Explaining Bowen’s Reaction Series
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Bowen’s reaction series shows us the pattern in which certain minerals form as an
igneous melt cools.
Consists of the discontinuous series and the continuous series at higher
temperatures, but once the temperature lowers to a certain point the two branches
merge. As the melt continues to cool, potassium (K) feldspar, muscovite, and quartz
will crystalize in that order.
The Discontinuous Reaction Series
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Includes mafic (iron & magnesium) minerals - olivine, pyroxene, amphibole, and biotite.
If the chemistry of the melt is correct (if there is enough silica in the melt), each mineral
will DISCONTINUOUSLY (abruptly) change to the next mineral lower in the series as
the temperature drops.
As you go down Bowen’s Reaction Series, the minerals have an increased concentration
of silica in their composition.
EXAMPLE: In a basaltic/mafic melt, olivine will be the first mafic mineral to form
within the melt. When the temperature is low enough to form pyroxene, all of the olivine
will react with the melt to form pyroxene, and pyroxene will crystallize out of the melt.
As the melt continues to cool, pyroxene will react with the melt to form amphibole and
amphibole will crystallize. At the crystallization temperature of biotite, all of the
amphibole will react to form biotite and biotite will crystallize.
From the reasoning above, all igneous rocks should only have biotite, but we know that it
is not true.
o If olivine has crystalized in the melt and there is not enough silica to form
pyroxene, then the reaction will not occur and olivine will remain.
o If you are crystallizing olivine and the temperature drops too fast for the olivine to
react with the melt to form pyroxene (the magma is erupted onto the surface by a
volcano), the reaction will not have time to occur, the rock will solidify quickly,
and the olivine will remain in the rock.
The Continuous Reaction Series
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Shows the chemical change of plagioclase mineral as temperature decreases.
As you go down Bowen’s Reaction Series, the minerals have an increased concentration
of silica in their composition.
At the highest temperature, plagioclase is concentrated with 100% calcium (Ca).
At the lowest temperature, plagioclase is concentrated with 100% sodium (Na).
As the melt cools, the plagioclase changes CONTINUOUSLY (gradually) changes from
being highly concentrated in Ca to highly concentrated in Na.
EXAMPLE: In a basaltic/mafic magma, at the highest temperature, plagioclase
crystals with 100% Ca and 0% Na would form. As the temperature starts to drop,
plagioclase crystals react with the melt to form 99% Ca and 1% Na. Half way down
the series, plagioclase crystals would have a concentration of 50% Ca and 50% Na.
At the lowest temperature, plagioclase has 100% Na and 0% Ca.
Fractional Crystallization
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Sometimes when a melt is cooling, FRACTIONAL CRYSTALIZATION will occur. This
is the process that explains how multiple igneous rocks can be created from the same
magma melt.
EXAMPLE: The first minerals to crystalize in a basaltic/mafic melt is olivine and Ca
plagioclase. The olivine and plagioclase crystals are denser than the melt, so they sink to
the bottom, forming a layer. The melt will not be able to react with the olivine and Ca
plagioclase because they are isolated from the melt. As temperature continues to drop,
pyroxene and Ca-Na plagioclase will crystalize in the melt. They will also sink to the
bottom because they are denser than the melt. This leaves us with a layer pyroxene and
Ca-Na plagioclase on top of a layer of olivine and Ca plagioclase. Notice that by
crystalizing these minerals, we are removing Fe, Mg, and Ca from the melt. This leaves
the melt concentrated in Na, K, and Si. This process of new mineral crystallization and
sinking can continue down Bowen’s reaction series.