What Causes Melting in Different Tectonic Settings: 18-Geol-A1 Mineralogy and Petrology, December 2018, Q9

Question 9: What Causes Melting in Different Tectonic Settings (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 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.

(1) Mid-ocean ridge (divergent boundary)

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.

(2) Subduction zone (convergent boundary, volcanic arc)

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.

Topic. The mantle solidus-crossing mechanisms and their tectonic settings.

Key relations. Decompression melting: reduce $P$ at constant $T$ (ridges). Flux melting: lower the solidus by adding $\text{H}_2\text{O}$ (arcs). Heat-source melting: raise source $T_p$ (hotspots, not required to answer this question).

Why this works. Every occurrence of mantle melting on Earth reduces to crossing the peridotite solidus by one of these routes, which is why the same two-mechanism contrast explains why a mid-ocean ridge (a "cold" divergent boundary in a bulk sense) still generates enormous volumes of basalt while an arc, sitting above a much cooler subducting slab, also melts — each setting reaches the solidus by a different route.

Common pitfall. Attributing arc magmatism to frictional heating of the subducting slab; flux (water-induced) melting of the overlying mantle wedge, not slab friction, 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 12: What Causes Melting in Different Tectonic Settings

Question 9: What Causes Melting in Different Tectonic Settings (Part 2 – 10 marks)

(1) Mid-ocean ridge (divergent boundary)

(2) Subduction zone (convergent boundary, volcanic arc)