Mechanisms of Mass Transfer in Sediment-Rich Mélanges in Modern Subduction Zones

Sediment-rich mélange diapirs have been suggested to transport key chemical slab signatures and volatiles to arc magma sources. Here, we assess the phase equilibria, buoyancy and implications for chemical geodynamics of a previously unexplored hydrous shaly-rich mélange (5–10 wt.% H₂O) with minor ultramafic component, from deep forearc to subarc depths (2–3 GPa and 700–1,150°C). The solidus lies between <645°C and 700°C and upon partial melting, produce dacitic to rhyolitic melts coexisting with a low-density pyroxenite enriched in mica, amphibole, quartz, garnet and accessory minerals. Our analysis shows that instabilities are likely in warm, slow-subducting and thinner channels with low viscosities compared to cold and fast subducting slabs where diapirism is likely limited, as they require extremely large channel thicknesses. Diapirism can occur in tectonic slabs with heat sources, such as nearby slab tears or plumes. However, those mélanges lose buoyancy upon thermal equilibration at temperatures above 850°C. While smaller diapirs may densify and stagnate near the channel, larger diapirs may maintain buoyancy, allowing them to remelt beneath the overriding lithosphere. Aqueous fluids and low-degree melts prevail near the channel, transferring high Large Ion Lithophile Element (LILE)/High-Field Strength Element (HFSE) ratios to arc magma sources, which resemble those found in arc lavas. High degree melting of mantle-wedge diapirs may explain arc lava diversity but not their ubiquitous high LILE/HFSE signatures. Overall, diapirism is highly conditional and likely originate in hot slabs. Thus, aqueous fluids and partial melts are the dominant mass-transfer agents of slab signatures to arc magma sources.