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BLUESCHIST
Garcia, Marie Anne
Juayang, Noemi
Villacampa, Jedi
Review:
Metamorphism
• It is a sub solidus process which leads to the
changes of both mineralogy and composition
of a rock. Due to both extreme physical and
chemical conditions which is different from a
planet’s surface wherein sedimentary process
occurs. Partial melting, crystallization, and
recrystallization occurs.
Metamorphic facies
• Metamorphic facies are recognizable terranes or zones with
an assemblage of key minerals that were in equilibrium
under specific range of temperature and pressure during a
metamorphic event.
Facies:
• Low T - Low P : Zeolite
• Mod - High T - Low P : Prehnite-Pumpellyite
• Low T - High P : Blueschist
• Mod to High T - Mod P : Greenschist - Amphibolite Granulite
• Mod - High T - High P : Eclogite
Intro
• Equilibrium is the condition of a system in
which competing influences are balanced.
Equilibrium in metamorphism:
• In a chemical process, chemical equilibrium
is the state in which the chemical activities
or concentrations of the reactants and
products have no net change over time.
• In a chemical reaction, when reactants are mixed together in a
reaction vessel (and heated if needed), the whole of reactants do
not get converted into the products. After some time (which may
be any amount of time, shorter than millionths of a second or
longer than the age of the universe), the opposing reactions will
have equal reaction rates, creating a dynamic equilibrium in which
the ratio between reactants and products will appear fixed. This is
called chemical equilibrium.
• to be at equilibrium the rates of the forward and backward
(reverse) reactions have to be equal. In this chemical equation
with arrows pointing both ways to indicate equilibrium, A and B
are reactant chemical species, S and T are product species, and α,
β, σ, and τ are the stoichiometric coefficients of the respective
reactants and products. The equilibrium position of a reaction is
said to lie far to the right if, at equilibrium, nearly all the reactants
are used up and far to the left if hardly any product is formed from
the reactants.
• The Second law of thermodynamics is an
expression of the universal principle of
decay observable in nature. This decay is
measured and expressed in terms of a
physical property called entropy, stating
that the entropy of an isolated system can
never decrease. It will tend to increase over
time if not at thermodynamic equilibrium,
reaching a maximum value when
equilibrium is reached
• Entropy is a macroscopic property of a system that is a measure of
the microscopic disorder within the system. It is an important part
of the second law of thermodynamics. Thermodynamic systems
are made up of microscopic objects, e.g. atoms or molecules,
which "carry" energy. According to the second law of
thermodynamics, the thermodynamic entropy is a measure of the
amount of energy which does no work during energy conversions.
• Enthalpy (denoted as H) is a measure of the energy associated
with a system. It can be thought of as the amount of energy
required to create a system plus the amount of energy required to
make room for it by displacing its environment. Enthalpy simplifies
certain calculations of energy transfer, as it is not affected by
energy transferred to the environment through the expansion of
the system. Enthalpy is sometimes described as the "heat content"
of a system for a given pressure.
Blueschist
Blueschist
• Is a rock that forms by the metamorphism of
basalt and rocks with similar composition at high
pressures and low temperatures, approximately
corresponding to a depth of 15 to 30 kilometers
and 200 to ~500 degrees Celsius. The blue color
of the rock comes from the presence of the
mineral glaucophane.
• Blueschists are typically found within orogenic
belts as terranes of lithology in faulted contact
with greenschist or rarely eclogite facies rocks.
Petrology
• Blueschist, as a rock type, is defined by the
presence of the minerals glaucophane + (
lawsonite or epidote ) +/- jadeite +/- albite or
chlorite +/- garnet +/- muscovite in a rock of
roughly basaltic composition.
• Blueschist often has a lepidoblastic, nematoblastic
or schistose rock microstructure defined primarily
by chlorite, phengitic white mica, glaucophane,
and other minerals with an elongate or platy
shape.
Petrology
• Experimental work had shown that the minerals in blue schist form only
under very unusual metamorphic conditions.
These conditions are a pressure range equivalent to a depth of 15 to 30
kilometers in the crust and a very cool temperature, only 200 to 400
degrees centigrade.
That's the approximate cooking temperature of a kitchen oven or toaster.
• At a depth of 15 to 30 kilometers, however, the temperature is normally
about twice as hot, 500 to 750 degrees centigrade.
So the only way that rocks can be metamorphosed to blue schist facies , is
to be quickly shoved down to those extreme depths and then rapidly
brought back up before the rocks have time to heat up completely. And
that's exactly what happens where two tectonic plates are colliding in a
subduction zone.
In fact, blue schist bearing rocks normally occur in long linear zones that
mark ancient plate subduction boundaries.
Ref: USGS CMG InfoBank: Blue Schist
Blueschist facies
•
Blueschist facies is determined by the particular Temperature-Pressure
conditions required to metamorphose basalt to form blueschist. Felsic
rocks and pelitic sediments which are subjected to blueschist facies
conditions will form different mineral assemblages than metamorphosed
basalt.
Blueschist mineralogy varies by rock composition, but the classic equilibrium assemblages
of blueschist facies are:
Basalts:
glaucophane + lawsonite and/or epidote + albite + titanite +/- garnet +/- quartz jadeite +
quartz - diagnostic of pressures ~> 10 kbar
Ultramafic rocks:
serpentinite/lizardite +/- talc +/- zoisite
Pelites: kyanite + zoisite +/- pargasite or phengite +/- albite +/- quartz +/- talc +/- garnet
Blueschist facies
• Granites: kyanite +/- paragonite +/- chlorite +/- albite +/- quartz +/pargasite or phengite
• Calc-silicates: Various
• Limestones and marble: calcite transforms to aragonite at high
pressure, but typically reverts to calcite when exhumed
• Blueschist facies generally is considered to form under pressures of
>0.6 GPa, equivalent to depth of burial in excess of 15-18 km, and at
temperatures of between 200 to 500 °C. This is a 'low temperature,
high pressure' prograde metamorphic path and is also known as the
Franciscan facies series, after the west coast of the United States
where these rocks are exposed.
• Continued subduction of blueschist facies oceanic crust will
produce eclogite facies assemblages in metamorphosed basalt
(garnet + omphacitic clinopyroxene). Rocks which have been
subjected to blueschist conditions during a prograde trajectory will
gain heat by conduction with hotter lower crustal rocks if they
remain at the 15-18km depth. Blueschist which heats up to greater
than 500 °C via this fashion will enter greenschist or eclogite facies
temperature-pressure conditions, and the mineral assemblages will
metamorphose to reflect the new facies conditions.
• Thus in order for blueschist facies assemblages to be seen at the
Earth's surface, the rock must be exhumed swiftly enough to
prevent total thermal equilibration of the rocks which are under
blueschist facies conditions with the typical geothermal gradient.
• Blueschists and other high-pressure subduction
zone rocks are thought to be exhumed rapidly by
flow and/or faulting in accretionary wedges or
the upper parts of subducted crust, or may
return to the Earth's surface in part owing to
buoyancy if the metabasaltic rocks are associated
with low-density continental crust (marble,
metapelite, and other rocks of continental
margins).
History and etymology
• In 1962, Edgar Bailey of the U.S. Geological
Survey introduced the concept of "blueschist"
into the subject of metamorphic geology. His
carefully constructed definition established the
pressure and temperature conditions which
produce this type of metamorphism.
Thank you!
Gwapa, lage c master
diri?
Marie Anne Garcia
Thank you!
Noeme. Juayang
Thank you!
Jedi, Villacampa