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GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 20
Metamorphic Reactions and
Petrogenetic Grids
April 6, 2016
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
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
TYPES OF METAMORPHIC REACTIONS
PHASE TRANSFORMATIONS
Isochemical phase transformations that depend on
temperature and pressure only
Al2SiO5
CaCO3
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)
TEXTURE IS A
GUIDE
to discriminate
incomplete reaction
vs. equilibrium
Rock w/ Ky+Sil+And
may indicate a
field
rather than an
invariant
point
TYPES OF METAMORPHIC REACTIONS
EXSOLUTION
Albite-rich perthite exsolution (h)
Orthoclase host (g)
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
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
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)
TYPES OF METAMORPHIC REACTIONS
DEVOLATILIZATION
The equilibrium curve
represents equilibrium
between the reactants and
products under watersaturated 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
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
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+....)
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 CaCO3 + 2 SiO2
Tr
Cal
Qtz
= 5 CaMgSi2O6 + 3 CO2 + H2O
Di
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.
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
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
TYPES OF METAMORPHIC REACTIONS
OXIDATION/REDUCTION (REDOX)
Involves a change in oxidation state of an element
Fe+3 (Ferric) = oxidized
MH - 6 Fe2O3 = 4 Fe3O4 + O2
FMQ - 2 Fe3O4 + 3 SiO2 = 3 Fe2SiO4 +
QIF - Fe2SiO4 = 2 Fe0 + SiO2 + O2
Hematite(Fe2O3 ): 2Fe+3,
O2 Magnetite (Fe O ): 2 Fe+3 + 1 Fe+2
3 4
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.
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
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.
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
TYPES OF METAMORPHIC REACTIONS
CONTINUOUS REACTIONS
Recall the igneous situation with solid solution
“Melt-in”
isograd?
Temperature
is strongly
X-dependent
Winter (2001) Fig. 6-10.
Isobaric T-X phase
diagram at atmospheric
pressure After Bowen
and Shairer (1932),
Amer. J. Sci. 5th Ser.,
24, 177-213
CONTINUOUS VS. DISCONTINUOUS REACTIONS
Consider the reaction: Chl + Ms + Qtz  Grt + Bt + H2O
F = C – f + 2 = 5 – 4 + 2 = 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
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
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.
REACTIONS AND CHEMOGRAPHICS
We can use chemographics to infer reactions.
Consider the 2-component system MgO-SiO2
P
e
r
F
o
E
n
M
g
O
Q
t
z
S
i
O
2
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
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.
REACTIONS AND CHEMOGRAPHICS
2A + B + C = X
At P/T
Conditions B
At P/T
Conditions A
X-in
Isograd
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
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
Increasing Grade
Above the Isograd
PETROGENETIC GRIDS
GRAPHICALLY PORTRAYING SEVERAL REACTIONS IN
PRESSURE – TEMPERATURE SPACE
Simple One Component System
Metastable
Extensions
Univariant
Curves can
be labelled
by Absent
Phase
PETROGENETIC GRIDS
MULTISYSTEMS f > 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)
PETROGENETIC GRIDS
MULTISYSTEMS
Theoretically Possible vs.
Geologically Probable
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. 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”).