Magma Types Based on Chemistry
50% SiO2
gabbro/basalt
60% SiO2
diorite/andesite
70% SiO2
granite/rhyolite
Processes That Change Magma Compositions
Primary Process
Partial Melting of Different Materials (e.g. mantle, oceanic
crust, continental crust, etc.)
Secondary Processes
- “Magmatic Differentiation”
•
Fractional Crystallization
•
Assimilation/Magma Mixing
•
Combined Process (AFC)
Partial Melting of Different Starting Materials
Peridotite (mantle rock) Basaltic Magma
Basaltic (oceanic crust) Andesitic Magma
Andesitic (continental crust) Rhyolitic Magma
Mantle Melting and the Origin of Basaltic Magma
Primary Basaltic Magmas
• Mantle melts with no subsequent modification by secondary
processes (fractional crystallization, assimilation, mixing).
• Criteria:
• MgO = 11 – 17 wt%
• Cr > 1000 ppm
• Ni > 400-500 ppm
• Rarely observed, “Holy Grail” of basalts, very important for
petrologists and geochemists, fingerprint of mantle source!
Processes That Change Magma Compositions
Primary Process
Partial Melting of Different Materials (e.g. mantle, oceanic
crust, continental crust, etc.)
Secondary Processes
- “Magmatic Differentiation”
•
Fractional Crystallization
•
Assimilation/Magma Mixing
•
Combined Process (AFC = Assimilation + Fractional Crystallization)
Fractional Crystallization
magma composition progressively changes as crystals are
physically “removed” from the magma.
Bowen’s Reaction Series
• Theoretical order in which common igneous crystallize,
and generally speaking correct!
• Used in early 1900’s (by N.L. Bowen) to explain the origin of
granite from primary basaltic magmas. True only in rare cases.
Fractional Crystallization
Remaining Magma
is Andesitic
Basaltic Magma
Palisades Sill – (western shore of Hudson River, NJ)
Palisades Sill – (western shore of Hudson River, NJ)
Sandstone
Diorite
Gabbro (or
“diabase”)
Olivine
gabbro
Basaltic intrusion
245–275 m (800–900 ft)
Basalt
Sodium-rich
plagioclase feldspar;
Fe-rich pyrox, hbl
Calcium-rich
plagioclase feldspar
and pyroxene;
Olivine, plag, pyrox
Basalt
Sandstone
Basalt cooled quickly at the
edges of the intrusion.
Palisades
intrusion
~1200°C
Olivine
crystals
Magma with
composition A
Olivine
crystallizes
first.
Palisades
intrusion
~1200°C
~1100°C
Plagioclase
feldspar
Olivine
crystals
Pyroxene
Olivine
Magma with
composition A
Olivine
crystallizes
first.
Magma with
composition B
Pyroxene and
plagioclase
feldspar
crystallize.
Palisades
intrusion
~1200°C
~1100°C
Plagioclase
feldspar
Olivine
crystals
~1050°C
Plagioclase
feldspar
Pyroxene
Pyroxene
Olivine
Magma with
composition A
Olivine
crystallizes
first.
Magma with
composition B
Pyroxene and
plagioclase
feldspar
crystallize.
Olivine
Magma with
composition C
Pyroxene and
plagioclase
gradually
change in
composition.
Palisades
intrusion
~1200°C
~1100°C
Plagioclase
feldspar
Olivine
crystals
Pyroxene
Pyroxene
Olivine
Magma with
composition A
Olivine
crystallizes
first.
Magma with
composition B
Pyroxene and
plagioclase
feldspar
crystallize.
~1000°C
~1050°C
Plagioclase
feldspar
Olivine
Magma with
composition C
Pyroxene and
plagioclase
gradually
change in
composition.
Plagioclase
feldspar
Pyx+Plag
Olivine
Magma with
composition D
Plagioclase
feldspar
continues to
crystallize.
Processes That Change Magma Compositions
Primary Process
Partial Melting of Different Materials (e.g. mantle, oceanic
crust, continental crust, etc.)
Secondary Processes
- “Magmatic Differentiation”
•
Fractional Crystallization
•
Assimilation/Magma Mixing
•
Combined Process (AFC = Assimilation + Fractional Crystallization)
Assimilation
Magma Mixing
Basaltic
magma
Rhyolitic
magma
Continental Crust
Basaltic
Blocks of continental
crust fall into basaltic
magma and dissolve.
+
Rhyolitic
basaltic magma +
assimilated blocks =
andesitic magma
HOT (1200°C)
basaltic magma
Heat transfer from
hot, basaltic magma
melts wall rock
Andesitic
magma
Assimilation + Fractional Crystallization (AFC)
Continental Crust
Blocks of continental
crust fall into basaltic
magma and dissolve.
basaltic magma +
assimilated blocks =
andesitic magma
HOT (1200°C)
basaltic magma
Heat transfer from
hot, basaltic magma
melts wall rock
magma composition progressively
changes as crystal are physically
“removed” from the magma
Harker
Diagrams
Trends usually indicate
that rocks are “related”
by some petrogenetic
process (ie. crystal
fractionation, mixing,
or AFC)
= Parental magma
= differentiation trend
Data for Crater
Lake Volcanics, Cascade
Range, Oregon
basalt andesite rhyolite
basalt andesite rhyolite
AFM
Diagram
basalt andesite rhyolite
andesite
basalt
rhyolite
= Parental magma
= differentiation trend
Data for Crater
Lake Volcanics, Cascade
Range, Oregon
basalt andesite rhyolite
Processes That Change Magma Compositions
Primary Process
Partial Melting of Different Materials (e.g. mantle, oceanic
crust, continental crust, etc.)
Secondary Processes
- “Magmatic Differentiation”
•
Fractional Crystallization
•
Assimilation/Magma Mixing
•
Combined Process (AFC = Assimilation + Fractional Crystallization)
Origin of Basaltic Magmas
1. Decompression melting of
mantle (peridotite) to produce
primary basaltic magmas
2. Fractional crystallization in
shallow crustal magma chambers
Origin of Andesitic Magmas
Modification of a Basaltic “Parent”
1. Crystallization in Magma Chambers
2. Assimilation of Continental Crust
3. Magma Mixing:
(basalt + rhyolite = andesite)
Origin of Granitic/Rhyolitic Magmas
Partial Melting of Continental Crust
(± AFC processes)
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