18-Geol-A1 Mineralogy and Petrology · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A1 Mineralogy and Petrology, 2019-Dec. Closed book; no calculator permitted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate structural classification, mineral chemistry and substitution, crystal systems, sulfide/carbonate ore mineralogy); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, tectonic settings of magmatism, metamorphic/metasomatic processes, volcanic and pyroclastic processes, plate-tectonic cycle).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Bowen's reaction series is a schematic model, based on Bowen's melting-crystallization experiments, of the order in which minerals crystallize from a cooling basaltic magma. It has two parallel branches: a discontinuous branch, on which each mafic mineral reacts with the remaining melt to be replaced by the next as temperature falls (olivine → orthopyroxene → clinopyroxene → amphibole → biotite), and a continuous branch, on which a single mineral, plagioclase, progressively changes composition without a discrete reaction, from Ca-rich to Na-rich. Both branches converge at low temperature on the last, most felsic phases: K-feldspar, muscovite and quartz.
The series is, at any instant, a snapshot of which minerals are in equilibrium with a melt of a given composition and temperature; because melt composition evolves continuously toward the residual, low-temperature end as earlier (higher-temperature) minerals crystallize and are removed, the series simultaneously predicts the sequence of mineral assemblages and, if crystals are ever separated from melt, the sequence of derivative rock compositions. A parent basaltic magma that crystallizes completely in place, with no crystal–melt separation, solidifies as basalt/gabbro, carrying the whole assemblage. But if early, dense minerals (olivine, then pyroxene) are physically removed — by gravitational settling into a cumulate pile in a layered mafic intrusion — the residual liquid is left depleted in Mg/Fe/Ca and enriched in Si/Na/K/Al, continuing to fractionate down the series toward andesitic/dioritic and, ultimately, granodioritic/granitic compositions. This process, fractional crystallization, is the single mechanism by which Bowen's series explains how the full compositional spectrum of igneous rock — ultramafic through felsic — can descend from one basaltic parent magma.
