CHAPTER 3 Igneous Rocks and Intrusive Activity
78
Bowen’ s Reaction Series and the Composition
of Igneous Rocks
Recall that ice freezes at a single temperature, whereas basaltic magma crystallizes over
a range of at least 200 °C of cooling. In a laboratory setting Bowen and his coworkers
demonstrated that as a basaltic magma cools, minerals tend to crystallize in a systematic
fashion based on their melting points. As shown in FIGURE 3.20, the first mineral to crystallize is the ferromagnesian mineral olivine. Further cooling generates calcium-rich
plagioclase feldspar as well as pyroxene, and so forth down the diagram.
During the crystallization process, the composition of the remaining liquid portion of
the magma also continually changes. For example, at the stage when about a third of the
magma has solidified, the melt will be nearly depleted of iron, magnesium, and calcium,
because these elements are major constituents of the earliest-formed minerals. The removal
of these elements causes the melt to become enriched in sodium and potassium. Further,
because the original basaltic magma contained about 50 percent silica (SiO 2 ), the crystallization of the earliest-formed mineral olivine, which is only about 40 percent silica, leaves
the remaining melt richer in SiO 2 . Thus, the silica component of the melt becomes enriched
as the magma evolves.
Bowen also demonstrated that if the solid components of a magma remain in contact
with the remaining melt, they will chemically react and change mineralogy as shown in
Figure 3.20. For this reason, this arrangement of minerals became known as Bowen’s
reaction series. As you will see, in nature the earliest-formed minerals can be separated
from the melt, thus halting any further chemical reaction.
The diagram of Bowen’ s reaction series in Figure 3.20 depicts the sequence in which
minerals crystallize from a magma of basaltic composition under laboratory conditions.
Evidence that this highly idealized crystallization model approximates what can happen in
nature comes from the analysis of igneous rocks. In particular, we find that minerals that
form in the same general temperature regime depicted on Bowen’ s reaction series are found
together in the same igneous rocks. For example, notice in Figure 3.20 that the minerals
quartz, potassium feldspar, and muscovite, which are located in the same region of Bowen’ s
diagram, are typically found together as major constituents of the plutonic igneous rock
granite.
MAGMATIC DIFFERENTIATION. Bowen demonstrated that minerals
crystallize from magma in
a systematic fashion. But
how do Bowen’ s findings
account for the great
diversity of igneous rocks?
It has been shown that, at
one or more stages during
the crystallization of
magma, a separation of
various components can
occur. One mechanism
that allows this to happen
is called crystal settling.
This process occurs when
the earlier-formed minerals
are more dense (heavier)
than the liquid portion
and sink toward the bottom of the magma chamber, as shown in FIGURE
3.21. (Crystallization can
also occur along the cool margins of a
magma body.) When the remaining melt
solidifies—either in place or in another
location if it migrates into fractures in the
surrounding rocks—it will form a rock
with a mineralogy much different from the
parent magma (Figure 3.21). The
formation of one or more secondary
magmas from a single parent magma is
called magmatic differentiation.
One consequence of magnetic differentiation of basaltic (mafic) magma is that it
Temperature
Regimes
Composition
(rock types)
High temperature
(~1200°C)
Low temperature
(~750°C)
Olivine
Pyroxene
Amphibole
Biotite mica
D is c o n t in
u
o
u
s
S
e
r
ie
s
o
f
C
r
y
s
t
a
ll
iz
a
t
io
n
Ultramafic
(peridotite/
komatiite)
Calciumrich
P la g io c la s e f e ld s p a r
C o n t in u o u s S e r ie s
o f C r y s t a ll iz a t io n
Potassium feldspar
Muscovite mica
Quartz
Sodiumrich
+
+
Mafic
(gabbro/basalt)
Intermediate
(diorite/andesite)
Felsic
(granite/rhyolite)
Cooling magma
Bowen's Reaction Series
FIGURE 3.20 Bowen’s reaction series shows the sequence in which minerals crystallize from a magma. Compare this
figure to the mineral composition of the rock groups in Figure 3.10. Note that each rock group consists of minerals that
crystallize in the same temperature range.
D I D Y O U K N O W ?
Quartz watches actually contain quartz
crystals that allow them to keep time.
Before quartz watches, timepieces used
some sort of oscillating mass or tuning
fork. Cogs and wheels converted this
mechanical movement to the
movement of the hands. It turns out
that if voltage is applied to a quartz
crystal, it will oscillate with a
consistency that is hundreds of times
better for timing than a tuning fork.
Because of this property, and modern
integrated-circuit technology, quartz
watches are now built so cheaply that
they are sometimes given away in
cereal boxes. Modern watches that
employ mechanical movements are very
expensive indeed.
78
Bowen’ s Reaction Series and the Composition
of Igneous Rocks
Recall that ice freezes at a single temperature, whereas basaltic magma crystallizes over
a range of at least 200 °C of cooling. In a laboratory setting Bowen and his coworkers
demonstrated that as a basaltic magma cools, minerals tend to crystallize in a systematic
fashion based on their melting points. As shown in FIGURE 3.20, the first mineral to crystallize is the ferromagnesian mineral olivine. Further cooling generates calcium-rich
plagioclase feldspar as well as pyroxene, and so forth down the diagram.
During the crystallization process, the composition of the remaining liquid portion of
the magma also continually changes. For example, at the stage when about a third of the
magma has solidified, the melt will be nearly depleted of iron, magnesium, and calcium,
because these elements are major constituents of the earliest-formed minerals. The removal
of these elements causes the melt to become enriched in sodium and potassium. Further,
because the original basaltic magma contained about 50 percent silica (SiO 2 ), the crystallization of the earliest-formed mineral olivine, which is only about 40 percent silica, leaves
the remaining melt richer in SiO 2 . Thus, the silica component of the melt becomes enriched
as the magma evolves.
Bowen also demonstrated that if the solid components of a magma remain in contact
with the remaining melt, they will chemically react and change mineralogy as shown in
Figure 3.20. For this reason, this arrangement of minerals became known as Bowen’s
reaction series. As you will see, in nature the earliest-formed minerals can be separated
from the melt, thus halting any further chemical reaction.
The diagram of Bowen’ s reaction series in Figure 3.20 depicts the sequence in which
minerals crystallize from a magma of basaltic composition under laboratory conditions.
Evidence that this highly idealized crystallization model approximates what can happen in
nature comes from the analysis of igneous rocks. In particular, we find that minerals that
form in the same general temperature regime depicted on Bowen’ s reaction series are found
together in the same igneous rocks. For example, notice in Figure 3.20 that the minerals
quartz, potassium feldspar, and muscovite, which are located in the same region of Bowen’ s
diagram, are typically found together as major constituents of the plutonic igneous rock
granite.
MAGMATIC DIFFERENTIATION. Bowen demonstrated that minerals
crystallize from magma in
a systematic fashion. But
how do Bowen’ s findings
account for the great
diversity of igneous rocks?
It has been shown that, at
one or more stages during
the crystallization of
magma, a separation of
various components can
occur. One mechanism
that allows this to happen
is called crystal settling.
This process occurs when
the earlier-formed minerals
are more dense (heavier)
than the liquid portion
and sink toward the bottom of the magma chamber, as shown in FIGURE
3.21. (Crystallization can
also occur along the cool margins of a
magma body.) When the remaining melt
solidifies—either in place or in another
location if it migrates into fractures in the
surrounding rocks—it will form a rock
with a mineralogy much different from the
parent magma (Figure 3.21). The
formation of one or more secondary
magmas from a single parent magma is
called magmatic differentiation.
One consequence of magnetic differentiation of basaltic (mafic) magma is that it
Temperature
Regimes
Composition
(rock types)
High temperature
(~1200°C)
Low temperature
(~750°C)
Olivine
Pyroxene
Amphibole
Biotite mica
D is c o n t in
u
o
u
s
S
e
r
ie
s
o
f
C
r
y
s
t
a
ll
iz
a
t
io
n
Ultramafic
(peridotite/
komatiite)
Calciumrich
P la g io c la s e f e ld s p a r
C o n t in u o u s S e r ie s
o f C r y s t a ll iz a t io n
Potassium feldspar
Muscovite mica
Quartz
Sodiumrich
+
+
Mafic
(gabbro/basalt)
Intermediate
(diorite/andesite)
Felsic
(granite/rhyolite)
Cooling magma
Bowen's Reaction Series
FIGURE 3.20 Bowen’s reaction series shows the sequence in which minerals crystallize from a magma. Compare this
figure to the mineral composition of the rock groups in Figure 3.10. Note that each rock group consists of minerals that
crystallize in the same temperature range.
D I D Y O U K N O W ?
Quartz watches actually contain quartz
crystals that allow them to keep time.
Before quartz watches, timepieces used
some sort of oscillating mass or tuning
fork. Cogs and wheels converted this
mechanical movement to the
movement of the hands. It turns out
that if voltage is applied to a quartz
crystal, it will oscillate with a
consistency that is hundreds of times
better for timing than a tuning fork.
Because of this property, and modern
integrated-circuit technology, quartz
watches are now built so cheaply that
they are sometimes given away in
cereal boxes. Modern watches that
employ mechanical movements are very
expensive indeed.