Melting Mechanisms by Tectonic Setting: 18-Geol-A1 Mineralogy and Petrology, May 2017, Q9

Question 9: Melting Mechanisms by Tectonic Setting (Part 2 – 10 marks)

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Mantle melting requires crossing the peridotite solidus, which can happen three fundamentally different ways: reducing pressure at constant temperature (decompression melting), adding a solidus-lowering component such as water (flux melting), or raising the temperature of the source itself (heat-source melting). Each tectonic setting is characterized by a distinct combination of these mechanisms.

Melting mechanisms by tectonic setting
Setting	Mechanism(s)	Explanation
(a) Divergent plate boundary	Decompression melting	Passive asthenospheric upwelling beneath a spreading ridge undergoes near-adiabatic decompression, crossing the mantle solidus (whose $dT/dP$ is steeper than the adiabat) with no change in temperature, generating MORB. Essentially the sole mechanism — no unusual heat source or flux is required.
(b) Island arc / continental margin arc	Flux (fluid-induced) melting; secondary decompression of wedge diapirs; slab melting in rare hot-slab cases	Dehydration of the subducting slab (breakdown of serpentine, chlorite, amphibole) releases $\text{H}_2\text{O}$ into the overlying mantle wedge, lowering the peridotite solidus and inducing melting without added heat; the resulting lower-density, hydrous diapirs then rise and undergo some decompression melting; where the slab itself is young/hot, direct partial melting of the subducted slab (adakite-type melting) can also occur.
(c) Continental rift valley	Decompression melting, often plume-enhanced	Lithospheric extension and thinning causes passive (or, above a plume, active) upwelling and decompression of asthenosphere through thinned continental lithosphere; active rifts above a mantle plume add a heat-source component (elevated $T_p$), combining decompression with an anomalously hot source.
(d) Intraplate oceanic island	Heat-source (thermal) melting via decompression of an anomalously hot plume	A mantle plume brings anomalously hot material ($T_p$ elevated above ambient mantle) to shallow depth; because its solidus is crossed at greater depth than for normal-temperature mantle, this is fundamentally decompression melting, but of a source made hotter by the plume's excess heat rather than by lithospheric thinning alone.

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

Key relations. Decompression melting: crossing the solidus by reducing $P$ at constant $T$. Flux melting: lowering the solidus by adding $\text{H}_2\text{O}$. Heat-source melting: raising $T$ (plume $T_p$) to cross the solidus at greater depth.

Why this works. Every occurrence of mantle melting on Earth can be classified by which of these three solidus-crossing mechanisms dominates, which is why the same three-mechanism framework explains settings as different as mid-ocean ridges, arcs, rifts and hotspots — and why more than one mechanism can operate simultaneously, as in a plume-affected rift.

Common pitfall. Attributing arc magmatism to "friction-generated heat" from the subducting slab; flux (water-induced) melting of the mantle wedge, not frictional heating, is the dominant mechanism.

Source. Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed., §12–13 (partial melting mechanisms, tectonic settings of magmatism).

Question 9 of 13: Melting Mechanisms by Tectonic Setting

Question 9: Melting Mechanisms by Tectonic Setting (Part 2 – 10 marks)