Partial Melting Mechanisms by Tectonic Setting: 18-Geol-A1 Mineralogy and Petrology, December 2016, Q11

Question 11 (Part 2, Q5): Partial Melting Mechanisms by Tectonic Setting (Part 2 – 10 marks)

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 it is pushed above its solidus by one (or a combination) of three routes: raising temperature, lowering pressure (decompression), or lowering the solidus itself (adding a flux such as water). Each tectonic setting below favours a different combination.

Melting mechanisms by tectonic setting
Setting	Melting mechanism(s)
(a) Divergent plate boundary (mid-ocean ridge)	Decompression melting. Passive/adiabatic upwelling of mantle peridotite as plates diverge reduces pressure at roughly constant potential temperature, driving the mantle across its (dry) solidus; the dominant, essentially sole, mechanism generating MORB.
(b) Island arc / continental margin arc	Flux (hydration) melting. The subducting slab dehydrates (breakdown of hydrous minerals in altered oceanic crust and sediment) and releases $\text{H}_2\text{O}$ into the overlying mantle wedge; water strongly depresses the peridotite solidus, inducing melting without requiring extra heat or major decompression. Minor decompression melting from wedge corner-flow can contribute secondarily.
(c) Continental rift valley	Decompression melting, as lithospheric thinning allows passive asthenospheric upwelling, often supplemented by elevated potential temperature where rifting is associated with a mantle plume; small-degree melting of fertile, locally volatile-enriched subcontinental lithospheric mantle can also contribute.
(d) Intraplate oceanic island (hotspot)	Heating plus decompression from a mantle plume. Anomalously hot upwelling mantle (elevated potential temperature relative to ambient asthenosphere) crosses the solidus at greater depth than normal and continues to melt by decompression as it rises through the plume conduit — the elevated potential temperature is what distinguishes hotspot decompression melting from ordinary ridge decompression melting (and produces thicker oceanic crust/higher magma flux at the hotspot).

Topic. The three fundamental mantle-melting mechanisms and their tectonic-setting expression.

Key relations. Melting requires $T > T_{\text{solidus}}(P, X)$; decompression lowers the effective solidus depth at constant $T$; flux (H$_2$O) addition lowers $T_{\text{solidus}}$ directly; excess potential temperature raises $T$ above ambient.

Why this works. Because the peridotite solidus is a function of pressure, volatile content and (implicitly) potential temperature, every tectonic setting that produces magma can be classified by which of these three levers it pulls — ridges and rifts by decompression, arcs by flux addition, hotspots by excess heat (usually combined with decompression) — and this classification is what allows a magma's major/trace-element and isotopic signature to be used to infer its tectonic setting even where the setting itself is no longer preserved.

Common pitfall. Attributing arc magmatism to decompression melting by analogy with ridges; arc magmas are generated primarily by flux melting from slab-derived water, which is why arc basalts are systematically more hydrous and have a distinct subduction-related trace-element signature (e.g. Nb-Ta depletion) that MORB lacks.

Source. Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed., §10, §13 (partial melting mechanisms, MORB/arc/OIB/rift petrogenesis).

Question 11 of 13: Partial Melting Mechanisms by Tectonic Setting

Question 11 (Part 2, Q5): Partial Melting Mechanisms by Tectonic Setting (Part 2 – 10 marks)