Lithospheric Mantle Metasomatism by Reactive Hydrous Infiltration

Earth’s volatile budget calculations indicate the need for an additional upward flux of water released through subduction processes, beyond what is accounted for by arc magmatism. This excess water may diffuse or be channeled through various mechanisms. Lithospheric mantle metasomatism via reactive hydrous infiltration is investigated as a critical process shaping Earth's magmatic, chemical, and geodynamic evolution. Fluid-driven metasomatism may play a more significant role in subduction zone and intraplate magmatism than traditionally acknowledged, acting as a primary agent of mantle transformation. In subduction zones, volatile-rich fluids released from dehydrating slabs infiltrate the mantle wedge, lowering the solidus temperature and enabling flux melting. These fluids may also function as agents of chemical transport. Similarly, in intraplate settings, hydrous fluids can introduce incompatible elements and hydrous minerals, altering mantle fertility and geochemistry.

Thermodynamic and transport models are integrated to examine metasomatic processes in the Earth's lithospheric mantle, particularly under conditions relevant to intraplate volcanism. Thermodynamic calculations generate lookup tables for essential variables such as phase densities, fluid incorporation into minerals, and fluid concentrations across pressure-temperature-composition (P-T-X) space, using Gibbs Free Energy minimization via the Thermolab tool. The transport model employs continuum mechanics principles for a two-phase system of fluid or melt and solid phases, with numerical implementation using finite difference methods to solve conservation laws.

Key metasomatic reactions, including dunitization, serpentinization, amphibolitization, and phlogopitization, are explored through thermodynamic and reactive transport models, revealing their impacts on mantle porosity and mineralogy. Dunitization enhances porosity, facilitating melt transport and the formation of high-permeability pathways such as dunite channels. Serpentinization reduces porosity, potentially clogging transport pathways, though its reverse reaction releases volatiles critical for arc magmatism. Amphibolitization reduces porosity while stabilizing amphiboles, providing insights into fluid-driven mantle metasomatism in the oceanic lithosphere. Phlogopitization highlights the significance of high-pressure metasomatic processes in modifying thick cratonic lithospheres and generating protoliths for alkaline and potassic magmatism.

This study emphasizes the underestimated role of water in magmatic processes, extending beyond its facilitation of melting to its crucial role in metasomatic enrichment, heat transfer, and compositional modification. The findings provide a framework for understanding magmatism’s multistep progression, from mantle enrichment to intraplate volcanic activity, and lay the groundwork for advanced two-dimensional models incorporating coupled thermo-hydro-mechanical-chemical (THMC) processes, with accurate porosity evolution.