Numerical modeling of trace element transportation in subduction zones: implications for geofluid processes

This study presents the first numerical model for trace element transportation associated with dehydration and fluid migration from the subducting slab and aims to incorporate both fluid dynamical processes (e.g., flow mode and mass fluxes) in subduction zones and associated geochemical evidence (e.g., chemical compositions of arc lavas). The model includes temperature and flow structures associated with slab subduction and mantle-fluid two-phase flow, as well as phase relations of hydrous phases (e.g., dehydration-hydration reactions and melting) and trace element partitioning among the phases (solid, aqueous fluid, and melt). The model calculations show that if instantaneous chemical equilibrium is achieved associated with porous flow of slab-derived fluid, the elements expelled with the ascending fluid (e.g., Pb) are absorbed into the down-going hydrated mantle layer developed above the slab. As a result, these elements are considerably depleted in the resultant magma generated by fluid-flux melting in the core part of the mantle wedge, and it therefore fails to reproduce the geochemical characteristics of arc lavas. In contrast, if disequilibrium element transport (e.g., associated with channel flow) is assumed when the hydrated mantle layer liberates the fluid, then the key elements are delivered to the melting region to reproduce certain arc lava signatures. These results suggest that disequilibrium fluid transport in the wedge mantle, such as through channels, plays an important role in element cycling in subduction zones.