18-Geol-A1 Mineralogy and Petrology · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2016-Dec. Closed book; no calculator permitted. Part 1 requires all six 10-mark short-answer questions (60 marks); Part 2 instructs "answer 4 of the 7 ten-mark questions" (40 marks).
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (mineral/silicate structural classification, ore mineralogy); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, phase diagrams, metamorphic reactions and facies, AFM projections, volcanic processes, ophiolites and oceanic crust, subduction-zone/rift/hotspot melting); Nesse, Introduction to Optical Mineralogy, 4th ed. (index-mineral optics).
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 (apices A = $\text{Al}_2\text{O}_3-(\text{K}_2\text{O}+\text{Na}_2\text{O})$, F = $\text{FeO}+\text{MnO}$, M = $\text{MgO}$) is used for igneous suites, as well as pelites, and it is the classic tool for separating the tholeiitic and calc-alkaline differentiation trends by whether the trend shows strong iron enrichment.
Tholeiitic magmas crystallize Fe-Ti oxides (magnetite) late, because they evolve at comparatively low (mantle-buffered) oxygen fugacity; with early-crystallizing phases (plagioclase, low-Ca pyroxene, olivine) removing relatively little Fe, the residual liquid becomes progressively Fe-enriched before oxide saturation finally intervenes. On the AFM diagram this produces a pronounced trend toward the F apex before curving back toward A — the diagnostic "tholeiitic iron-enrichment trend." Typical of MORB, oceanic tholeiites and some primitive arc tholeiites.
Calc-alkaline magmas crystallize Fe-Ti oxides early (favoured by the higher oxygen fugacity typical of hydrous, subduction-related magmas), which removes Fe from the liquid continuously through the crystallization history and suppresses iron enrichment. On the AFM diagram the trend instead runs more directly toward the A apex (progressive silica/alumina enrichment) without a pronounced F-ward excursion. Typical of subduction-zone andesite–dacite–rhyolite arc suites.
Alkaline magmas are distinguished less by their AFM trend (which can also show iron enrichment) than by silica undersaturation and alkali (Na$_2$O+K$_2$O) enrichment relative to both other series, best displayed on a total-alkali-silica (TAS) diagram rather than AFM; they typically form by low-degree partial melting of enriched mantle in intraplate or continental-rift settings (e.g. basanite, nephelinite), where small melt fractions preferentially incorporate the more incompatible alkali elements.
