution of Fe and Mg in garnet vs. biotite at approximately 500oC (a) and 800oC (b). From Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1. MSA. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

6. Redox Reactions Involves a change in oxidation state of an element
6 Fe2O3 = 4 Fe3O4 + O2 2 Fe3O4 + 3 SiO2 = 3 Fe2SiO4 + O2 At any particular pressure these become oxygen buffers Fig Isobaric T-fO2 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 (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Note the range of fO2 in which iron occurs principally in silicate minerals (i.e. most natural rocks).

7. Reactions Involving Dissolved Species
Minerals plus ions neutral molecules dissolved in a fluid One example is hydrolysis: 2 KAlSi3O8 + 2 H+ + H2O = Al2Si2O5 (OH)4 + SiO2 + 2 K+ Kfs aq. species kaolinite 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 Ci Isobaric T-Ci diagrams Isobaric and isothermal Ci - Cj diagrams Reaction above might be handled by a T vs. CK+/CH+ diagram

Reactions and Chemographics
We can use chemographics to infer reactions Fo + Qtz = En Mg2SiO4 + SiO2 = Mg2Si2O6 En + Per = Fo Mg2Si2O6 + 2 MgO = 2 Mg2SiO4 Per + Qtz = Fo or En If we know the chemographics we can determine that a reaction is possible (and can dispense with balancing the reaction) It doesn’t mean that they will under any set of conditions (thermodynamic equilibrium dictates that)

What reaction does this ternary system allow? Fig From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

What reaction is possible between A-B-C-D? A chemographic diagram for some metamorphic zone Remember that coexisting phases are connected with tie-lines A + B + D or A + B + C Fig a. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

A + B = C + D Below the isograd At the isograd Above the isograd
Fig From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. A + B = C + D At the isograd Above the isograd Note that phi = 4 at the isograd with crossing tie-lines Then have new groupings : A + C + D or B + C + D No new minerals become stable- simply different associations The groupings follow from the reaction: If A > B then B consumed first, and A remains with new C & D -> A + C + D C + D cannot coexist below the isograd, and A + B cannot coexist above it If a chemographic diagram is a projection, the approach still works, but you will have to balance the reaction with other components For example, if the previous diagram is projected from quartz, SiO2 will have to be added to one side of the A + B = C + D reaction to balance it properly This is called a tie-line flip, and results in new groupings in the next metamorphic zone

Petrogenetic grid for mafic rocks
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-Al2O3-SiO2-H2O-(Na2O) system (“C(N)MASH”). Winter (2001) 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-XCO2 phase diagram fro 2 reactions in the CaO-MgO-SiO2-H2O-CO2 system at 0.5 GPa, assuming ideal mixing of non-ideal gases, calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure The Al2SiO5 T-P phase diagram from Figure 21-9 (without H2O). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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 (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure A portion of the P-T phase diagram for SiO2 (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.

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 (2001) 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,

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,

Chapter 26: Metamorphic Reactions

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