18-Geol-A1 Mineralogy and Petrology · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exam — 18-Geol-A1 Mineralogy and Petrology. 3 hours, closed book, no calculator permitted. Two parts, twelve ten-mark short-answer questions in total: Part 1 (Q1–5) requires all five questions (50 marks); Part 2 (Q6–12) is printed as "answer 5 of the 7" on one page and "answer 5 of the 5" on another (the paper's own instructions disagree on the count) — every question in both parts is solved in full below so this set also serves as a complete study reference. This sitting is treated as undated because the paper is internally inconsistent about its own date: the first-page footer reads "May 2018" (matching the 18-Geol-A1 code, in use from December 2018 onward) while a later page's footer reads "19-Geol-A1 / May 2019". No exam date is asserted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate/oxide structural classification, mineral chemistry, solid solution and exsolution, crystal systems); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, tectonic settings of magmatism and melting, ophiolites, LIPs, anatexis, contact/thermal metamorphism).
There is no numeric given data anywhere in this qualitative/descriptive paper.
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 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). Two contrasting tectonic settings illustrate two of these 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.
The two settings are a useful contrasting pair precisely because they achieve the same outcome — crossing the peridotite solidus — by opposite means: the ridge lowers pressure while holding the solidus itself fixed, whereas the arc lowers the solidus itself while barely changing pressure at the melting depth. A third mechanism, raising the source's own potential temperature ($T_p$) as in a mantle plume beneath a hotspot or Large Igneous Province, is not required to answer this question but completes the set of three ways mantle rock can ever cross its solidus.
