Laboratory Manual for Introductory Geology 4e

EXERCISE 5.8
Insights from Melting and Crystallization Experiments
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Results from melting experiments help explain several aspects of igneous rock texture and composition. Remember that
crystallization is the exact opposite of melting. Answer the following questions:
(a) The photomicrographs below show textural features displayed by plagioclase (left) and by pyroxene and
amphibole (right), minerals found in many igneous rocks. Considering the continuous and discontinuous branches
of Bowen’s reaction series, suggest an explanation for the origin of these textures.
Photomicrographs of common mineral textures in igneous rocks.
Ca-rich
2 mm 2 mm
px
Na-rich
am
Compositionally zoned plagioclase grain with Ca-rich core
and progressively Na-rich rim.
Pyroxene grain (px; dark green) with amphibole rim (am;
lighter green).
(b) Considering Bowen’s reaction series, is it likely that minerals with high melting temperatures will occur in rocks with
minerals with low melting temperatures? Explain.
(c) List the four major compositional igneous rock groups in order from lowest magma temperature to highest.
1.
2.
3.
4.
126 CHAPTER 5 USING IGNEOUS ROCKS TO INTERPRET EARTH HISTORY