Chapter 26: Metamorphic Reactions If we treat isograds as reactions, we can: Understand what physical variables might affect the location of a particular.

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 SiO2 or Al2SiO5 or graphite-diamond or calcite-aragonite

• Simplest: depend on temperature and pressure only

1. Phase Transformations

Figure 26.1. 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, 28-38. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

1. Phase Transformations

Figure 26.15. 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 (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 6.16. T-X phase diagram of the system albite-orthoclase at 0.2 GPa H2O

pressure. After Bowen and Tuttle (1950). J. Geology, 58, 489-511. 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


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

Figure 27-1. 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.


• 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

Discontinuous reaction: + 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)

Figure 26.2(a). 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 (2010) 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 26.2. 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 (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


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. 26.2 I calculated the curves for the latter case on the basis of ideal H2O-CO2 mixing

4. Devolatilization Reactions• An important point arising from Fig. 26.2 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 26.4. 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 (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


Figure 26.4. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 26.2. Winter (2010) 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

Figure 26.3. 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


• 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


P-T phase diagram for the reaction Cal + Qtz = Wo + CO2

Figure 26.5. 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 (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: 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 DiFigure 26.6. 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, 542-567. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

5. Continuous Reactions

Figure 26.8. 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 ReactionsTwo 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)

5. Continuous Reactions2. 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. 6.10. Isobaric T-X phase diagram at atmospheric pressure After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24, 177-213. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

“Melt-in” isograd?

5. Continuous ReactionsDiscontinuous reactions occur at a constant grade

Chl + Ms + Qtz Grt + Bt + H2O (26.11)

in KFASH F = C – + 2 = 5 – 4 + 2 = 1

5. Continuous ReactionsIf Chl + Ms + Qtz Grt + Bt + H2O (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-XMg

diagram representing the simplified metamorphic reaction Chl + Qtz Grt + H2O. 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 Xbulk

Fig. 26.9b. Schematic P-T “pseudosection” for a specific bulk composition (in this case for Mg/(Mg + Fe) = 0.5 and quartz-excess) 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 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 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 Xbeen done) or chlorite. The boundaries and contours would change for a different Xbulkbulk (i.e. different X (i.e. different XMgMg), and even the field assemblages ), and even the field assemblages

might change: for example the higher temperature garnet + quartz field would be garnet + chlorite if the SiOmight change: for example the higher temperature garnet + quartz field would be garnet + chlorite if the SiO 22 content were so low that content were so low that

quartz were consumed before chlorite by the reaction. 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

– 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 27.6. AFM projections showing the relative distribution of Fe and Mg in garnet vs. biotite at approximately 500 oC (a) and 800oC (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 Fe2O3 = 4 Fe3O4 + O2

2 Fe3O4 + 3 SiO2 = 3 Fe2SiO4 + O2

• At any particular pressure these become oxygen buffers

Fig. 26.10. 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. 469-488. 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 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 ChemographicsWe can use chemographics to infer reactions

MgO SiO2

Per Fo En Qtz

• Any two phases in a binary system can react to from a phase between them

• 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 it)

Reactions and ChemographicsWhat reaction does this ternary system allow?

Fig. 26.12. From Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Reactions and ChemographicsA + B + C = X

below x-in isograd

above x-in isograd

Reactions and ChemographicsWhat reaction does this system allow?


Reactions and ChemographicsWhat reaction is possible between A-B-C-D?

Fig. 26.14a. 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


Petrogenetic GridsP-T diagrams for multicomponent systems that show a set

of reactions, generally for a specific rock type

Petrogenetic grid for mafic rocks

Fig. 26.19. 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 (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. 26.7. 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 (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


Figure 26.15. The Al2SiO5 T-P phase

diagram from Figure 21-9 (without H2O).

Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


Figure 26.16. 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.


Figure 26.17. 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.

Text figures that I don’t have time to

cover in my 1-semester class

Figure 26.18. 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 26.20. 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, 244-267.

Figure 26.21. 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, 244-267.



Figure 26.21. 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, 244-267.

Chapter 26: Metamorphic Reactions If we treat isograds as reactions, we can: Understand what physical variables might affect the location of a particular.

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