The Formation of Igneous Rocks

1
The Formation of Igneous Rocks

• Magma Genesis, Transport, and Modification

2
Igneous Rocks Review

• With your neighbors, discuss and answer
• What is fractionation? How does this process
  occur? Why is it important in understanding how
  igneous rocks form and evolve?
• What is Bowens Reaction Series (BRS)? Sketch as
  much as you can remember about BRS.
• What is the geothermal gradient? Why is it
  important in discussing the formation of igneous
  rocks?

3
Igneous Rocks Importance and Occurrence

• Importance
• Make up gt90 of earths crust
• Occur on all terrestrial planets
• Volcanic hazards
• Economic deposits (diamond, Cr, Ni, Co, Mo, Sn,
  W, Ti, Li)

• Occurrence
• Mantle (ultramafic, plutonic)
• Oceanic crust (SIMA - mafic)
• Continental crust (SIAL - intermediate to felsic)
• Specific rock types closely tied to tectonic
  setting

Mt. Vesuvius, Naples
4
Introduction to Igneous Rocks

• Understanding igneous rock petrography requires
• Knowledge of characteristic igneous rock textures
  and structures
• Knowledge of characteristic compositions (mineral
  assemblages)
• Understanding igneous rock petrogenesis requires
• Understanding the origin of magma
• Understanding magmatic evolution and cooling
  history
• Understanding magma transport, storage, and
  eruption mechanisms
• Understanding relationships between igneous rock
  formation and tectonic settings

5
Igneous Rock Textures

• Texture is largely determined by rate of cooling
• Intrusive (plutonic)
• Crystallize slowly below the earths surface
• Holocrystalline
• Phaneritic
• Extrusive (volcanic)
• Crystallize more quickly at the earths surface
• Porphyritic to aphanitic
• Glassy
• Vesicular
• Pyroclastic

6
Igneous Rock Composition

• Natural range of igneous rock compositions
• Reflected in mineral associations (assemblages)

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Igneous Rock Compositions

• As a magma cools, all minerals have a
  characteristic crystallization (melting)
  temperature and crystallize from either
• All liquid or
• Some liquid some crystals
• Crystals interact with the liquid and change
  composition as cooling takes place
• Crystallization temps depend on magma composition

• Bowens Reaction Series
• Predicts the order in which minerals crystallize
  from a cooling magma
• Idealized model for equilibrium crystallization
  in a magmatic system
• Discontinuous mineral crystallization series
• Continuous mineral crystallization series
• Composition of natural magmas determines the
  extent to which crystallization follows BRS

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Composition BRS
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Igneous Rock Classification

• Based on
• Texture
• Phaneritic
• Aphanitic/ porphyritic
• Vesicular
• Glassy
• Composition (essential minerals)
• Ultramafic
• Mafic
• Intermediate
• Felsic (silicic)

10
Petrogenesis The Formation of Igneous Rocks

• Magma genesis
• Source rock
• Mechanism to cause melting and create magma
• Mechanism to transport magma from source to
  crystallization site
• Brittle dikes and fractures
• Ductile diapirs
• Magma cooling and crystallization to form rock
• Compositional modification (fractional and
  equilibrium crystallization, assimilation,
  mixing)
• Combination of these processes produces the
  diversity of igneous rocks on earth

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The Formation of Igneous Rocks

• Closely tied to tectonic setting and processes
• Divergent boundaries ( hot spots) primitive
  magma
• Convergent boundaries recycled magma

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What is Magma?

• Magma
• Liquid (molten rock) crystals dissolved
  gasses (volatiles)

Also varies with temperature and water content
13
Magma Fractionation

• Separation of two fractions in a source
  material through
• Partial melting and/or
• Partial crystallization

• Two fractions are
• Different in composition
• Different from the original material
• Separate through gravity settling/upward movement
  of melt

14
Origin of Magma Partial Melting

• Bowens Reaction Series
• Idealized model for equilibrium melting or
  crystallization

• Minerals have different melting/ crystallization
  temps
• Minerals lower on BRS melt first
• Hydrous minerals melt first
• Melt will generally be LESS mafic than starting
  rock
• Solid residue will generally be MORE mafic than
  starting rock

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The Origin of Magma

• Magma genesis requires
• Source rock
• Mantle (ultramafic)
• Oceanic crust (mafic)
• Continental crust (intermediate to felsic)
• Unlikely that any preexisting rock will melt 100
  to make magma
• Magma sources
• Primitive magma mantle rock
• Recycled magma crustal mantle rock

16
The Origin of Magma

• Magma genesis
• The mantle and crust are SOLID
• Seismic evidence
• Xenolith evidence
• Experimental evidence
• Requires mechanism to cause melting (anatexis)
• Increase heat
• Lower rock melting temperature

• Average crustal geothermal gradient 20ºC/km

17
The Origin of Magma

• Raising the geothermal gradient (increase heat)
• Frictional heat
• Caused by faulting
• Localized heating and melting

• Decompression melting
• Caused by convection
• Hot, deep material rises faster than heat is lost
  to surroundings
• Raises local geothermal gradient
• Process responsible for generating primitive
  magma at DIVERGENT MARGINS and HOT SPOTS

18
The Origin of Magma

• Lowering the solidus
• Flux melting addition of component(s) that
  lower rock melting temperature

• Caused by adding volatiles (H2O, CO2)
• Sea water in pore spaces in rock
• Water bound in hydrous minerals (clay,
  serpentine, mica, amphibole)
• Water is released as rocks reach higher temps
  pressures, melts surrounding rocks
• Process responsible for generating recycled magma
  at CONVERGENT MARGINS

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The Origin of Magma

• Adding hot material (raise geothermal gradient)
• Crustal anatexis melting of continental crust
• Basaltic magmas generated in mantle rise into the
  crust
• Lower density can prevent magma from rising to
  surface
• Hot magma heats and partially melts surrounding
  crustal rocks
• Process responsible for generating felsic magmas
  at CONVERGENT MARGINS and CONTINENTAL RIFTS

20
Magma Transport

• Partial melting of source rock (via
  decompression, flux melting, or crustal anatexis)
  produces
• Magma
• Crystal residue
• Magma migrates upward due to density contrast
• Magma less dense than surrounding rocks
• Two major mechanisms of movement
• Brittle fractures and dikes
• Ductile diapirism

Basalt dike
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Magma Transport

• Fracturing opens spaces that magma can invade
• Fracturing caused by
• Tensional stress (rock pulled apart by tectonic
  forces)
• Expansion of magma and upward buoyancy
• Process most common in mafic magmas (basalt)
• Process common in brittle crust (upper crust
  extension)

• Allows rapid transport of magma (scale of days to
  years)

Dike and volcanic neck, Ship Rock, NM
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Magma Transport

• Diapir elliptical to tear-shaped mass that
  rises toward the surface
• Upward migration due to density contrast
• Magma less dense than surrounding rocks
• Process most common in intermediate to felsic
  magmas, particularly granite bodies
• Process common in mantle and ductile lower crust

• Slower transport, scale of years to many
  thousands of years

23
Magma Modification and Evolution

• After origin and transport, magmas are commonly
  modified before/during crystallization to form
  rocks
• DIFFERENTIATION processes that modify
  composition of the magma
• Magma mixing
• Assimilation
• Crystal Fractionation
• CRYSTALLIZATION solidification of magma to form
  rock
• Equilibrium crystallization (Bowens Reaction
  Series)
• Fractional crystallization

24
Magma Modification and Evolution

• Magma Mixing (or mingling)
• Two magmas of different compositions blend
  together to form a single magma
• New magma has a composition partway between the
  two original magmas
• Incomplete mixing mingling

• Assimilation
• Magma can melt, react with, and/or dissolve
  surrounding rocks
• Difficult to fully assimilate melted rock due to
  density and viscosity contrasts

25
Magma Modification and Evolution

• Crystallization (equilibrium or fractional)
  changes the composition of the liquid
• Elements are preferentially partitioned into
  certain minerals

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Magma Modification and Evolution

• Equilibrium crystallization

• Crystals that form remain in direct contact with
  melt
• Crystals and melt continually equilibrate
• Composition of the system is constrained by the
  bulk composition of the original melt
• Example equilibrium crystallization of
  plagioclase feldspar

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Magma Modification and Evolution

• Bowens Reaction Series
• idealized model for equilibrium
  crystallization (and melting)

• Crystallizing minerals are in equilibrium with
  the melt
• Melt changes composition as crystals form and
  melt cools
• Earlier formed crystals will no longer be in
  equilibrium with the melt, and will be dissolved
  to form new minerals
• Process gives rise to many (BUT NOT ALL) diverse
  igneous rocks

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Magma Modification and Evolution

• Natural examples of BRS reactions
• Early forming crystals are generally euhedral
• Early forming crystals may be surrounded by later
  forming crystals
• Early forming crystals may be resorbed,
  suggesting a reaction with the melt to form later
  crystals (olivine, left photo)
• Minerals that undergo solid solution may be zoned
  (plagioclase, right photo)

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Magma Modification and Evolution

• Fractional crystallization

• Crystals that form are immediately removed from
  the melt
• Floating or sinking of crystals
• Filter pressing
• Flow segregation
• Armoring of crystals
• Composition of the system is NOT constrained by
  the original bulk composition
• Example fractional crystallization of
  plagioclase feldspar

30
Magma Modification and Evolution

• Natural examples of fractional crystallization
  processes

• Cumulate textures, suggesting crystal removal by
  settling (layered mafic intrusions)

• High-temp crystals may have a rim of low-temp
  crystal

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Igneous Rock Formation

• Few (perhaps no) igneous rocks are simple
  crystallized melts of a parent rock
• Igneous rocks represent a complex interplay of
• Source rock type and conditions of melting
• Fractionation of magma from its source
• Modification by contamination (mixing,
  assimilation)
• Fractionation as magma cools and crystallizes
  (likely some each of equilibrium and fractional
  crystallization)
• Conditions of cooling (volcanic vs. plutonic)

32
Collaborative Activity

• Answer the following and turn in (1 per group)
• Use the plagioclase feldspar binary T-X diagram
  to determine the equilibrium crystallization of a
  melt with starting bulk composition of 40 An,
  and answer the questions on the worksheet.
• Use the plagioclase feldspar binary T-X diagram
  to determine the fractional crystallization of a
  melt with starting bulk composition of 60 An,
  and answer the questions on the worksheet.
• How might you distinguish a rock formed by the
  process in question 1 from a rock formed by the
  process in question 2 (what physical and/or
  chemical characteristics would you look for)?
  Answer on back of worksheet
• Begin Homework Part 1 (modeling of fractional
  crystallization in a MM magma chamber).