Download LECTURE 17 - Introduction to Metamorphism 2

Introduction to Metamorphism 2
IN THIS LECTURE
• Importance of Understanding Metamorphic Mineral
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Assemblages
Progressive Nature of Metamorphism
Stable Mineral Assemblages
The Phase Rule in Metamorphic Systems
The MgO-H2O system
What’s so Important About
Metamorphic Mineral Assemblages
• Equilibrium mineral assemblages can tell us about
P-T conditions
• P-T conditions tell us about tectonic environments
• P-T conditions combined with geochronology tells
us about how tectonic environments evolve through
time
• Optical Mineralogy is the starting point!!
The Progressive Nature of Metamorphism
• A rock at a high metamorphic grade probably progressed
through a sequence of mineral assemblages rather than
hopping directly from an unmetamorphosed rock to the
metamorphic rock that we find today
• All rocks that we now find must also have cooled to surface
conditions. Therefore, at what point on its cyclic P-T-t
path did its present mineral assemblage last equilibrate?
• The preserved zonal distribution of metamorphic rocks
suggests that each rock preserves the conditions of the
maximum metamorphic grade (temperature)
The Progressive Nature of Metamorphism
• Prograde reactions are endothermic and easily driven
by increasing T
• Devolatilization reactions are easier than
reintroducing the volatiles
• Geothermometry indicates that the mineral
compositions commonly preserve the maximum
temperature
Stable Mineral Assemblages in
Metamorphic Rocks
Equilibrium Mineral Assemblages
• At equilibrium, the mineralogy (and the composition of
each mineral) is determined by T, P, and X
• “Mineral paragenesis” refers to such an equilibrium
mineral assemblage
• Relict minerals or later alteration products are thereby
excluded from consideration unless specifically stated
The Phase Rule in Metamorphic Systems
• Phase rule, as applied to systems at equilibrium:
F=C-P+2
the phase rule
P is the number of phases in the system
C is the number of components: the minimum number of
chemical constituents required to specify every phase
in the system
F is the number of degrees of freedom: the number of
independently variable intensive parameters of state
(such as temperature, pressure, the composition of
each phase, etc.)
• Remember we’ve seen this already with igneous systems
The Phase Rule in Metamorphic Systems
• Pick a random point anywhere on a phase diagram
– Likely point will be within a divariant field and not on a
univariant curve or invariant point
• The most common situation is divariant (F = 2),
meaning that P and T are independently variable
without affecting the mineral assemblage
Remember this diagram?
The Phase Rule in Metamorphic Systems
• If F  2 is the most common situation, then the
phase rule may be adjusted accordingly such that
F=C-P+2 2
and therefore P  C
• This is Goldschmidt’s mineralogical phase rule, or
simply the mineralogical phase rule
The Phase Rule in Metamorphic Systems
If C has been determined for a particular rock then
there are three potential situations according to the
phase rule
1. P=C
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2. P<C
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This is the standard divariant situation in metamorphic
rocks
The rock probably represents an equilibrium mineral
assemblage from within a metamorphic zone
A situation that commonly arises in systems that display
solid solution.
We’ve seen this already with the binary phase diagrams for
the albite-anorthite system
Albite-Anorthite Phase Diagram
The Phase Rule in Metamorphic Systems
If C has been determined for a particular rock then there are three
potential situations according to the phase rule
1. P = C
• This is the standard divariant situation in metamorphic rocks
• The rock probably represents an equilibrium mineral
assemblage from within a metamorphic zone
2. P < C
• A situation that commonly arises in systems that display solid
solution.
• We’ve seen this already with the binary phase diagrams for
the albite-anorthite system
3. P > C
• A more interesting situation, and at least one of three
situations must be responsible
The Phase Rule in Metamorphic Systems
For P > C then the following three situations could apply
1. F < 2
2. Equilibrium not attained
3. Choice of C not correct
If (1) applies then the sample was
collected from a location right on
a univariant reaction curve or
invariant point
The P-T phase diagram for the system Al2SiO5 calculated using the program
TWQ (Berman, 1988, 1990, 1991). Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
The Phase Rule in Metamorphic Systems
2. Equilibrium is not attained
• The situation with (2) is more important in terms of optical
mineralogy.
• The phase rule only applies to systems that are in equilibrium.
• If equilibrium is not attained or maintained then there could be
any number of minerals co-existing
• Unfortunately, this is often the case, especially in rocks that
have been partially retrogressed or rocks in the blueschist and
eclogite facies.
• Optical Mineralogy is the main tool for decided which minerals
represent the equilibrium mineral assemblage
The Phase Rule in Metamorphic Systems
3. The number of components was not correct
•
Some guidelines for an appropriate choice of C
– Begin with a 1-component system, such as CaAl2Si2O8 (anorthite),
there are 3 common types of major/minor components that we can
add
a) Components that generate a new phase
o
Adding a component such as CaMgSi2O6 (diopside), results in
an additional phase: in the binary Di-An system diopside
coexists with anorthite below the solidus
b) Components that substitute for other components
o
Adding a component such as NaAlSi3O8 (albite) to the 1-C
anorthite system would dissolve in the anorthite structure,
resulting in a single solid-solution mineral (plagioclase) below
the solidus
o
Fe and Mn commonly substitute for Mg
o
Al may substitute for Si
o
Na may substitute for K
The Phase Rule in Metamorphic Systems
3. The number of components was not correct
(cont.)
c) “Perfectly mobile” components
o
Either a freely mobile fluid component or a component
that dissolves in a fluid phase and can be transported
easily
o
The chemical activity of such components is commonly
controlled by factors external to the local rock system
o
They are commonly ignored in deriving C for
metamorphic systems
The Phase Rule in Metamorphic Systems
• Consider the very simple metamorphic system, MgOH2O
– Possible natural phases in this system are periclase (MgO),
aqueous fluid (H2O), and brucite (Mg(OH)2)
– How we deal with H2O depends upon whether water is
perfectly mobile or not
– A reaction can occur between the potential phases in this
system:
MgO + H2O  Mg(OH)2
Per + Fluid = Bru
– As written this is a retrograde reaction (occurs as the rock cools
and hydrates)
The System MgO-H2O
Cool to the temperature of the reaction curve, periclase reacts
with water to form brucite: MgO + H2O  Mg(OH)2
The System MgO-H2O
Reaction: periclase coexists with brucite:
P=C+1
F = 1 (2nd reason to violate the mineralogical phase rule)
To leave the curve, all
the periclase must be
consumed by the
reaction, and brucite is
the solitary remaining
phase
F = 1 and C = 1 again
The Phase Rule in Metamorphic Systems
Once the water is gone, the excess periclase remains stable
as conditions change into the brucite stability field
Thus periclase can be
stable anywhere on the
whole diagram, if
water is present in
insufficient quantities
to permit the reaction
to brucite to go to
completion
The Phase Rule in Metamorphic Systems
At any point (other than on the univariant curve itself) we would expect
to find two phases, not one
P = brucite +
periclase below
the reaction curve
(if water is
limited), or
periclase + water
above the curve
The Phase Rule in Metamorphic Systems
How do you know which way is correct?
• The rocks should tell you
– The phase rule is an interpretive tool, not a predictive tool, and
does not tell the rocks how to behave
– If you only see low-P assemblages (e.g. Per or Bru in the MgOH2O system), then some components may be mobile
– If you often observe assemblages that have many phases in an
area (e.g. periclase + brucite), it is unlikely that so much of the
area is right on a univariant curve, and may require the number
of components to include otherwise mobile phases, such as H2O
or CO2, in order to apply the phase rule correctly