Igneous and Metamorphic Petrology: Overview of Fundamental Concepts

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Igneous and Metamorphic PetrologyOverview of
Fundamental Concepts

• What role is played by energy in its various
  forms to create magmatic and metamorphic rocks?
• What is the source of internal thermal energy in
  the Earth? How does this drive rock-forming
  processes?
• What is the role of the Earths mantle?
• How does mantle convection focus rock forming
  processes in specific tectonic settings?
• What are the most significant properties of rocks
  and does each tell us about rock-forming
  processes?
• How does a petrologist study rocks to determine
  their nature and origin?

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Temperature ranges of Igneous and Metamorphic
Rocks

• Igneous Rocks formed by the cooling and
  solidification of magma, defined as mobile molten
  rock whose temperature is generally in the range
  of 700-1200C (1300-2200F). Most magmas are
  dominated by silicate melts on Earth.
• Metamorphic Rocks formed by the reconstitution
  of pre-existing rocks at elevated temperatures
  well beneath the surface of the Earth. Lower
  bound of temperature range is poorly defined, but
  usually gt 200C. Upper range bounded by melting
  (700C), above which we are in the igneous
  realm.

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crust
obvious from space that Earth has two
fundamentally different physiographic features
oceans (71) and continents (29)
from http//www.personal.umich.edu/vdpluijm/gs20
5.html
global topography
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Volumes of Igneous Rocks on Earth
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Forms of Energy

• Energy commonly defined as the capacity to do
  work (i.e. by system on its surroundings) comes
  in many forms
• Work defined as the product of a force (F) times
  times a displacement acting over a distance (d)
  in the direction parallel to the force
• work Force x distance
• Example Pressure-Volume work in volcanic
  systems.
• Pressure Force/Area VolumeArea x distance
• PV ( F/A)(Ad) Fd w

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Forms of Energy

• Kinetic energy associated with the motion of a
  body a body with mass (m) moving with velocity
  (v) has kinetic energy
• E (k) 1/2 mass velocity2
• Potential energy energy of position is
  considered potential in the sense that it can be
  converted or transformed into kinetic energy.
  Can be equated with the amount of work required
  to move a body from one position to another
  within a potential field (e.g. Earths
  gravitational field).
• E (p) mass g Z
• where g acceleration of gravity at the surface
  (9.8 m/s2) and Z is the elevation measured from
  some reference datum

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Forms of Energy (cont.)

• Chemical energy energy bound up within chemical
  bonds can be released through chemical
  reactions
• Thermal energy related to the kinetic energy of
  the atomic particles within a body (solid,
  liquid, or gas). Motion of particles increases
  with higher temperature.
• Heat is transferred thermal energy that results
  because of a difference in temperature between
  bodies. Heat flows from higher T to lower T and
  will always result in the temperatures becoming
  equal at equilibrium.

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Heat Flow on Earth
An increment of heat, Dq, transferred into a body
produces a proportional incremental rise in
temperature, DT, given by
Dq Cp DT
where Cp is called the molar heat capacity of
J/mol-degree at constant pressure similar to
specific heat, which is based on mass
(J/g-degree).
1 calorie 4.184 J and is equivalent to the
energy necessary to raise 1 gram of of water 1
degree centigrade. Specific heat of water is 1
cal /g C, where rocks are 0.3 cal / g C.
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Heat Transfer Mechanisms

• Radiation involves emission of EM energy from
  the surface of hot body into the transparent
  cooler surroundings. Not important in cool
  rocks, but increasingly important at Ts gt1200C
• Advection involves flow of a liquid through
  openings in a rock whose T is different from the
  fluid (mass flux). Important near Earths
  surface due to fractured nature of crust.
• Conduction transfer of kinetic energy by atomic
  vibration. Cannot occur in a vacuum. For a
  given volume, heat is conducted away faster if
  the enclosing surface area is larger.
• Convection movement of material having
  contrasting Ts from one place to another. T
  differences give rise to density differences. In
  a gravitational field, higher density (generally
  colder) materials sink.

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Magmatic Examples of Heat Transfer
Thermal Gradient DT between adjacent hotter and
cooler masses
Heat Flux rate at which heat is conducted over
time from a unit surface area
Thermal Conductivity K rocks have very low
values and thus deep heat has been retained!
Heat Flux Thermal Conductivity DT
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Heat Conduction
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Definitions
Thermal conductivity is a property of materials
that expresses the heat flux f (W/m2) that will
flow through the material if a certain
temperature gradient ?T (K/m) exists over the
material. The thermal conductivity is usually
expressed in W/m.K. and called l. The usual
formula is f l ?T It should be noted that
thermal conductivity is a property that is
describes the semi static situation the
temperature gradient is assumed to be constant.
As soon as the temperature starts changing, other
parameters enter the equation.
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More Definitions
In case of changing thermal parameters, also the
heat capacity C (J/K.m3) starts playing a role.
The heat capacity is again a material property.
It expresses the fact that for changing the
temperature ?T (K) of a certain volume V (m3) of
material  energy E (J) must flow in or out. The
heat capacity is usually linked to the density
??(kg/m3) of the material. The heat capacity is
usually found in the textbooks a specific heat
capacity Cp (J/K.kg), which must be multiplied by
the density to get the full picture. C ?
Cp When dynamic processes are involved, the
change of temperature versus time, at known
boundary conditions is determined by both thermal
conductivity and heat capacity. a l / ? Cp
, where l is the thermal conductivity. The
thermal diffusivity a ( m2/s) is always
encountered in the equations multiplied by the
time t (s).
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models
from http//www.geo.lsa.umich.edu/crlb/COURSES/2
70
convection in the mantle
observed heat flow warm near ridges cold over
cratons
from http//www-personal.umich.edu/vdpluijm/gs20
5.html
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Earths Geothermal Gradient
Average Heat Flux is 0.09 watt/meter2
Geothermal gradient DT/ Dz 20-30C/km in
orogenic belts Cannot remain constant
w/depth. At 200 km, would be 4000C ! 7C/km in
trenches Viscosity, which measures resistance
to flow, of mantle rocks is 1018 times tar at
24C !
Approximate Pressure (GPa10 kbar)
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examples from western Pacific
blue is high velocity (fast) interpreted as
slab
note continuity of blue slab to depths on
order of 670 km
from http//www.pmel.noaa.gov/vents/coax/coax.htm
l
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example from western US
all from http//www.geo.lsa.umich.edu/crlb/COURS
ES/270
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Cartoon of Earths Interior
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Earths Energy Budget

• Solar radiation 50,000 times greater than all
  other energy sources primarily affects the
  atmosphere and oceans, but can cause changes in
  the solid earth through momentum transfer from
  the outer fluid envelope to the interior
• Radioactive decay 238U, 235U, 232Th, 40K, and
  87Rb all have t1/2 that gt109 years and thus
  continue to produce significant heat in the
  interior this may equal 50 to 100 of the total
  heat production for the Earth. Extinct
  short-lived radioactive elements such as 26Al
  were important during the very early Earth.
• Tidal Heating Earth-Sun-Moon interaction much
  smaller than radioactive decay
• Primordial Heat Also known as accretionary heat
  conversion of kinetic energy of accumulating
  planetismals to heat.
• Core Formation Initial heating from short-lived
  radioisotopes and accretionary heat caused
  widespread interior melting (Magma Ocean) and
  additional heat was released when Fe sank toward
  the center and formed the core

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Gravity, Pressure, and the Geobaric Gradient

• Geobaric gradient defined similarly to geothermal
  gradient DP/D? in the interior this is related
  to the overburden of the overlying rocks and is
  referred to as lithostatic pressure gradient.
• SI unit of force is the Newton
• SI unit of pressure is the Pascal, Pa and 1 bar
  (1 atmosphere) 105 Pa

Force mass acceleration kg(m/s2) kg m
s-2 N Pressure Force / Area P F/A
(mg)/A and r (density) mass/volume (kg/m3) P
(in Pa) (kg m/s2)/m2 kg/m1s2 kg m-1 s-2
Nm-2
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Earth Interior Pressures

• P rVg/A rgz, if we integrate from the surface
  to some
• depth z and take positive downward we get
• DP/Dz rg

Rock densities range from 2.7 (crust) to 3.3
g/cm3 (mantle) 270 bar/km for the crust and 330
bar/km for the mantle At the base of the crust,
say at 30 km depth, the lithostatic
pressure would be 8100 bars 8.1 kbar 0.81 GPa

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Changing States of Geologic Systems

• System a part of the universe set aside for
  study or discussion
• Surroundings the remainder of the universe
• State particular conditions defining the energy
  state of the system

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Definitions of Equilibrium