Fluid-mediated forearc mantle metasomatism: Insights from petrological-thermomechanical modeling

At subduction zones, mantle metasomatism induced by metamorphic fluids is known to have a noticeable control on the rheology, seismicity, volcanic activity and heat/mass transfer. Although geophysical, geochemical and petrological observations have greatly improved our knowledge about this process, significant uncertainties remain concerning the chemical composition and the extent of element redistribution within the mantle wedge. This is mainly due to the complexities of multicomponent fluid speciation and fluid migration dynamics, as well as to the uncertainties concerning thermal structure and rheological behavior of subduction zones. Herein, we incorporate forward thermodynamic modeling (Backcalc algorithm, Galvez et al. (2015), and MAGEMin software, Riel et al. (2022)) of fluid-rock chemical interactions with a 2D thermomechanical code (I2VIS, Gerya and Yuen (2003)) to present the first-order redistribution patterns of rock-forming elements.

The composition of multicomponent H₂O-CO₂ fluids emanated from slabs evolve with slab depth from diluted Si-Na solution to relatively complicated (alkali+Ca) aluminosilicate-rich solution. The associated alteration zones are characterized by a decrease in the phase proportions of antigorite, chlorite, and an increase of that of talc and carbonates with depth near the slab-mantle interface. Lithological boundaries with steep compositional gradients often undergo intensive fluid-mediated alterations, generating characteristic (talc-rich) metasomatites. This is because the slab-derived elements, such as C and Si, are mostly absorbed along the lithological boundaries. Ultimately, substantial carbon-poor fluids infiltration near sub-arc depths decomposes talc as slab surface temperatures approach the solidus. Talc together with antigorite and chlorite can potentially play a significant role in element circulation and mechanical properties (e.g., seismic activity) along the plate interface within subduction zones.

In summary, metasomatism induced by mobile volatile elements often results in notable petrological records wherever hydrous minerals are stable in the mantle wedge. The redistribution of non-volatile elements involving the addition of common peridotite phases(e.g. clinopyroxene) is stealthier except for the lithological boundaries. This improved petrological-thermomechanical modeling strategy provides a promising tool for studying the complex interplay among geodynamics of subduction zones, geochemical recycling of shallow planetary interior, and magmatic processes.