MSc Course

1. IgneousIgneous
PetrologyPetrology
Thanks to John WinterThanks to John Winter
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2. IntroductionIntroduction
Igneous petrology is the study of melts (magma) and
the rocks that crystallize from such melts,
encompassing an understanding of the processes
involved in melting and subsequent rise, evolution,
crystallization, and eruption or emplacement of the
eventual rocks.
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3. All terrestrial rocks that we now find at earths
surface were derived initially from the mantle,
although some have gone cycles of
sedimentation and metamorphism.
If these rocks have ultimate origin at depth, it
follows that we need to know what makes up the
earth if we want to understand their origin more
fully.
Compositionally there is crust, mantle and core
delineated by seismic discontinuities.
The Earth’s InteriorThe Earth’s Interior
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4. The Earth’s InteriorThe Earth’s Interior
Crust:
Oceanic crust: Basaltic
Thin: 10 km
Relatively uniform stratigraphy
= ophiolite suite:
sediments
pillow basalt
sheeted dikes
more massive gabbro
ultramafic (mantle)
Plate tectonics is creating oceanic crust at mid-ocean ridges and
consuming it at subduction zones, the oceanic crust is continually
being renewed and recycled.
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5. The Earth’s InteriorThe Earth’s Interior
Continental Crust
Thicker: 20-90 km average ~35
km
Highly variable composition
Average ~ granodiorite
Covers ~40% of earths surface
The amount of continental crust has been increasing over the past 4 Ga.
The stable continents (Cratons) consist of more ancient shield and stable platforms.
several marginal orogenic belts also flank many cratons.
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6. The Earth’s InteriorThe Earth’s Interior
Mantle: 83% of earth’s volume
Peridotite (ultramafic)
There are layers distinguished by more physical rather
than chemical differences.
80-220km-LVZ-1-5% Partial melting.
The melt probably forms thin discontinuous film along
mineral grain boundaries.
It makes mantle behave in a more ductile fashion.
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7. The Earth’s InteriorThe Earth’s Interior
Below the LVZ, 2 more seismic
discontinuities are encountered.
Upper to 410 km (olivine → spinel)
Low Velocity Layer 60-220 km
Transition Zone as velocity increases ~ rapidly
660 spinel → perovskite-type
SiIV
→ SiVI
Both these discontinuities result in abrupt increase in the density of the mantle,
accompanied by a jump in seismic velocities.
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8. The Earth’s InteriorThe Earth’s Interior
Below 660-km discontinuity, the velocities of seismic
waves increase fairly uniformly with depth.
At the very base of mantle is a ~200km thick
heterogenous layer of anomalously low seismic
velocity called the D” layer.
A thin ~40km, apparently discontinuous layer has been recently
resolved at the mantle-core boundary, clearly beneath central
Pacific.
The nature of these 2 layers is not clear, but their
properties are sufficiently anomalous to require more
than a thermal boundary perturbation, and they
probably represent layer of different composition,
probably dense ‘dregs’ of subducted oceanic
lithosphere that have settled at the base of lower
mantle.
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9. The Earth’s InteriorThe Earth’s Interior
Core:
Silicates of the mantle give way to a much
denser Fe rich metalic alloy with minor
amounts of Ni, S, Si, O etc.
Outer Core is liquid
No S-waves
Inner Core is solid
Figure 1.2 Major subdivisions of the Earth.
Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
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10. Figure 1.3 Variation in P and S wave velocities with depth. Compositional subdivisions of the Earth are on the left,
rheological subdivisions on the right. After Kearey and Vine (1990), Global Tectonics. © Blackwell Scientific. Oxford.
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11. The PressureThe Pressure
GradientGradient
P increases = ρgh
Hydrostatic pressure=lithostatic
pressure
Density increases with depth as the
rocks are compressed
Figure 1.8 Pressure variation with depth. From Dziewonski and
Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier
Science.
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12. The PressureThe Pressure
GradientGradient
P increases = ρgh
Nearly linear through mantle
~ 30 MPa/km
≈ 1 GPa at base of ave crust
Core: ρ incr. more rapidly
since alloy more dense
Figure 1.8 Pressure variation with depth. From Dziewonski and
Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier
Science.
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13. Heat SourcesHeat Sources
in the Earthin the Earth
1. Heat from the early accretion and differentiation of the Earth
(Secular cooling)
Primordial heat developed early in the history of the Earth from the
processes of accretion and gravitational differentiation
Some continued gravitational partitioning of iron in the inner core
may contribute some heat as well.
Estimates are 10-25% of total heat flux
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14. Heat SourcesHeat Sources
in the Earthin the Earth
2. Heat released by the radioactive breakdown of unstable
nuclides
• Most of the radioactive elements are concentrated in the continental
crust.
• 75-90% heat is produced that reaches the surface
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15. Heat TransferHeat Transfer
1. Radiation: Sufficiently transparent or translucent
material can transfer heat by radiation. It is the movement
of the particles/waves through another medium. Eg. Lamp
losing heat or surface of the earth losing heat.
2. Conduction: Opaque and rigid substances transfer heat
by conduction. It involves transfer of kinetic energy from
hotter atoms to adjacent cooler ones. It is efficient for
metals. Conduction is poor for silicate minerals but
relatively efficient in the core.
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16. Heat TransferHeat Transfer
3. Convection: If the material is more ductile and if it can be
moved, this is the process. It is the movement of material due
to density differences caused by thermal or compositional
variations.
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17. Heat TransferHeat Transfer
3. Convection: It affects ductile rocks and liquids.
• It may involve flow in a single direction, in which case the
moved hot material will accumulate at the top of the ductile
portion of the system.
• Convection may also occur in a cyclic motion, typically in a
closed cell above a localized heat source.
• Here, the heated material rises and moves laterally as it cools
and is pushed aside by later convective matter.
• Material pushed to the margin cools, contracts, and sinks
towards the heat source, where it becomes heated and cycle
continues. 18

18. Heat TransferHeat Transfer
3. Advection:
• It involves the transfer of heat with rocks that are otherwise in motion.
• For eg, if a hot region at depth is uplifted by either tectonism, induced flow
or erosion and isostatic rebound, heat rises physically with the rocks.
• All rocks, whether igneous, sedimentary or metamorphic,
owe their origins directly or indirectly to the advective
cooling of the earth.
• The rise of magma into the crust or its extrusion onto the
surface is an obvious example of advection.
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19. The Geothermal GradientThe Geothermal Gradient
• Convection works well in the liquid core and in the
somewhat fluid mantle.
• It is also a primary method of heat transfer in hydrothermal
systems above magma bodies.
• Beyond these areas, conduction and advection are the only
available methods of heat transfer.
• Transfer of heat controls metamorphism, melting,
crystallization, as well as the motion, mixing and mechanical
properties of Earth materials.
• We will consider heat transfer later.
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20. The Geothermal GradientThe Geothermal Gradient
Diagrammatic cross-section through the
upper 200-300 km of the Earth showing
geothermal gradients reflecting more
efficient adiabatic (constant heat content)
convection of heat in the mobile
asthenosphere (steeper gradient in blue) )
and less efficient conductive heat transfer
through the more rigid lithosphere
(shallower gradient in red). The boundary
layer is a zone across which the transition
in rheology and heat transfer mechanism
occurs (in green). The thickness of the
boundary layer is exaggerated here for
clarity: it is probably less than half the
thickness of the lithosphere.
Convection in the mantle redistributes heat more faster than conduction 21

21. The Geothermal GradientThe Geothermal Gradient
A similar example for thick
(continental) lithosphere.
The curved part of the
geotherm represents a zone of
gradual transition – boundary
layer – where, as temperature
rises, rocks become
progressively more ductile and
begin to participate in
convection.
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22. The Geothermal GradientThe Geothermal Gradient
Notice that thinner lithosphere
allows convective heat transfer to
shallower depths, resulting in a
higher geothermal gradient across
the boundary layer and lithosphere.
TP is the potential temperature. It
permits comparison of (estimated)
temperatures at depth from one
locality to another. Because
temperature varies with depth, one
must select some reference depth. In
this case the surface was chosen.
One simply extrapolates
adiabatically from the T and P in
question to the surface. Potential temperature, is the hypothetical surface temperature a deeply
buried rock would end up at if it were transported to the surface along
the adiabat passing through PT , without melting.
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23. The Geothermal GradientThe Geothermal Gradient
Figure 1.10 Temperature contours calculated for an oceanic plate generated at a mid-ocean
ridge (age 0) and thickening as it cools. The 1300o
C isotherm is a reasonable approximation
for the base of the oceanic lithosphere. The plate thickens rapidly from zero to 50 Ma and is
essentially constant beyond 100 Ma. From McKenzie et al. (2005). 26

24. The Geothermal GradientThe Geothermal Gradient
Figure 1.11 Estimates of
oceanic (blue curves) and
continental shield (red curves)
geotherms to a depth of 300 km.
The thickness of mature (>
100Ma) oceanic lithosphere is
hatched and that of continental
shield lithosphere is yellow.
• Because the thermal
conductivity of shallow crustal
rocks is low, heat transfer is
slow. Extrapolating this leads to
very high geothermal gradient.
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25. The Geothermal GradientThe Geothermal Gradient
• Shallow gradients in old oceanic
lithosphere range from about 17-
200
C/Km, gradually steepening to about
70
C/Km with depth.
• Shallow shield gradients range from
about 12-180
C/Km, transitioning to
about 4-60
C/Km below about 30Km.
• The oceanic v/s shield temperature
differences are essentially
restricted to the lithosphere and
boundary layer, so the oceanic and
continental curves converge by
about 250-300Km where
convection has homogenizing
influence.
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26. The Geothermal GradientThe Geothermal Gradient
Figure 1.12 Estimate of the geothermal
gradient to the center of the Earth (after
Stacey, 1992). The shallow solid portion is
very close to the Green & Ringwood (1963)
oceanic geotherm in Fig. 1–11 and the
dashed geotherm is the Jaupart & Mareschal
(1999) continental geotherm.
• Nearly adiabatic convective
heat flow deeper than about
300Km results in a linear
geothermal gradient of approx.
0.30
C/Km.
• The gradient shallows across
the D” layer and then becomes
steep in the metallic core,
where the thermal conductivity
and convection is very high.
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27. The Geothermal GradientThe Geothermal Gradient
• The density contrast across
the core-mantle boundary
prohibits the core material
from rising into the mantle,
so heat can only be
transferred upward by
conduction, resulting in a
thermal boundary layer
across which the thermal
gradient is estimated to be
300 – 10000
C.
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28. The geothermal gradientThe geothermal gradient
The rate of convective flow in the mantle is measured
in terms of cms/year.
Slabs of cooler oceanic lithosphere, mostly mantle
rock, too viscous to convect within themselves, sink
into the underlying hotter and relatively less dense
underlying mantle.
Computed tomography of the Earth using seismic
waves has shown that at least some subducting
lithospheric slabs sink all the way through the mantle
and come to rest on top of the dense metallic core
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29. Even though the rate at which the mantle
convects, on the order of a few
centimeters/year, seems minuscule, it is far
greater than the rate of heat transfer by
conduction.
The fact that lithospheric slabs can sink
convectively over tens of millions of years
about 2800 km through the hotter mantle but
still maintain a recognizable cooler T
throughout their approximately 100-km
thickness demonstrates how slowly they are
conductively heated.
Conversely, if heat conduction were more
rapid than convection, the subducting slabs
would absorb heat and lose their density
contrast and identity before sinking very far.
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30. 33

31. Dynamic cooling of the earthDynamic cooling of the earth
Previous figure was intended to illustrate the thermal
structure at shallow levels of Earth’s interior in stabilized
areas, similar to a static, chemically and physically
stratified earth with continents and oceans.
The earth is far more dynamic.
Temperature variations that drive convective flow are
concentrated in thin boundary layers that are much smaller
than the overall circulation pattern.
The upper boundary of this system is a cool thermal
boundary layer with low viscosity: the lithosphere.
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32. Dynamic cooling of the earthDynamic cooling of the earth
In laboratory models of convecting fluids, cool dense upper
thermal boundary layer descend as either cylindrical
downwellings or as larger networks of partially connected
tabular-shaped downwelling slabs, depending on the
rheology of the material.
It generally leads to slab-like downwelling.
Mantle convection and plate tectonics are inseparable
manifestations of the heat-driven dynamic cooling process of
the Earth, given its present thermal state and rheological
properties.
For the Earth, plate tectonics is mantle convection.
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33. Plate tectonicsPlate tectonics
The negative buoyancy of the descending plates has been called slab pull,
dense subducting slab pulls the rest of the plate after it.
The motion of the lithospheric plates down and away from the elevated mid-
oceanic ridges is called as ridge-push.
Both are body forces affecting whole plate.
As the subducting slab descends, the slab cant pull significantly on the rest
of the plate, which would simply fault and break. 36

34. Plate tectonicsPlate tectonics
Instead, as it sinks, the slab sets up circulation patterns in the mantle that
exert a sort of suction force.
As the slab descends from the surface, the pressure behind it is lowered,
which is immediately compensated by feeding more plate into the
subduction zone.
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35. Dynamic cooling of the earthDynamic cooling of the earth
Ridge push is gravitational sliding of a plate off the elevated
ridge, not an active mantle push by ascending mantle flow,
which would buckle the plate.
Plate motion velocities are roughly proportional to the
percentage of plate’s perimeter that corresponds to
subduction zones and not to ridges (Forsyth & Uyeda, 1975)
The predominance of slab pull suggest that mantle
upwelling at divergent plate boundaries, which moves
heat upward, is essentially a passive response to plate
separation and descent.
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36. TheThe
GeothermalGeothermal
GradientGradient
Fig 1.13. Pattern of global heat flux variations compiled from
observations at over 20,000 sites and modeled on a spherical
harmonic expansion to degree 12. From Pollack, Hurter and
Johnson. (1993) Rev. Geophys. 31, 267-280.
Cross-section of the mantle based on a seismic tomography model. Arrows represent
plate motions and large-scale mantle flow and subduction zones represented by dipping
line segments. EPR =- East pacific Rise, MAR = Mid-Atlantic Ridge, CBR = Carlsberg
Ridge. Plates: EA = Eurasian, IN = Indian,
PA = Pacific, NA = North American, SA = South American, AF = African, CO = Cocos.
From Li and Romanowicz (1996). J. Geophys. Research, 101, 22,245-72.
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37. TheThe
GeothermalGeothermal
GradientGradient
Thermal structure in a 3D spherical mantle
convection model (red is hot, blue is cold).
J. H. Davies and H.-Peter Bunge
http://www.ocean.cf.ac.uk/people/huw/AG
U99/mantlecirc.html
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38. Dynamic cooling of the earthDynamic cooling of the earth
What happens at other deeper boundary?
The density more than doubles from lower mantle to the core,
thus preventing convection across the boundary, regardless of
thermal differences.
The liquid outer core must be internally convecting to produce
Earth’s magnetic field.
The D” layer may be the resulting mantle side thermal boundary
layer, across which the temperature gradient is estimated to be
300 to 10000
C.
Convection within the mantle results from thermal instabilities as the
heated by D” layer becomes positively buoyant.
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39. Dynamic cooling of the earthDynamic cooling of the earth
The viscosity is much lower than that of the plates at such elevated
temperatures, and the instabilities take the alternative form of rising
cylindrical plumes rather than slabs.
These plumes are more of an active upwelling, in comparison to the more
passive shallow mantle rise in response to plate separation.
The plumes appear to be largely independent of plate tectonics,
but in many cases are sufficiently vigorous to penetrate the
lithosphere and reach the Earth’s surface, where they result in
hotspot activity.
Plate tectonics thus appears to be the method by which a largely internally
heated and viscous mantle cools by convection, whereas plumes essentially
cool the core and have an increasing effect on lower mantle.
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40. Dynamic cooling of the earthDynamic cooling of the earth
Even general model of mantle convection depends on the
influence of 660-km phase transition.
Some investigators consider the density contrast across the
boundary insufficient to impede convection, so there is whole
mantle convection.
Others believe the 660-km transition can impede convection,
so that warm, buoyant mantle material in rising portions of
deep convection cells would be less buoyant than the material
above transition, causing the rising material to spread laterally
instead.
If so, heat can transfer across the boundary only by
conduction, inducing convection in the upper mantle. 45

41. Figure 1.14. Schematic diagram of a 2-
layer dynamic mantle model in which the
660 km transition is a sufficient density
barrier to separate lower mantle convection
(arrows represent flow patterns) from upper
mantle flow, largely a response to plate
separation. The only significant things that
can penetrate this barrier are vigorous
rising hotspot plumes and subducted
lithosphere (which sink to become
incorporated in the D" layer where they
may be heated by the core and return as
plumes). Plumes in core represent relatively
vigorous convection (see Chapter 14). After
Silver et al. (1988).
Mantle dynamicsMantle dynamics
Is the 660 km transition a
barrier to whole-mantle
convection?
Maybe?
Partly?
No?
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42. Mantle dynamicsMantle dynamics
The 660-km boundary
layer in such models
allows some material
transfer but impedes
wholesale mantle
convection from
homogenizing the full
vertical extent of the
mantle and thus permits
the composition of the
shallow and deep mantle
to evolve independently.
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43. Plate Tectonic - Igneous GenesisPlate Tectonic - Igneous Genesis
1. Mid-Ocean Ridges
2. Intracontinental Rifts
3. Island Arcs
4. Active Continental
Margins
5. Back-Arc Basins
6. Ocean Island Basalts
7. Miscellaneous Intra-
Continental Activity
kimberlites, carbonatites,
anorthosites...
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