18-Geol-A1 Mineralogy and Petrology · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2017-Dec. Closed book; no calculator permitted. Part 1 requires all five 10-mark short-answer questions (50 marks); Part 2's page-1 header says "5 of the 8" while the page-3 instructions say "5 of the 7" and list exactly 7 questions (a source discrepancy noted on the exam page itself).
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate/sulfide/carbonate structural classification, mineral chemistry and formulas); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, metamorphic agents/facies, volcanic processes, layered intrusions and cumulates, partial melting, ophiolites); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (carbonate mineral diagnostics).
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.
Mantle peridotite melts only when local conditions cross its solidus, and there are just three fundamentally different ways to make that happen: lower the pressure at roughly constant temperature (decompression melting), lower the solidus itself by adding a flux such as water (flux melting), or raise the temperature of the source (heat-source melting). Three contrasting tectonic settings illustrate all three mechanisms.
Passive asthenospheric upwelling beneath a spreading centre undergoes near-adiabatic decompression. Because the mantle solidus has a steeper $dT/dP$ slope than the mantle adiabat, rising material eventually crosses the solidus with essentially no change in temperature, generating MORB. This is pure decompression melting — no unusual heat source or fluid flux is needed.
The subducting slab progressively dehydrates as it heats up (breakdown of serpentine, chlorite and amphibole), releasing $\text{H}_2\text{O}$ into the overlying, otherwise sub-solidus mantle wedge. This water drastically lowers the peridotite solidus, triggering flux melting without requiring any unusual heat input; the resulting hydrous, buoyant melt then rises to feed arc volcanism.
A mantle plume delivers anomalously hot material (elevated mantle potential temperature $T_p$) toward the surface. Because it starts hotter, this material crosses the (unchanged) peridotite solidus at greater depth than normal-temperature mantle would, and continues melting through decompression as it rises — fundamentally a decompression process, but of a source made hotter by the plume's excess heat, i.e. heat-source-enhanced decompression melting, producing intraplate volcanism far from any plate boundary (e.g. Hawai'i).
