The Green Dog That Did Not Bark: Broader Impacts of Metasomatic Underplating

Seismic methods suggest that the oceanic crust beneath hotspot islands is often underlain by a layer with seismic velocities intermediate to crust and mantle. The "metasomatic underplating" (MSUP) hypothesis argues that crustal fractures develop during hotspot magma ascent to allow seawater to infiltrate and to serpentinize the sub-Moho mantle. Assuming 8-km thickness with Vp=7.4 km/s for the underplated layer at the Hawaii swell and 35% serpentinization with density 3.1 gm/cm³, one can estimate 1-km of seawater is chemically bound within a typical underplated layer. If a serpentinized mantle layer underlies hotspot tracks and/or aseismic ridges, their buoyancy can induce flat-slab behavior within subduction zones, e.g., central Chile, Peru, weaken slab rheology, and foster slab tears. Geochemically, MSUP may produce more serpentine and talc, and less brucite and magnetite, if seawater equilibrates with silica in the gabbroic oceanic crust as it descends to the Moho. The restricted solubility of carbonate with temperature may induce seawater CO₂ to sequester in the crust as carbonate concretions or intergrowths. Consumption of seawater H₂O by MSUP serpentinization raises its salinity so that Fe cations stabilize in chloride complexes and depart the open system. Alkali cations contribute to the sodic metasomatism of pyroxenes, analogous to alterations in abyssal peridotites. Chemical interaction between main-stage hotspot magmas and the underplated layer is minimal, but the signature of the latter may be evident at Hawaii in postshield trachyte magmas and xenoliths within scattered rejuvenated-stage alkali basalts. Metasomatized crust and mantle beneath hotspot tracks become buoyant ribbons trapped within cool dense slabs, and are likely avenues for water and CO₂ to reach the mantle transition zone. Dehydrated MSUP mantle will resemble a mixture of peridotite and pyroxenite, and could source the alkali basalts in "secondary" hotspots, as well as suggest an Archean process that formed the EM1 and EM2 mantle reservoirs. Subducted MSUP crust becomes volatile-rich eclogite as it descends, vulnerable to melting in subduction zones to form adakite magmas. Deeply subducted MSUP crust chemically resembles the ancient carbonated-eclogite mantle source inferred for HIMU end-member OIBs.