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 the most significant chemical potential gradients, which leads to fluid-mediated metasomatic reactions and chemical transport. As subducting sediment and basaltic crust often contain silica in various forms, the Si-metasomatism of mantle rocks is thought to occur along the subduction zone interface. However, growing evidence from the geochemistry of altered rocks and thermodynamic modelling has revealed the presence of multi-component fluids at the slab interface. Here, we review the laboratory experiments, geochemical models, and natural observations that improve our understanding of mass transport and metasomatic reactions at the crust–mantle interface, focusing on the relative mobility of Mg and Si. Hydrothermal experiments using analogues for the boundary between mantle (olivine) and crust (quartz or plagioclase) under vapor-saturated pressures indicate that Si is preferentially transported from crust to mantle, whereas Mg is immobile. This result is consistent with the distribution of talc rocks in oceanic lithosphere. On the other hand, at the contact between ultramafic (e.g., serpentinite) and crustal (pelitic schist or basaltic rocks) rocks in high-pressure metamorphic terranes, a large volume of chlorite rocks form in the crustal rocks, and the volume of chlorite often exceeds talc in serpentinites. Geochemical modeling reveals that in the shallow part of a subduction zone, the dissolved Si content of fluids in equilibrium with pelitic schist (CSi,crust) is significantly higher than the dissolved Mg content of fluids in equilibrium with mantle peridotite (CMg,mantle); however, CMg,mantle becomes dominant at depth, resulting in the Mg-metasomatism of crustal rocks to form chlorite rocks. This Mg-metasomatism is more widespread in warmer subduction zones (e.g., the Nankai and Cascadia subduction zones) than in colder subduction zones (e.g., in Northeast Japan). In addition, the infiltration of CO2-bearing fluid can form talc (along with carbonates) in ultramafic rocks without Si-metasomatism. Variations in the relative mobility of Si and Mg at the subduction zone interface produce variations in the overall solid volume change of mantle (expansion or contraction), the types of sheet silicates (talc versus chlorite), and the fluid budget (dehydration or hydration) during metasomatic reactions, which affects the pore fluid pressure, frictional strength of the subduction megathrust, and the location of seismicity around the mantle wedge corner.