Page 157 - Laboratory Manual in Physical Geology

Temperature Ca in Magma Rock
Regimes Bowen’s Reaction Series Feldspar Composition Types
High ~1400°C
temperature Olivine (green)
(first to > 70% Ultramafic Peridotite
crystallize) Komatiite
~1100°C
Pyroxene
(dark gray-
Continuous Series
green) 50–70% Mafic Gabbro
Basalt
~900°C Plagioclase feldspar
Amphibole
(dark gray 30–50% Intermediate Diorite
Discontinuous Series
to black)
Biotite Andesite
(black mica)
~800°C
Potassium feldspar (K-spar)
Low + < 30% Felsic Granite
temperature Muscovite (brown mica) Rhyolite
+
(last to
crystallize) ~500°C Quartz
No magma
remaining

FIGURE 5.6 Bowen’s Reaction Series —A laboratory-based conceptual model of one way that different kinds of igneous rocks can

differentiate from a single, homogeneous body of magma as it cools. See text for discussion.
cools to about 1100° C, then the olivine starts to react with This phenomenon is known as partial melting . When minerals
it and dissolve as pyroxene (next mineral in the series) starts of Bowen’s Reaction Series are heated, they melt at diff erent
to crystallize. More cooling of the magma causes pyroxene temperatures. The plagioclase feldspars melt continuously from

to react with the magma as amphibole (next mineral in the about 1100–1500° C, but the ferromagnesian minerals, quartz,
series) starts to crystallize, and so on. If the magma cools too and K-feldspar melt discontinuously. K-feldspar melts at about
quickly, then rock can form while one reaction is in progress 1250° C, pyroxene at 1400° C, quartz at 1650° C, and olivine
and before any remaining reactions even have time to start. at 1800° C. Because feldspars tend to melt at lower temperatures
than the ferromagnesian minerals, partial melting of an igneous
rock tends to produce magma of more felsic composition than
Continuous Crystallization of Plagioclase (Right
Branch). The right branch of Bowen’s Reaction Series the original rock from which it melted. So when a rock like
( FIGURE 5.6 ) shows that plagioclase feldspar crystallizes con- basalt partially melts, it tends to form a magma that is more felsic

tinuously from high to low temperatures (~1100–800° C), and would cool to form andesite.
but this is accompanied by a series of continuous change in
the composition of the plagioclase. The high temperature Magmatic Diff erentiation. Bowen’s Reaction Series is an
plagioclase is calcium rich and sodium poor, and the low example of one way that more than one rock type can form
temperature plagioclase is sodium rich and calcium poor. from a single body of magma. It was generated under controlled
If the magma cools too quickly for the plagioclase to react laboratory conditions. There is no known natural location
with the magma, then a single plagioclase crystal can have a where an ultramafic magma evolved to a felsic one according
more calcium rich center and a more sodium rich rim.
to Bowen’s Reaction Series. However, there are many examples
where parts of Bowen’s Reaction Series have occurred in nature.
Crystallization of Quartz (Bottom of the Series). Finally, Bowen’s continuous series of crystallization leads
notice what happens at the bottom of Bowen’s Reaction to the depletion of calcium and sodium from the magma,

Series ( FIGURE 5.6 ). At the lowest temperatures, where the last so the composition of the magma changes. However,
crystallization of magma occurs, the remaining elements form along the discontinuous series, early-formed mafic min-
abundant potassium feldspar (K-spar), muscovite, and quartz. eral crystals in a cooling body of magma have been shown
to react with the magma at lower temperatures to form
Partial Melting and Bowen’s Reaction Series. When new mafic minerals. If this recycling of elements occurred
a plastic tray of ice cubes is heated in an oven, the ice perfectly, then the concentrations of iron and magnesium
cubes melt long before the plastic tray melts (i.e., the ice in the magma would never change. In nature, some of
cubes melt at a much lower temperature). As rocks are the early-formed crystals either settle out of the magma
heated, their different mineral crystals also melt at different or are encrusted with different minerals before they can
temperatures. Therefore, at a given temperature, it is pos- react, so they can no longer react with the original magma.
sible to have rocks that are partly molten and partly solid. This is called fractional crystallization . On the other hand,

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