18-Geol-A1 Mineralogy and Petrology · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A1 Mineralogy and Petrology, 2019-Dec. Closed book; no calculator permitted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate structural classification, mineral chemistry and substitution, crystal systems, sulfide/carbonate ore mineralogy); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, tectonic settings of magmatism, metamorphic/metasomatic processes, volcanic and pyroclastic processes, plate-tectonic cycle).
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.
MORB (mid-ocean ridge basalt) is the tholeiitic basalt that erupts along the global mid-ocean ridge system and constitutes most of Earth's oceanic crust. It is remarkably uniform in major-element composition worldwide and, in its most common form (N-MORB, "normal" MORB), is depleted in the most incompatible trace elements (K, Rb, Ba, light rare-earth elements) relative to primitive mantle, because it is derived from mantle that has already lost an earlier melt fraction.
As two oceanic plates diverge at a spreading centre, asthenospheric mantle peridotite passively rises to fill the gap. This ascent is close to adiabatic (no significant heat is gained or lost on the timescale of upwelling), but pressure drops rapidly as the material rises. Because the mantle solidus has a much steeper $dT/dP$ slope than the adiabat the rising mantle follows, the two curves eventually cross: the peridotite's temperature exceeds its own (pressure-dependent) solidus, and it begins to partially melt — typically 5–15% partial melting — with no unusual heat source or fluid flux required. The resulting basaltic melt segregates from its peridotite source, rises buoyantly, and erupts or intrudes to build the oceanic crust: pillow basalt flows (layer 2A), a sheeted dike complex (layer 2B) and gabbro (layer 3), the classic ophiolite-sequence stratigraphy.
Because melting stops once the melt fraction has extracted enough heat and incompatible elements to bring the residual solid mantle back below its own (now-shifted) solidus, only a limited degree of partial melting occurs at any given depth interval — this is why N-MORB is depleted, not primitive: it represents the extracted melt fraction, leaving behind a refractory, incompatible-element-poor harzburgitic residue that continues upward as the mantle root of the newly-formed oceanic lithosphere. Rarer, more enriched E-MORB reflects either a smaller melt fraction sampling a still-fertile mantle source or local mixing with a slightly enriched (e.g. plume-influenced) component beneath an otherwise normal ridge segment.
