Lecture Sixteen - Introduction to metamorphism Flashcards
What is metamorphism?
Metamorphic rock = A rock formed from a pre-existing rock (protolith), that undergoes mineralogical and textural change in response to modification of its physical or chemical environment.
Metamorphism = The process of forming new minerals that may grow at the expense of old ones, and/or changing the shape, size and arrangement of grains in the rock may.
Metamorphism is slow - the formation of metamorphic textures and minerals takes thoughsand to millions of years.
It involves different processes that can occur alone or in combination:
1) Recrystalisation.
2) Phase change.
3) Metamorphic reaction or ‘neocrystalisation.’
4) Pressure solution.
5) Plastic deformation.
What happens during recrystalisation?
Changes in the shape and size of grains without changing what the mineral is.
E.g. Sandstone recrystalises occurs when quartz grains grow into larger quartz grains.
What happens during phase change?
Transforms a grain of one mineral into a new one with the same composition but a different crystal structure.
E.g. Graphite to diamons - both are carbon compounds.
What happens during metamorphic reaction?
Also known as neocrystalisation.
Growth of new mineral crystals that differ from the ones in the starting rock (protolith).
Chemical reactions ‘digest’ minerals (reactants) producing new minerals (products).
What happens during pressure solution?
When a rock is squeezed more strongly in one direction than another.
Occurs at relatively low P and T, with water present.
Minerals dissolve when their surfaces are pressed against other grains.
Grains may become elongated in this way.
What happens during plastic deformation?
High P and T cause some minerals to behave like soft plastic without breaking.
Squeezing and streaching may produce flattened or elongate grains.
No change in the composition or crystal structure of the mineral.
How can heating cause metamorphism?
Atoms vibrate rapidly, streaching and bending chemical bonds when the atoms in minerals are heated up.
Increased heating allows bonds to strech and break allowing movememnt of atoms within grains (recrystallisation) or in/out of grains forming new bonds (neocrystalisation).
Metamorphism occurs between the diagenesis (process that form sedimentary rocks) and melting (process which forms igneous rock).
200-850 degrees C (can be up to 1200 degrees C if correct composition and water pressure).
Deapth of metamorphic termperatures can greatly vary.
E.g. 500 degrees C reaced easily next to shallow igneous intrusions, but 20-25km depth in normal continental crust.
How can a change in pressure cause metamorphism?
Near the Earths surface where P is low, minerals have a fairly open structure (large spaces between atoms).
If these minerals experience extreme pressure (e.g. burial or compression), denser minerals form -> phase change and/or neocrystallisation.
Weight of overlying rock in the crust -> every km in depth increases P by ~0.3kbar (3000 times P at earths surface). So 12kbar = ~36km depth.
Link pressure with temperature, how are they related with relation to metamorphism?
Temperature increases with depth:
Primary sources of heat in the earth are:
1) Radioactive decay of U, K and Th.
2) Crystallisation of the earths inner core.
heat from these primary sources in redistributed by radiation, convection and magma migration.
The rate of increase in temperature with depth is known as the geothermal gradients, which averages ~25 degrees C/km, but can be 10-60 degrees /km depending on the tectonic setting.
Pressure increases with depth:
P = phg (p=density, g=gravity and h=height).
Density of the continental crust is ~2700kg/m^3.
Gravity is 9.8m/s/s.
Note that the density of rock increases with depth, so P does not increase linearly.
Different roks form at different P and T conditions in the crust.
We can identify them by mineral content.
And then work out the P and T conditions it formed under.
Used this information to work out a tectonic setting.
Mineral stability changes with P and T, so a metamorphic rock at 8km will be different to a metamorphic rock at 20km.
E.g. Al2SiO2 can exist as three different minerals, by having different elemental arrangements within the minerals it forms.
How can meteorite impacts cause metamorphism (shock metamorphism)?
Meteorite impacts are one way we can create (local) high pressure and temperatures over a very short time scale.
Meteorite impacts can metamorphose the rocks on which they land - shock metamorphism.
One way of identifying the occurrences and location of past meteorite impacts is to look for the metamorphism effects on the earth rocks that are produced.
Shock metamorphism = Creates very high pressures (and increases temperatures) over a short time period. Ultra high pressure minerals can form as well as ‘shocking’ existing ones.
How can differential stress cause metamorphism?
Often rocks are squeezed more in one direction than another.
There are two kinds of differential stress:
1) Normal stress = Push (compression) or pull (tension) perpendicular to a surface. Push - flattens a material, pull - stretches a material.
2) Shear stress = Moves one part of material sideways relative to another.
Rocks subjected to differential stress (and high P and T) change shape without breaking.
Internal structure of the rock changes - Preferred minerals orientation of inequant grains.
Inequant grains can be platy (pancake shaped) or elongate (cigar shaped).
Preferred orientation of inequant minerals in a rock -> planar fabric or layering. This is a type of metamorphic folliation.
How can hydrothermal fluids cause metamorphism?
Metamorphic reactions usually occur in the presence of hydrothermal fluids (e.g. water is common in the crust).
E.g. Water, carbon dioxide, methane, hydrogen sulfide (O2 and H+ are also important).
The presence or absence of these fluids strongly affects the chemical stability fo many minerals.
These fluids are chemically active (can dissolve certain minerals).
Water in hydrothermal fluids comes from ground water or from other metamorphic reactions.
Water can be absorbed during metamorphic reactions.
E.g. many minerals contain water, if there is no water present these minerals are not able to form.
E.g. If there is no free oxygen present, oxygen-bearing minerals are not able to form.
Fluids can accelerate metamorphic reactions - atoms can move faster through a liquid than a solid.
Fluids passing though a rock can pick up dissolved ions and drop off others - changes over all chemicl composition of a rock.
This is called metasomatism.
Hydrothermal fluids can carry away dissolved material and deposit (precipitate) it elsewhere -> veins (cut across pre-existing rock).
What is metasomatism?
When fluids pass though a rock pick up dissolved ions and drop off others -> changes overall chemical composition of a rock.
Hydrothermal fluids can carry away dissolved material and deposit (precipitate) it elsewhere -> veins (cut across pre-existing rock).
Explain the common sequence of the metamorphosis of a shale.
Shale is not a metamorphic rock, it is sedimentary.
Made of clay, and formed in an ancient lake or ocean.
It is a common protolith.
If we increase P and T -> Slate.
Slate is fine grained and cleaved metamorphic rock.
Cleavage results from alignment of fine grained minerals (e.g. muscovite or chlorite micas).
If we increase the P and T again -> Schist.
Strongly foliated crystaline metamorphic rock.
Foliation results from alignment of platy and fine grained minerals (muscovite, chlorite, biotite, hornblent (an amphibole)).
Increase the P and T again -> Gneiss.
Course grained metamorphic rock with alternating ligth and dark bands.
These bands are often due to different mineral concentrations.
Higher T - courser (larger crystals formed).
Increase P and T again -> Migmatite.
Part igneous and part metamorphic - rocks begin to melt.
