Metamorphic Reactions

(Chapter 26)

last update:08/18/05

Isograd Reactions

If we treat isograds as reactions, however, we can then:

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

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

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, and crystals of one polymorph may remain as metastable relics in the stability field of another

Coexisting polymorphs may therefore represent non-equilibrium states, such as overstepped equilibrium curves or polymetamorphic overprints

2. Exsolution

3. Solid-Solid Net-Transfer Reactions

Involve solids only

Differ from polymorphic transformations in that they involve solids of differing composition, and thus matter must diffuse from one site to another in order 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 = (Mg,Fe)₃Al₂Si₃O₁₂ + Ca(Mg,Fe)Si₂O₆ + SiO₂

Opx           +    Plag        =       Gnt       +    Cpx      + Qtz

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

4. Devolatilization Reactions

Among the most common metamorphic reactions

Concentrate on H₂O-CO₂ systems, but the principles involved may be applied to any reaction involving volatiles

The reactions are 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 (p_H2O). This dependence is easily demonstrated by applying Le Châtelier’s principle to the reaction at equilibrium

Suppose H₂O is withdrawn from the system at some point on the water-saturated equilibrium curve: p_H2O < P_lithostatic

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

 

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 CO₂ or some other volatile phase or dissolved salts

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-X_fluid 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-X_fluid diagram must be created for a specified pressure

The shape of ~ all dehydration curves on T-X_fluid diagrams is similar to the curve above

They have a maximum temperature at the pure H₂O end, and a slope that is gentle at high X_H2O, but increasingly steep toward low X_H2O, becoming near vertical at very low X_H2O

The temperature of the reaction can thus be practically any temperature below the maximum representing p_H2O = P_lith

One should take great care to constrain the fluid composition, if at all possible, before using a dehydration reaction to indicate metamorphic grade

Decarbonation reactions may be treated in an identical fashion

For example, the reaction:

CaCO₃ + SiO₂ = CaSiO₃ + CO₂ 

Cal  + Qtz   =   Wo

can be shown on a T-X_CO2 diagram, with the maximum thermal stability of the carbonate mineral assemblage occurs at pure X_CO2

5. Continuous Reactions

Two reasons for variation of garnet appearance:

1. Such contrasting composition that the garnet reaction is different

Example: in some rocks garnet may be created by the (unbalanced) reaction:

Chl + Ms + Qtz ® Grt + Bt + H₂O 

Whereas in more Fe-rich and K-poor pelites, garnet might be generated by an (unbalanced) reaction involving chloritoid:

Chl + Cld + Qtz ® Grt + H₂O 

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

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 

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

Continuous reactions occur when F ³ 1, and the reactants and products coexist over a temperature (or grade) interval

If Chl + Ms + Qtz ® Grt + Bt + H₂O 

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

6. 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

6. Redox Reactions

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 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

What reaction is possible between A-B-C-D?

This is called a tie-line flip, and results in new groupings in the next metamorphic zone

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

Petrogenetic Grids

P-T diagrams for multicomponent systems that show a set of reactions, generally for a specific rock bulk composition