18-Geol-A1 Mineralogy and Petrology · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2017-May. Closed book; no calculator permitted. Part 1 requires all five 10-mark short-answer questions (50 marks); Part 2 lists eight questions with instructions to answer "5 of the 7" (a source discrepancy noted on the exam page itself).
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate structural classification, mineral chemistry/formulas); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation and mixing, metamorphic agents/facies, volcanic processes, phase equilibria and AFM projections, magma viscosity, layered intrusions, tectonic melting mechanisms).
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.
The AFM diagram ($\text{A} = \text{Na}_2\text{O}+\text{K}_2\text{O}$, $\text{F} = \text{FeO}+\text{Fe}_2\text{O}_3$(total Fe), $\text{M} = \text{MgO}$) plots the liquid line of descent of an evolving magma series and is the standard tool for separating these families.
Fe-Ti oxide (magnetite) crystallization is delayed (favoured by lower magmatic $f_{\text{O}_2}$, typical of relatively anhydrous mid-ocean-ridge and oceanic-plateau magmas), so early fractional crystallization removes little Fe. The liquid is driven strongly toward the F apex (marked Fe-enrichment) before eventually turning toward A — the classic tholeiitic "Fe-enrichment trend."
Early crystallization of Fe-Ti oxides (favoured by the higher $f_{\text{O}_2}$ and water content typical of subduction-zone/arc magmas) removes Fe from the liquid from an early stage, strongly suppressing Fe-enrichment. The trend curves more directly toward the A apex with comparatively little excursion toward F — the diagnostic contrast with the tholeiitic trend, and characteristic of most arc (Cascades, Andes) volcanic suites.
Silica-undersaturated magmas with high total alkalis relative to silica occupy a field displaced toward the A apex from the outset of crystallization, reflecting derivation from small-degree partial melts of enriched mantle, typically in intraplate/rift settings rather than arc or ridge settings.
