Metamorphic Petrology
Francis, 2014
4.3 Ga “Faux Amphibolite”
Reactions
The processes of progressive metamorphism are
dominated by de-watering and de-carbonation
reactions, with the final production of anhydrous
mineral assemblages at the highest metamorphic grades.
Mixed Volatiles and Buffering
800
Periclase
700
Per + CO2
Mag
Per + H2 O
Bru
2
CO
H
ag
+
2
O
+
Br
u
600
M
Temp
o
C
MBP
Brucite
A
500
400
0.0
Magnesite
0.1
XCO2 in fluid
0.2
Thermobarometry
garnet – biotite thermometer
garnet – plagioclase barometer
Eastern Acadian Terrane - Low pressure – high
temperature ”Buchan” style metamorphism with
the highest grade zones being cored by granitic
intrusions.
Counter clockwise P-T path is
interpreted to reflect crustal extension in the preAcadian continental margin.
Western
Acadian
Terrane
Regional
“Barrovian” style metamorphism with clockwise
P-T metamorphic paths interpreted to be the late
over thrusting of the Eastern Acadian terrane
during the main Acadian deformation.
Acadian Orogeny
Solid - Solid Reactions
Single Component Systems:
SiO2
When a solid consist of 2 coexisting minerals
(phases):
F = C–P+2 = 1 -2+2 = 1
Such a system is invariant at any given pressure,
and thus a single component solid phase will melt
at 1 unique temperature at any specified pressure.
The boundary between the 2 phases in P - T space
will be a univariant line with a slope approximated
by:
dG = - SdT + VdP = 0
dP/dT = S/ V
This is also true for solid - liquid phase boundaries
because, to a first approximation, Ho and So are
constant for small changes in temperature (true for
all reactions not involving a relatively
compressible vapour phase).
1) Solid - Solid Reactions
andalusite
Al2Si05
sillimanite
Al2Si05
From the phase rule, we know that this is reaction is univariant and thus can be represented by a line in P-T space:
F = C - P + 2 = 1
At equilibrium:
G
(P,T)
=
H (1bar,T)
o
- T So(1bar,T) + (P-1) V = 0
dG (P,T) = 0 = - So dT + VdP
S, V,
and H vary little with T and P for solid - solid reactions because the changes in the reactants with T and P tend to parallel
those in the products. Thus the above equation approximates that of a straight line in P - T space with a slope of:
dP/dT
V
S
=
S/ V
= - 3.3 bar / K
= - 1.44 Joules / bar mol
= 4.72 Joules / mol K
1) Solid - Solid Reactions
Similarly:
albite
NaAlSi3O8
jadeite
+ quartz
NaAlSi2O6 + SiO2
dP/dT
=
V
=
=
S
S/ V = 25.02 bar / oK
- 1.734 Joules / bar mol
- 43.39 Joules / mol K
Most solid - solid reactions have positive slopes in P - T space because the higher temperature side of the reaction typically has both
higher entropy and volume.
F=C–P+2
F=2–3+2
F=1
Jd
univariant
Jd
Jd
Ab
Qtz
Qtz
Ab
Qtz
2) Simple Dehydration and Other Devolatilization Reactions
Water
analcite + qtz
NaAlSi2O6 H2O + SiO2
albite
+ H2O
NaAlSi3O8 + H2O
pyrophyllite
Al2Si4O10(OH)4
kyanite + 3 qtz + H2O
Al2Si05 + 3SiO2 + 2H2O
paragonite + qtz
NaAl2(Al,Si3)O10(OH)2
Al-silicate + albite
+ H2O
Al2Si05
+ NaAlSi3O8 + H2O
muscovite + qtz
KAl2(Al,Si3)O10(OH)2
sillimanite + K-felds + H2O
Al2Si05
+ KAlSi3O8 + H2O
Dehydration Reactions - the general case :
Hhydrous
G
(P,T)
Aanhydrous + water
=
G (P,T)
o
+ RTln (aH2O)(aA) = 0
(aH)
If the solids have constant compositions and water behaves as an ideal gas, then:
G
G
o
(P,T)
(P,T)
=
=
G (P,T)
o
H (1bar,T)
o
+ 0
- T So(1bar,T) + (P-1) V = 0
At relatively low pressures, both V and S are
positive, and the reaction has a positive slope:
(dP/dT = S/V)
With increasing pressure, however, H2O
compresses, the V of the reaction decreases
and the slope of the reaction increases and
may even become negative because typically
VA < VH.
Dehydration Reactions - the general case :
Hhydrous
G
Aanhydrous + water
(P,T)
=
G (P,T)
o
+ RTln (aH2O)(aA) =
(aH)
If H and A are pure phases, but the fluid
phase is diluted by another component such
as CO2, then the maximum thermal stability
of H is reduced by an amount given by:
G
(P,T) =
G
o(P,T)
G (P,T)
o
+ RTln (aH2O) = 0
= - RTln (aH2O)
Assuming water behaves as an ideal gas:
G (P,T)
o
= - RTln (XH2O)
0
3 ) CO2 - Decarbonation reactions
dolomite + qtz
CaMg(CO3)2 + SiO2
diopside
CaMgSi2O6
calcite + qtz
CaCO3 + SiO2
wollastonite + CO2
CaSiO3
+ CO2
Similar to the general case for dehydration reactions:
Ccarbonate
G
(P,T)
Ddecarbonated + carbon dioxide
=
G (P,T)
o
+ RTln(aCO2)(aD)
(aC)
Again, if C and D are pure phases, and CO2 is an ideal gas:
G (P,T)
o
= - RTln(XCO2)
=
0
+ CO2
+ CO2
Mixed Volatile Reactions
Reduced water pressure (PH2O < Ptotal), either because a system is open to water or because other
components are present in the fluid phase reducing the activity of water, will shift simple dehydration
reactions to lower pressures. If, however, there are reactions involving the additional components
that are diluting water in the fluid phase, more dramatic effects occur. This is particularly true for
CO2, whose presence in the fluid phase may stabilize carbonates at the expense of Ca and Mg bearing
silicate.
5CaMg(CO3)2 + 8SiO2 + H2O
dolomite
qtz
Greenwood’s Classification of Mixed Volatile Reactions:
bB + ……
1)
2)
3)
4)
5)
6)
dD + ….. + mH20 + nCO2
m=n=0
m > 0, n = 0
m = 0, n > 0
m = n > 0; 4a) m = 1, n = 3
m > 0, n < 0
m < 0, n > 0
Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2
tremolite
calcite
4) Net Transfer Reactions:
Reactions that cause a change in the number of moles of minerals are termed net-transfer
reactions. Net transfer reactions with large ΔV make the best geobarometers. Net-transfer
reactions are also the most useful for fieldwork, as they typically mark the appearance or
disappearance of a phase that can be mapped in the field as a metamorphic “isograd”.
Tie-line switching (2D) or piercing
plane (nD) reactions
Terminal reactions at which phases
appear or disappear
B + C = A appearance
A = D + E disappearance
Three Component Systems:
F = 3 - P + 2
F = 2, if P =3
F = 1, if P = 4, univariant
Temperature
Tie-line switching (2D)
or
piercing plane (nD) reactions
Pressure
F = 3 - P + 2
Terminal reactions at which phases
appear or disappear
F = 2, if P =3
F = 1, if P = 4, univariant
B + C = A appearance
D = A + B + C disappearance
Temperature
Pressure
5)
Exchange Reactions:
Mg2SiO4 + 2FeSiO3
forsterite ferrosillite
Fe2SiO4 + 2MgSiO3
fayalite enstatite
Go(P,T) = -RTlnK = -RTln(aFa)(aEn)2
(aFo)(aFs)2
Mg3Al2Si3O12 + KFe3AlSi3O10(OH)2
pyrope
annite
Fe3Al2Si3O12 + KMg3AlSi3O10(OH)2
almandine
phlogopite
2Fe7Al4Si4O15(OH)12 + 7Mg2Al9Si4O23(OH)
Fe-chlorite
Mg staurolite
2Mg7Al4Si4O15(OH)12 + 7Fe2Al9Si4O23(OH)
Mg chlorite
Fe staurolite
Exchange reactions typically have small volume
changes and tend to be relatively insensitive to
pressure.
They commonly make the best
geothermometers for calculating the last
temperature of equilibrium of the mineral
assemblage.
6) Continuous Reactions:
(Mg,Fe)7Al4Si4O15(OH)12 + KAl2AlSi3O10(OH)2
chlorite
muscovite
K(Mg,Fe)3AlSi3O10(OH)2 + Fe2Al9O6(SiO4)4(O,OH)2
biotite
staurolite
+
water
Reactions that have a variance of 2, or more, are
termed continuous reactions and describe a region in
P - T space. Many reactions are continuous with
increasing metamorphic grade because of Fe-Mg
exchange between reactants and products. Other
continuous reactions involve the exchange of Na and
Ca, or other substituting cation pairs.
Pure
exchange reactions are always continuous, and many
net transfer reactions are also continuous because
they are sensitive to concomitant exchange reactions.
7) Discontinuous Reactions:
(Mg,Fe)7Al4Si4O15(OH)12 + KAl2AlSi3O10(OH)2 + Fe2Al9O6(SiO4)4(O,OH)2
chlorite
muscovite
staurolite
K(Mg,Fe)3AlSi3O10(OH)2 + Al2SiO5
biotite
kyanite + water
Reactions that have a variance of 1 are discontinuous and
describe a line in P - T space. Most discontinuous reactions are
terminal or piercing plane net-transfer reactions that are not
sensitive to exchange reactions.
Multi-Component Systems
The curves for the dehydration reactions presented for metapelites and metabasites are drawn for average
bulk compositions of shale and basalt respectively, and assume that the activity of water is 1 and Ptotal =
PH2O. The majority of these reactions, however, are not univariant and their positions are sensitive to
variations in bulk composition and the controls on the composition of the fluid phase. Commonly the effect
of additional components on a reaction can be predicted in a qualitative way by considering whether they
dissolve preferentially in the product or the reactant phases, the stability of the phase(s) that preferentially
accepts the additional component is enhanced at the expense of that which does not. These preferences can
be predicted on the basis of the crystal-chemical preferences of minerals.
The equilibrium constant :
K =
A + 3B
activities of products / activities reactants
2C + D
Le Châtelier’s Principle or the Law of Mass action enables us to use such equilibria to predict the effect of
other chemical components on reactions of interest:
K
= RTln ([aC]2[aD])
([aA][aB]3)
Bulk Compositional Effects:
Fe and Mg
In general, increasing Fe shifts most reactions to lower pressures and increasing Mg shifts them to
higher pressures. There are exceptions, however, particularly in the case of Fe preferentially
partitioning into garnet - higher Fe contents tend to increase the stability field of garnet, to both
lower and higher pressures. For the same reason, Mn has a similar effect on increasing the
stability field of garnet at the expense of other minerals.
Ca, Ti, Fe3+
The presence of Ca in metapelites may have a similar effect on garnet as Fe2+, whereas Ti and Fe3+
, in contrast, partition preferentially into the phyllosilicates biotite and chlorite, thus enhancing
their stability.
Additional components permit the existence of more mineral phases compared to that present at equivalent
degrees of freedom in the simpler system. In general, for an assemblage to exist across a 3-D space (and thus a
range of P & T), the number of phases is less than or equal to the number of components:
Goldschmidt’s mineralogical phase rule: C  P.
This follows from the phase rule that requires 2 or more degrees of Freedom:
FFreedom = CComponents - PPhases +
2
Metamorphic Facies : A metamorphic facies is the set of mineral assemblages that are stable
over a given range of P and T. The actual mineral assemblage within this set of possible mineral
assemblages, the one that a given rock exhibits is a function of its chemical composition. The
delineation of the metamorphic facies commonly used today is a matter of historical development that
predates actual experimental determination of pressures and temperatures. The division of the P-T
metamorphic regime into the following metamorphic facies developed from field observations on the
persistence of certain mineral assemblages for specific bulk compositions in geographic and thus P-T
space:
Zeolite
- zeolites or clay minerals, calcite and/or
quartz-filled amygdules
Greenschist - green minerals: chlorite, actinolite,
epidote
Blueschist
- blue amphibole, aragonite
Amphibolite - dark amphibole (hornblende),
staurolite, garnet
Granulite
- absence of hydrous minerals
and thus schistocity
Eclogite
- pyropic garnet & jadeiitic
clinopyroxene – high pressure