EXTENSION MODULE 4.1
Bowen’s Reaction Series
This module illustrates the reactions observed to occur between minerals and magma, and the order of these reactions
during magma crystallization that was described by Norman Bowen in the 1920s.
A. The Reaction Principle
Olivine
Order of crystallization
Amphibole
Na-rich
plagioclase
feldspar
Biotite
Potassium feldspar
Muscovite
Quartz
Late
▲ Figure EM4.1-1 A diagram of the reaction series.
Bowen used this diagram in 1928 to illustrate two different mineral series that result from reactions
between early crystallized minerals and magma. Arrows show the chemical reactions that produce new
minerals (discontinuous series) and gradual changing composition within the plagioclase feldspars (continuous series) during crystallization of a silicate magma.
feldspars. Relatively calcium-rich (Ca) plagioclase
feldspar crystallizes first in a cooling magma. As temperature decreases during crystallization, plagioclase
feldspar reacts with the remaining liquid to become
progressively more sodium rich (Na), as illustrated
schematically in Figure EM4.1-2. The sodium-rich
Initial crystal is
calcium-rich
Calcium-rich
Plagioclase
crystal
C. Understanding Each Series
Magma
Crystal becomes
more sodium-rich
as it grows
The two sides of the reaction series diagram (EM4.11) depict two different types of reactions between
minerals and melt. Bowen named these reactions the
continuous and discontinuous series.
The continuous series involves only the plagioclase
EM4.1-1
Smith/Pun How Does Earth Work? Second Edition, Pearson Prentice Hall © 2010
Sodium-rich
Figure EM4.1-2 Visualizing
the continuous series.
The first plagioclase feldspar crystals to form in a cooling magma
are relatively rich in calcium. As
the magma cools, the feldspar
crystals react with the melt and
become more sodium rich as they
continue to grow. If crystallization
takes a sufficiently long time,
then the reactions will homogenize the crystal into a single composition. Otherwise, the center of
each crystal remains more calcium rich while the edges are more
sodium rich.
▲
Why should a mineral, once it crystallizes from magma,
then react with the magma? The key thing to remember is that the crystallizing minerals do not have the
exact same composition as the magma. For example,
quartz consists entirely of silica, but no magma is 100
percent silica. Another example is olivine, which commonly crystallizes first in cooling mafic magma. Olivine
contains less silica and more magnesium than mafic
magma. As more and more olivine crystals form, the
remaining liquid contains progressively more silica and
less magnesium than before the olivine crystallized.
Eventually, the magma is sufficiently silica rich and
magnesium poor that the olivine crystals are no longer
stable and these crystals react with the magma to form
pyroxene, which contains less magnesium and more silica than the original olivine.
Discontinuous series
Ca-rich
plagioclase
feldspar
Composition of plagioclase feldspar
B. Why Reactions Happen
Continuous series
Pyroxene
Decreasing temperature
During crystallization of silicate magmas, reactions take
place between crystallized minerals and the remaining
molten liquid. These reactions change the compositions
of the minerals or consume one mineral while producing a new mineral. As a result of these reactions, minerals appear and disappear in a specific order as crystallization and reactions progress. This reaction principle,
which explains the minerals found in igneous rocks, is
based on both laboratory experiments and evidence of
mineral reactions preserved in real rocks. Norman L.
Bowen presented the principle in 1928, and his research
was supported by nearly two decades of research at the
Carnegie Institution of Washington.
Figure EM4.1-1 diagrammatically illustrates the
mineral reactions. In continuous reactions, the mineral composition changes gradually as crystallization
occurs. In discontinuous reactions, one mineral reacts
with the liquid to produce a new mineral at the
expense of the first one.
Early
Olivine crystal
Magma
Olivine reacts with magma
to form pyroxene
Olivine
Figure EM4.1-3 Visualizing
the discontinuous series.
Olivine crystallizes early in mafic
magma. As crystallization proceeds
with decreasing temperature, the
olivine reacts with the magma to
form calcium-poor pyroxene. The
temperature does not change until
the reaction is completed. After the
reaction consumes all of the
olivine, the temperature continues
to decrease and pyroxene continues to crystallize.
▲
Olivine crystallizes
from cooling magma
Decreasing temperature
feldspar also contains more silica and less aluminum
than the calcium-rich feldspar. The descriptive term
“continuous” emphasizes that the reactions are ongoing and modify the composition within single crystals
of plagioclase feldspar.
The discontinuous series describes reactions
between the common Fe-Mg silicate minerals—
olivine, pyroxene, amphibole, and biotite. After crystallization of each mineral, reaction with the remaining liquid causes that mineral to dissolve and be
replaced by the next mineral shown beneath it on the
diagram (EM4.1-1). Figure EM4.1-3 schematically
illustrates how olivine and magma react to form
pyroxene. The term “discontinuous” applies to reactions in which one mineral is consumed and a new
mineral formed, unlike the continuous series where
compositional change happens continuously within a
single mineral group.
Pyroxene
Pyroxene crystal
D. Illustrations in Real Rocks
Bowen realized that reactions like those produced in
laboratory experiments were observed in many volcanic rocks. Rapid cooling of lava flows and nearsurface magma intrusions can prevent the reactions
from being completed. Therefore, Ca-rich plagioclase can be surrounded by more Na-rich plagioclase
(continuous series) and Fe-Mg silicates that formed
early can be surrounded by those that formed by
later reaction (discontinuous series). Figure EM4.14 illustrates these reactions in real rocks.
Plagioclase crystal has a calciumrich center surrounded by more
sodium-rich zones.
This plagioclase feldspar in this
andesite has distinct zones.
Analyses show that the interior of
the crystal is calcium rich and the
outer zones are sodium rich. these
zones reflect the reactions of the
continuous series.
E. Relationship of the Reaction
Series to Rock Composition
Where crystallization begins and ends on Bowen’s
diagram depends on the initial composition of the
magma and whether or not fractional crystallization
occurs (see Section 4.7 of text). A mafic magma may
begin with crystallization of olivine and Ca-plagioclase and be completely crystallized by the time pyroxene and slightly more sodium-rich plagioclase form.
Quartz, potassium feldspar, and muscovite typically crystallize if the magma has not completely solidified after formation of sodium-rich feldspar and
biotite (see Figure EM4.1-1). These minerals do not
form as a result of chemical reactions but simply crystallize from the remaining magma, which typically is
very rich in silica, potassium, and aluminum. These
three minerals are not, therefore, part of either the
continuous or discontinuous series.
2 mm
The olivine crystals in this gabbro
have rims of pyroxene,
documenting the reaction of the
olivine with the magma to produce
pyroxene at the time crystallization
of the rock ended.
Olivine reacted with
magma to form a rim
of pyroxene.
▲
Figure EM4.1-4 Reactions observed in rocks.
These microscope photographs show evidence for reactions in
igneous rocks. (Source: Aurora Pun)
EM4.1-2
Smith/Pun How Does Earth Work? Second Edition, Pearson Prentice Hall © 2010
Early
Putting It Together
■
■
■
Continuous series
Discontinuous series
Ca-rich
plagioclase
feldspar
Olivine
Bowen’s reaction series diagrammatically illustrates the minerals that commonly occur together in rocks in accordance with rock and laboratory studies that show early minerals to react
with magmas during crystallization.
In the continuous series, reactions produce
gradual changes in the composition of plagioclase feldspar during magma crystallization.
In the discontinuous series, an earlier formed
magnesium-rich (and calcium-poor) silicate
mineral is consumed by a reaction that produces
a different magnesium-rich silicate.
Rock composition
Mafic
Intermediate
plagioclase
feldspar
Felsic
Biotite
Late
Potassium feldspar
Muscovite
Quartz
EM4.1-3
Smith/Pun How Does Earth Work? Second Edition, Pearson Prentice Hall © 2010
Figure EM4.1-5 Relating Bowen’s reaction series to rock
composition.
The broad horizontal bands on the diagram show the ranges of
approximate boundaries between the mineral compositions of
mafic, intermediate, and felsic rocks. The types of minerals found in
each rock type, also illustrated in text Figure 4.3, result from mineral
reactions. Olivine, typical of mafic rocks, does not occur with quartz,
typical of felsic rocks, because these two minerals form at different
stages in magma crystallization.
▲
Figure EM4.1-5 illustrates how the reaction series
explains why mineral associations are different for
each compositional group. Minerals widely separated
on the diagram (e.g., Mg-rich olivine and quartz) are
not expected to be found together because they do not
form at the same stage of magma crystallization or in
magmas of comparable composition. Typically, only
adjacent minerals on the diagram are found together.
Bowen used an analogy to a childhood game that was
popular in his time to explain the mineral associations:
“Suffice it to say here that those minerals that belong to
the same general period of crystallization tend to be
associated and those belonging to remote periods ordinarily fail to associate. The controlling factors are thus
analogous to those which determine that little girls ordinarily play ‘London Bridge’ with other little girls, occasionally with their mothers, seldom with their grandmothers and never with their great grandmothers.”