Si- versus Mg-metasomatism at the crust–mantle interface: insights from experiments, natural observations and geochemical modeling

Abstract The slab–mantle interface in subduction zones is one of the geological boundaries with most significant chemical potential gradients, which leads to fluid-mediated metasomatic reactions and transport. As subducting sediment basaltic crust often contain silica various forms, Si-metasomatism mantle rocks thought occur along zone interface. However, growing evidence from geochemistry altered thermodynamic modelling has revealed presence multi-component fluids at slab Here, we review laboratory experiments, geochemical models, natural observations that improve our understanding mass transport crust–mantle interface, focusing on relative mobility Mg Si. Hydrothermal experiments using analogues for boundary between (olivine) (quartz or plagioclase) under vapor-saturated pressures indicate Si preferentially transported mantle, whereas immobile. This result consistent distribution talc oceanic lithosphere. On other hand, contact ultramafic (e.g., serpentinite) crustal (pelitic schist rocks) high-pressure metamorphic terranes, a large volume chlorite form rocks, exceeds serpentinites. Geochemical modeling reveals shallow part zone, dissolved content equilibrium pelitic ( C Si,crust ) significantly higher than peridotite Mg,mantle ); however, becomes dominant depth, resulting Mg-metasomatism rocks. more widespread warmer Nankai Cascadia zones) colder Northeast Japan). In addition, infiltration CO 2 -bearing fluid can (along carbonates) without Si-metasomatism. Variations produce variations overall solid change (expansion contraction), types sheet silicates (talc versus chlorite), budget (dehydration hydration) during reactions, affects pore pressure, frictional strength megathrust, location seismicity around wedge corner.