Metasomatismo del mantello dell'avambraccio mediato dai fluidi: approfondimenti dalla modellazione termomeccanica petrologica

Mantle metasomatism and associated fluid-aided mass transfer have wide-ranging implications for geochemical, rheological and dynamical aspects of the shallow Earth interior. However, large uncertainties remain due to the inaccessibility of metasomatic processes over geologic timescales, scarcity of samples and the fragmentary nature of outcrops. Despite chemically open-system behavior has been evidenced from geochemical and petrological records, most geodynamical modelling approaches rely on a chemically closed-system. Taking advantage of improved numerical techniques, and thermodynamic theories, we developed a new petrological–thermo–mechanical code to overcome such a closed-system limitation. This code use thermodyamically predicted rock solubility to monitor the mass exchange between rocks and coexisting fluids, and thermomechanically determined fluid and solid flow to simulate advection and potential dynamic feedbacks. Comparison with previously published numerical results justifies the robustness of this new method in spite of the differences in the employed solid solution models, thermodynamic database and coarser resolution (0.5$\times$0.5km), which cause minor compositional differences mainly due to smoothing effects along lithological boundaries. We utilize this new code to explore the first-order redistribution patterns of rock-forming elements and newly formed metasomatic minerals as a function of convergence rates, slab ages and subducted sediment compositions. Youngest and slowest subduction zones with an abundant water supply from the slab experience the most notable element mobility. The progressive thermal cooling of subduction zones enhances the extent of mantle alteration. Lithological boundaries with steep compositional gradients are often affected by intensive fluid-mediated alterations and formation of metasomatinites as slab-derived elements are mostly absorbed along such boundaries. The multicomponent fluid compositions emanated from slabs evolve with slab depth, and associated alteration zones are characterized by a decrease in the phase proportions of antigorite, chlorite, and an increase of that of talc with depth near the slab-mantle interface. C- and Si-bearing fluids infiltration facilitates the formation of talc-rich metasomatinites at cold slab top conditions, whereas substantial carbon-poor fluids infiltration decomposes talc as slap top temperatures approach the solidus. Talc together with antigorite and chlorite likely play a significant role in element circulation as well as in controlling the bulk mechanical properties and seismicity along the plate interface at subduction zones. Overall, the obtained results are consistent with geophysical observations, petrological records, petrological experiments and reaction path model results. This work provides a promising tool for exploring the complex interplay among geodynamics of subduction zones, geochemical recycling occurring within the shallow planetary interior, and magmatic processes.