1. Geol 2312 Igneous and Metamorphic Petrology
Lecture 20
Metamorphic Reactions and
Petrogenetic Grids
April 6, 2016

2. 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

3. Types of Metamorphic Reactions
Phase Transformations
Exsolution Reactions
Solid-Solid Net Transfer Reactions
Devolatilization Reactions
Ion Exchange Reactions
Oxidation/Reduction (redox) Reactions
Reactions involving Dissolved Species
Continuous Reactions

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

5. 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

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

7. 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

8. 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

9. 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)

10. 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

11. 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

12. 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+....)

13. 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 CaCO SiO Tr Cal Qtz
= 5 CaMgSi2O6 + 3 CO2 + H2O Di

14. 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.

15. 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

16. 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

17. 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 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.

18. 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

19. Types of Metamorphic Reactions Continuous Reactions Involving Solid Solution Phases
An idealized field area of steeply dipping meta-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.
WHY?
Winter (2001) Figure 26-8.

20. Types of Metamorphic Reactions Continuous Reactions
Two possible reasons:
1. Each unit has contrasting compositions such 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)
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 or 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
Let’s evaluate the second situation

21. Types of Metamorphic Reactions Continuous Reactions
Recall the igneous situation with solid solution
“Melt-in” isograd?
Temperature is strongly
X-dependent
Winter (2001) Fig Isobaric T-X phase diagram at atmospheric pressure After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24,

22. Continuous vs. Discontinuous Reactions
Consider the reaction: Chl + Ms + Qtz  Grt + Bt + H2O
F = C – f + 2 = 5 – = 1 (univariant)
But effectively F=0 since T
and P are controlled by
moving along a geothermal
or field gradient
Therefore, the reaction occurs
at a particular T for a given
gradient and would be considered
DISCONTINUOUS

23. Continuous vs. Discontinuous Reactions
However, when considering the solid solution of Mg and Fe in garnet, biotite, and chlorite, the general reaction is
(Fe,Mg)Chl + Ms + Qtz  (Fe,Mg)Grt + (Fe,Mg)Bt + H2O
This is a continuous reaction and we expect to 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 will vary across the interval, and the proportions of the minerals will change until one of the reactants disappears with increasing grade

24. Continuous Reactions
Perhaps a more realistic way to portray this continuous reaction (minus the K component) is this.
Thus, the offsets in the idealized field area may be due to differences in the Mg/Fe ratios of the different rock layers.

25. Reactions and Chemographics
We can use chemographics to infer reactions. Consider the 2-component system MgO-SiO2
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)
However, thermodynamics determines whether such a reaction is probable

26. Reactions and Chemographics
For a ternary system, any phase that is coplanar with three other phases can be related by a chemical reaction
2A + B + C = X
Again, whether this reaction is probable under natural conditions of P & T depends on Thermodynamics.
If the diagram represents a projection from another phase or component, then that phase is implied in the reaction.
For component D, the reaction might be:
2A + B + C + #D = X
However, the amount of D in the
reaction would have to be figured out by balancing the reaction.

27. Reactions and Chemographics
2A + B + C = X
At P/T
Conditions B
At P/T
Conditions A
X-in
Isograd

28. Reactions and Chemographics
Another Possibility
If phase X can be defined by two different reactions:
2A+B+C = X
D+E = X
Then, the reaction:
2A+B+C = D+E
is also true

29. Reactions and Chemographics
A Tie-line Flip - results in new groupings in the next metamorphic zone
Because A+B and C+ D share a common tie-line, they can be related by the reaction:
A + B = C + D
Below the Isograd
At the Isograd
Above the Isograd
Increasing Grade

30. Curves can be labelled by Absent
Petrogenetic Grids Graphically Portraying Several Reactions in Pressure – Temperature Space
Simple One Component System
Univariant
Curves can be labelled by Absent
Phase
Metastable
Extensions

31. Petrogenetic Grids Multisystems  > C +2
One-Component System
Four Phases (ABDE)
Four invariant points
(labelled by the absent phase)
Seven univariant lines
(labelled by the absent phases)
Four divariant fields
(labelled by stable phase)

32. Petrogenetic Grids Multisystems
Theoretically Possible vs. Geologically Probable

33. Petrogenetic Grids Multisystems
Many Petrogenetic Grids will overlay grids of multiple
component systems that pertain to a specific protolith type
Petrogenetic Grid for Mafic Rocks
Lawsonite
Actinolite
P-T Range of Rock with Lawsonite + Actinolite + Pyrophyllite
Pyrophyllite
Winter (2001) 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”).