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.
Komatiites are ultramafic ($\geq 18\,\text{wt\%}$ MgO, some varieties $>30\,\text{wt\%}$) volcanic rocks, diagnosed by spinifex texture — skeletal, radiating blade- or plate-like olivine and pyroxene crystals produced by very rapid quenching of a super-liquidus melt. They occur almost exclusively in Archean (>2.5 Ga) greenstone belts (e.g. Barberton, South Africa; Kambalda, Western Australia), erupted as extremely hot ($\approx 1600\,{}^{\circ}\text{C}$), very low-viscosity lava flows.
Komatiites require very high degrees of partial melting (commonly >30–50%) of anomalously hot mantle. This demands a much higher mantle potential temperature ($T_p$) than exists today — Archean $T_p$ is estimated at roughly $1600$–$1900\,{}^{\circ}\text{C}$ versus $\approx 1350\,{}^{\circ}\text{C}$ for the modern ambient mantle — generated within unusually hot upwelling mantle, such as an Archean plume head or a hotter, more vigorously convecting early-Earth mantle regime.
Earth's mantle has cooled secularly since the Archean as short-lived radiogenic heat-producing isotopes have decayed, so the ambient (and even plume) mantle potential temperature today is insufficient to reach the extreme degrees of partial melting required to generate a true komatiitic (as opposed to picritic/basaltic) melt composition. Modern mantle plumes instead produce picrites (elevated but sub-komatiitic MgO). The very rare Phanerozoic komatiite occurrences that do exist (e.g. Gorgona Island, Colombia, Cretaceous) are attributed to unusually hot, localized plume material and are not evidence of a return to Archean-style ambient mantle conditions.
