Download Metamorphism What and Why

EAS 2200
The Earth System
Spring 2011
Lecture 10
Metamorphism
What and Why
Metamorphism refers to mineralogical and textural changes that occur in rocks as a
consequence of changing conditions.
Many rocks at the surface of the Earth are out of chemical equilibrium (that is to say,
their minerals do not represent the most stable possible physiochemical assemblage).
When subjected to increased temperature (and pressure), the chemical components in
the rock (including both minerals and pore fluid) react in an attempt to achieve chemical
and textural equilibrium.
Metamorphism can also occur when “dry” rocks come in contact with water at elevated
temperature. (Reactions also occur between water and rock at low temperature, but this
is called weathering or diagenesis.)
Metamorphism, Temperature, and Reaction Rates
Metamorphism generally occurs in association with changes in temperature, particularly
increases in temperature.
This reflects the exponential increase in both reaction rates and diffusion rate with
temperature.
The relationship is given by the
Arrhenius Equation:
Rate ∝ e-E/RT
Where E is a barrier energy, R is
the gas constant, and T is temp.
Minerals in a rock can often
coexist in a metastable, nonequilibrium state at the surface
of the Earth almost indefinitely.
For example, the minerals that
crystallized from magma at
1000˚C.
Reaction rates increase
exponentially with increasing
temperature.
Consequently, once a rock
experiences elevated
temperature, it begins to react
and undergo chemical and
textural changes.
Textural Changes
The simplest kind of
metamorphism is where
existing minerals simply recrystallize and there is no change in mineralogy
Most common in monomineralic rocks.
Examples: Limestone -> Marble, Sandstone -> Quartzite
Almost always results in a coarsening of grain size.
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EAS 2200
The Earth System
Spring 2011
Lecture 10
Why does this happen?
Because atoms at the surface of a substance are in a different environment than those in the
interior, there is an excess energy at the surface, called the interfacial free energy. (Surface
tension is a manifestation of this energy.)
Ratio of surface area to volume decreases with increasing grain size. Consequently, increasing
the size of crystals reduces the interfacial energy.
Where more than one mineral is present, interfacial energy can sometimes be minimized if
only specific minerals or specific mineral faces are in contact.
When pressure is uniform, this is called static annealing
Stress and Pressure
Stress is force/area – same as pressure
Normal stress results from force applied perpendicular (normal) to a surface
Normal stress equal in all directions is pressure. Pressure resulting from the weight of
rocks above is called lithostatic pressure.
Shear stress results from force applied parallel to a surface.
Recrystallization under Deformation
When the pressure is not the same in all directions (i.e., non-lithostatic stress), some
grain boundaries will experience more pressure than others. This favors dissolution of
some crystal faces and growth of others.
This results in the preferred orientation of crystals and development of a type of fabric
known as foliation.
Under normal stress, minerals orient long axes perpendicular to the stress.
Under shear stress, minerals orient long axes parallel to stress.
Schists and Schistocity
Schistocity is an example of the foliation that develops as a result of preferred
orientation of minerals - primarily micas (and other sheet silicates).
Gneisses and Banding
Gneisses are characterized by bands of light (quartz and feldspar) and dark (micas and
amphiboles) minerals that were not originally present.
This also results from a minimization of free energy.
Nucleation requires less energy when the new crystal nucleates on an existing crystal of
the same kind. Hence new micas grow on old micas, new quartz on old quartz, etc.
Mineralogical Changes
Most often, metamorphism involves replacement of pre-existing minerals with new
ones.
This reflects a drive toward the equilibrium mineral assemblage for prevailing conditions.
As we have seen, individual minerals are generally stable over a limited range of temperature,
pressure, and system composition.
The equilibrium assemblage is the one that minimizes the energy of the system.
Many metamorphic reactions involve hydration or dehydration reactions.
Sedimentary rocks rich in clay minerals formed by weathering at the surface will progressively
dehydrate as metamorphic temperatures increase.
Igneous rocks, forming at high temperature in the virtual absence of water, will become
hydrated during metamorphism at lower temperature.
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EAS 2200
The Earth System
Spring 2011
Lecture 10
Hydration Reactions
Some examples of hydration reactions that occur during metamorphism of igneous
rocks
2 Plagioclase + Diopside + Hematite + Water = Epidote + Enstatite
2CaAl2Si2O8 + 2CaMgSi2O6 + Fe2O3 + H2O = 2Ca2(Al,Fe)3Si3O12(OH) + 2MgSiO3
3 Enstatite + 1 Quartz + Water = Talc
3MgSiO3 + SiO2 + H2O = Mg3Si4O10(OH)2
Many minerals formed by metamorphism of igneous rocks have a green color; hence
the term greenschist.
Metamorphism at Mid-Ocean Ridges
Hydrothermal systems at mid-ocean ridges hydrate the initially anhydrous (i.e., dry)
basalt, replacing original minerals with ones such as chlorite, epidote, and albite to form
greenschist. At higher temperature, these minerals are replaced by hornblende and
plagioclase to form amphibolite.
Dehydration Reactions
Hydrous minerals break down at elevated temperature.
Some examples of dehydration reactions that occur during metamorphism of
sedimentary rocks:
Pyrophyllite ↔ Quartz + Kyanite
2AlSi2O5(OH) ↔ 3SiO2 + Al2SiO5 + H2O
Muscovite + Quartz ↔ K-Feldspar + Sillimanite
KAl3Si3(OH)2 + SiO2 ↔ KAlSi3O8 + Al2SiO5 + H2O
Metamorphic Facies
As we have seen, individual minerals and assemblages of minerals are stable only over a
limited range of temperature and pressure (and composition).
This leads to the concept of facies.
A metamorphic facies is a set of metamorphic mineral assemblages (parageneses), each
for a specific rock composition, that form over a specific range of P and T.
By reproducing the mineral assemblages in the laboratory, it is possible to relate specific
facies to specific ranges of temperature and pressure.
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EAS 2200
The Earth System
Spring 2011
Lecture 10
Facies and Reactions
Metamorphic Grade
Metamorphic grade refers to the intensity of metamorphism.
Grade is strongly related to, but not identical to, temperature.
Prograde metamorphism occurs as temperature (and pressure) increase.
Retrograde metamorphism occurs as temperature decreases.
Metamorphic Zones
Index minerals are ones that are stable over only limited range of conditions and hence
useful in determining grade (as opposed to, say, quartz and feldspars).
In the field, we can map out regions where these index minerals are present or absent.
These regions correspond to zones.
Zones for pelitic* metamorphic rocks:
chlorite zone ((Fe,Mg)6(Si,Al)4O10(OH)8):
chlorite + muscovite + qtz
biotite zone:
chlorite + biotite + muscovite + qtz
garnet zone
chlorite + biotite + garnet + muscovite
staurolite ((Mg,Fe)2Al9Si4O22(OH)2) zone:
staurolite + muscovite + biotite + qtz
kyanite zone
kyanite + biotite + muscovite + qtz
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EAS 2200
The Earth System
Spring 2011
Lecture 10
sillimanite zone
garnet + biotite + sillimanite + muscovite + qtz
orthoclase zone
sillimanite + orthoclase + qtz and no muscovite (± melt )
Isograds
Isograds are lines of equal metamorphic intensity.
In practice, they correspond to lines where a specific mineral appears (often at the
expense of another).
Geothermometry & Geobarometry
Many minerals have variable compositions, which is to say they are solid solutions.
The exact composition of pairs these minerals will depend on temperature and pressure.
By analyzing individual mineral compositions and using thermodynamic models based
on laboratory experiments, it is often possible to determine the exact temperature and
pressure at which a mineral assemblage (i.e., a rock) equilibrated.
P-T metamorphic paths
Sometimes, rocks do not reach complete chemical and textural equilibrium. In these
cases, study of mineral chemistry and and textural relations between minerals can reveal
the pressure-temperature path that a rock has followed. This can be quite useful in
tectonic reconstructions.
Why does metamorphism occur?
There are two broad classes of metamorphism:
Contact metamorphism
This occurs when igneous intrusions heat the surrounding crustal rocks metamorphosing
them.
Usually restricted to a few km’s at most from intrusive body.
Regional metamorphism
This occurs when large area are carried to depth where the rocks experience higher
temperature and pressure.
Usually a result of compressive tectonic forces rather than mere burial by sedimentation.
Contact Metamorphism
Heat released as an intrusive magma body crystallizes and cools will diffuse into the
surrounding country rock. The result is a metamorphic aureole. Width can vary from
cm’s to km’s, depending on the size and depth of the intrusion.
Contact metamorphism is often “dry”, but intrusions can set up hydrothermal
circulation. Mid-ocean ridges and Yellowstone are examples.
At shallow depth, contact metamorphism produces hornfels metamorphic facies.
Regional metamorphism
Regional metamorphism occurs when broad regions are buried to considerable depth
(km’s). This usually happens as a result of compressive forces in continental collisions
or subduction zones. Consequently, deformation often accompanies metamorphism.
Metamorphism is closely related to tectonics and mountain-building, our next topics.
A particular kind of metamorphism occurring at low temperature but high pressure
produces blueschist and eclogite metamorphic facies and is associated with subduction.
The Franciscan complex of the central and northern California coast consists of a
chaotic mix that includes rocks metamorphosed under blueschist conditions. It formed
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EAS 2200
The Earth System
Spring 2011
Lecture 10
as oceanic crust and sediment accreted to North America from the subducting Farallon
plate during the Mesozoic.
Ultra-high Pressure Metamorphism
Over the last decade or two, many examples have been found of rocks metamorphosed
at pressures corresponding to depths of 100 to 200 km. These are known as ultra-high
pressure (UHP) metamorphic belts.
Very high pressure minerals such as coesite (SiO2) and diamond are diagnostic.
Dabie Shan in China is among the best examples of UHP metamorphism. It is though to
represent a peninsula-like piece of continental crust (e.g., Florida) carried into a
subduction zone. It eventually “popped” back to the surface under its own buoyancy,
like a submerged cork.
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