Dr. Helen Lang
Dept. of Geology & Geography
West Virginia University
SPRING 2015
GEOLOGY 285:
INTRO. PETROLOGY
Crystallization of Magmas
Igneous Rocks form by the
Crystallization of Magmas at or
beneath the Earth’s surface
Effects of cooling rate
• Magma cooled very suddenly freezes to
glass (basalt must cool more quickly than
rhyolite to form glass)
• Magma cooled pretty quickly has many
small crystals (microscopic or submicroscopic) - called aphanitic
• Magma cooled very slowly has big crystals
(visible to the naked eye) - called phaneritic
How might a porphyritic texture
develop?
Phenocrysts are nepheline (a feldspathoid)
What happens to a magma as it
cools?
• Pure liquids crystallize (freeze) at a single
temperature (melting T = freezing T)
– Water to ice at 0oC (32oF)
– SiO2 liquid to cristobalite at 1740oC
• Impure liquids (like magmas) crystallize
over a range of T, crystals are different
composition than liquid
– Different minerals form at different
temperatures as the magma cools
Sequence of mineral crystallization
and T at which different minerals
begin to crystallize depends on:
•
•
•
•
•
Magma composition
Pressure
Water pressure
Cooling rate
and many other factors
Bowen’s Reaction Series is a generalization
about “typical” Crystallization order
high T
olivine
Ca plagioclase
orthopyroxene
temperature
clinopyroxene
NaCa plagioclase
amphibole (Hb)
biotite
Na plagioclase
cooling
alkali feldspar
muscovite
low T
quartz
residual
phases
To really understand igneous crystallization, we
need to go beyond Bowen’s Reaction Series
• In the last 50+ years, geochemists have done
laboratory experiments on magma-like liquids
• One Method: Melt a rock, cool liquid to known T,
“freeze” it, view it under the microscope, identify
minerals - called “cook-and-look” method
• Better Method: Start with simple systems, e.g., liquid
with various proportions of pure Anorthite (Ca-plag,
CaAl2Si2O8) and pure Diopside, (pyroxene,
CaMgSi2O6), examine results at different Ts
Results are displayed on phase diagrams
• What is a phase? - a homogeneous portion of a
system that is physically distinct and mechanically
separable from the other parts of the system
(p = #phases)
• We also need to know about chemical composition
components = the minimum number of chemical
constituents necessary to describe the
compositions of all solids, liquids and gases
(phases) in a system(c = #components)
If the System of interest is:
• Quartz, Cristobalite, Tridymite, Stishovite,
Coesite and liquid SiO2
• All phases can be described with the single
chemical formula SiO2
• 1 chemical component, SiO2
• c=1
For more complex systems, finding the
components is a trial-and-error process
• for Olivines and corresponding liquids: Mg2SiO4(Ol
or liq.), Fe2SiO4(Ol or liq.), MgFeSiO4(Ol or liq.),
Mg1.5Fe0.5SiO4(Ol or liq.), etc.
– try elements:
Mg, Fe, Si, O
4
3
– try oxides:
MgO, FeO, SiO2
– try mineral end-members:
Mg2SiO4, Fe2SiO4
2*
*smallest number, therefore, correct # of components
Mg2SiO4
Fe2SiO4
For Olivines (Mg2SiO4 - Fe2SiO4) and
Orthopyroxenes (MgSiO3 - FeSiO3) and
corresponding liquids
• try mineral end-members:
Mg2SiO4, Fe2SiO4, MgSiO3, FeSiO3
• try oxides:
MgO, FeO, SiO2
• try elements:
Mg, Fe, Si, O
4
3*
4
*smallest number, therefore, correct # of components
The Phase Rule
• A generalization about phase diagrams
• by J. Willard Gibbs in the 1870s
• f = degrees of freedom, the number of
parameters that can be varied independently
without changing the phases present
• f=2+c-p
(f = #degrees of
freedom, c = #components, p = #phases)
• Examples will show what this means
Degrees of freedom or variance
•
•
•
•
f=0
f=1
f=2
f=3
invariant (point)
univariant (line)
divariant (field or area)
trivariant
One component system: SiO2
p=1
c=1
f=?
x
p=2
c=1
f=?
p=3
c=1
f=?
x
x
Real Magmas have many
components (8 or 9 major ones)
• We can’t draw diagrams that represent all
those components
• because that requires too many dimensions
• But we can learn a lot about crystallization
of real magmas (like basalts) by examining
several different 2-component systems!
2-component (Binary) Systems
• We’d need 3 dimensions to represent onephase fields (c=2, p=1, f=2+2-1=3; trivariant)
• But, we can only draw diagrams in 2
dimensions easily!
• Hold Pressure (P) constant, lose one degree of
freedom, only need 2 dimensions
• Phase rule (at constant P) becomes
fP = 1 + c - p
Most Common Minerals in Basalt are
Plagioclase and Clinopyroxene
Diabase (medium-grained “basalt”)
Simple 2-component system:
Diopside (CaMgSi2O6) - Anorthite (CaAl2Si2O8)
C=2
x
p=1
f=?
x
Eutectic
Di
p=2
f=?
p=3
f=?
An
Simplest representation of a basalt-like liquid (Cpx+Plag)
How do we use this diagram to tell
about crystallization of basalt?
Diopside-rich
liquid (X)
80% CaMgSi2O6 (Di)
20% CaAl2Si2O8 (An)
Draw isopleth=
line of constant
composition
What happens as
liquid cools?
X
How do we use this diagram to tell
about crystallization of basalt?
Y
Anorthite-rich
liquid (Y)
65% CaAl2Si2O8 (An)
35% CaMgSi2O6 (Di)
Draw isopleth=
line of constant
composition
What happens as
liquid cools?
Tie-lines and the Lever Rule
X
a
b
d
e
c
f
Animated Di-An Diagram
Courtesy of Professor Kenneth E. Windom
Department of Geological and Atmospheric Sciences
Iowa State University, Ames, Iowa
PowerPoint® presentation by
Kenneth E. Windom
1. A bulk composition of X cools to the liquidus, at which point An crystallizes.
2. Continued crystallization of An causes liquid composition to move toward Di.
3. When T=1274°C, liquid has moved to eutectic; Di begins to crystallize. No
change in temperature or composition of any of the 3 phases is permitted until
the liquid has completely crystallized.
4. At temperatures below 1274°C, only crystals of An and Di are present.
X
Textures of rocks in Di-An System
Plag and Pyroxene Phenocrysts
in fine-grained groundmass
Perfect Equilibrium Crystallization
• Crystals that have formed remain in contact
with the liquid and continually equilibrate
with it
• Constant bulk composition
• Makes little difference in Di-An system
• Contrasted with fractional crystallization,
where crystals are physically separated from
the liquid in which they were formed,
unable to react or equilibrate with liquid
More Complex 2-component system
Y
Z
p=3
f=?
At a Forsterite reacts with Liquid to form Enstatite
En melts incongruently
Olivine in Basalt reacted with Liquid to form Opx
Ol
Ol
Ol
Ol
Opx
Plagioclase, a 2-component system
with Complete Solid Solution
Crystallization of
Plagioclase in
Basalts (real
igneous plagioclase
is not pure An like
in the Di-An
system)
Animated Ab-An Diagram
Professor Ken Windom
Iowa State University
PowerPoint® presentation
by Kenneth E. Windom
Equilibrium Crystallization of the Plagioclase Feldspars
1. Liquid of composition X (An61) cools to the liquidus
2. Crystals of approximately An87 begin to form
3. Crystals have higher Ca/Na than liquid; ppt of crystals causes L
composition to become more sodic.
X
P=1 atm
4. Ratio of Ca/Na in both
crystals and liquid
decrease with decreasing
temperature; proportion
of crystals increases as
liquid decreases
5. Crystals of An61 cool
without further change in
composition
Composition
of last crystal
PowerPoint® presentation
by Kenneth E. Windom
Composition
of last liquid
Composition
of 1st crystal
Zoned Plagioclase
One component system: SiO2
x
x
x