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Incoming links: Slab melting
Past exam of the plate tectonics course of the University of Oxford 2023 Question 1 a Solution 2026-10-25
Choose an ocean–continent convergent margin. The essential relationship is recycling of oceanic lithosphere by subduction, together with production and modification of crust in a volcanic arc.
An original ocean–continent subduction cross-section, with slab recycling, fluid release and arc crust production
. Details. 
The incoming tectonic plate consists of thin oceanic crust, chiefly basalt and gabbro, above colder lithospheric mantle. As that oceanic lithosphere approaches the subduction zone, bending produces the subduction trench. Its cold interior can become denser than the surrounding mantle; the resulting slab pull helps sustain subduction. The descending slab carries both oceanic crust and lithospheric mantle into the interior. Thus the recycled unit is an entire portion of lithosphere, rather than just its crust.
At the subduction trench, some incoming sediment is scraped off and stacked by thrust faults into an accretionary prism. Material can also be added beneath the overriding plate by tectonic underplating. A forearc basin collects sediment between the accretionary prism and the volcanic arc. This is an accreting example: other convergent margins lose upper-plate material through tectonic erosion, so an accretionary prism is not universal.
The cold slab undergoes increasing pressure and metamorphism. Conversion of basaltic rocks and gabbro towards eclogite increases their density, while slab dehydration releases water from hydrated minerals. The rising fluid lowers the solidus of hot peridotite in the mantle wedge, causing flux melting. The usual source of arc magma is therefore the fluid-modified mantle wedge; extensive slab melting is not required. Thermal-petrological calculations show that the depth and amount of slab dehydration depend on the incoming slab's thermal structure.[ref]
Buoyant magma rises through the overriding plate, freezes in igneous intrusions or erupts in the volcanic arc. Fractional crystallization, interaction with existing continental crust and repeated intrusion can produce more evolved arc magmas and add new crust. Some material merely reworks old crust, and simultaneous tectonic erosion can remove it: volcanism alone does not measure net continental growth. Tectonic shortening, folding and thrust faults can thicken and raise the upper plate.
The Wadati-Benioff zone marks earthquakes along and within the cold descending slab. Shallow interface earthquakes reflect unstable slip where the plates are coupled; deeper earthquakes require mechanisms such as dehydration embrittlement and are not simply dry shallow friction operating at every depth. In contrast, the hot mantle wedge deforms mainly by material creep. Together, the subduction trench, accretionary prism, forearc basin, volcanic arc and inclined seismicity connect surface features to the processes recycling and creating crust.
Today the usual first melt is basaltic melt from the mantle wedge; in a hotter Archean subduction zone, melting of hydrated basaltic crust itself could generate sodic felsic melt of tonalite-trondhjemite-granodiorite composition. Distinguish a primary melt from the evolved melt ultimately added to the upper continental crust.
On the modern Phanerozoic Earth:
- Hydrothermal alteration introduces water into the incoming oceanic crust and locally into serpentinized mantle. Sediments also carry water in pore space and hydrous minerals.
- As the slab descends, heating and metamorphism break down hydrous minerals. Released fluids enter the hotter peridotite of the mantle wedge and lower its solidus, causing flux melting. Most ordinary modern slabs release fluids before extensive melting of their basaltic crust.
- The primary melt is predominantly hydrous basaltic magma. During ascent, storage and fractional crystallization, mafic minerals are removed and the remaining liquid evolves toward andesite, dacite and rhyolite compositions. Partial melting of recently emplaced mafic lower continental crust, magma mixing and some crustal contamination can contribute to felsic magma. Intrusive equivalents include diorite, tonalite and granodiorite.
This adds juvenile material derived from the mantle; simply remelting old continental crust is reworking, not net juvenile addition. The amount and depth of slab water release depend on its thermal history, as quantified in Subduction factory: 4. Depth-dependent flux of H2O from subducting slabs worldwide by Peter E. van Keken and others.
For the hotter Archean endmember posited in the question, a hotter mantle and a warm descending plate make it easier for hydrated basaltic crust to reach its solidus. Its partial melting can directly make silica-rich, sodium-rich tonalite-trondhjemite-granodiorite liquids, leaving a more mafic residue. Residual garnet and amphibole, and the presence or absence of residual plagioclase, help determine the rare-earth element and strontium signatures. These are not ultramafic liquids merely because the surrounding mantle was hotter.
The contrast is therefore commonly fluid-fluxed mantle melting followed by differentiation today, versus a greater potential contribution from direct melting of hydrated mafic crust in the hotter case. Tonalite-trondhjemite-granodiorite is not unique evidence for subduction: melting of thickened mafic crust can produce similar liquids. Nor are all modern arcs devoid of slab melting. These qualifications are central to The composite Archaean grey gneisses: Petrological significance, and evidence for a non-unique tectonic setting for Archaean crustal growth by Jean-François Moyen.