Thermo-Hydro-Mechanical-Chemical (THMC) reactive transport modeling of Mg isotope fractionation to constrain the timescales of fluid-driven rock transformation in the crust.

Fluid-rock interactions can induce significant chemical changes, resulting in metasomatic rock transformations or formation of metasomatic fronts when mass transfer is substantial. Among the chemical agents driving metasomatism, magnesium (Mg) plays a critical role in mafic/ultramafic rock systems, altering bulk composition and mineralogy. Additionally, the mass difference between ²⁴Mg and ²⁶Mg isotopes enables detectable kinetic fractionation.

This study examines a metasomatic reaction zone in the Voltri Massif (Ligurian Alps, Italy), formed through high-pressure diffusional metasomatism of metagabbro by Mg-rich fluids equilibrated with serpentinite. This zone is an ideal natural analogue for reactive fluid flow between the subducting hydrated mantle and the overlying mafic crust. Evidence for Mg metasomatism includes a continuous MgO gradient, transitioning from serpentinite (~40 wt.%) to metagabbro (~5 wt.%). Isotopic analysis reveals significant fractionation: δ²⁶Mg values range from +0.09‰ in serpentinite to -1.1‰ in the reaction zone, then increasing to -0.1‰ in metagabbro. This trend indicates kinetic isotope fractionation driven by Mg diffusion.

A reactive transport model incorporating viscous rheology is applied to investigate porosity- permeability evolution and estimate process duration. Integrating bulk rock major element and Mg isotope geochemistry with fully coupled Thermo-Hydro-Mechanical-Chemical (THMC) model for reactive transport and phase equilibria, we analyze geochemical and mineralogical transformations across the reaction zone. The model results are validated by fitting field-based geochemical and isotopic data, ensuring consistency with observed MgO gradients and δ²⁶Mg fractionation patterns. Systematic numerical simulations and analyses provide insights into the timescales of Mg metasomatism, shedding light on the dynamics of such metamorphic processes.