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 the formation of metasomatic fronts when mass transfer is substantial. Among the chemical agents driving metasomatism, magnesium (Mg) plays a critical role, particularly in mafic and ultramafic rock systems. Magnesium's transport not only alters bulk composition but also impacts mineral assemblages in affected rock volumes. Additionally, the large mass difference between ²⁴Mg and ²⁶Mg isotopes enables detectable kinetic fractionation in the rock record.

This study examines a metasomatic reaction zone within the Voltri Massif of the Ligurian Alps (Italy), formed through high-pressure (HP) diffusional metasomatism of a (meta)gabbroic body by Mg-rich fluids (with Ni and Cr) equilibrated with serpentinite. This zone serves as an ideal natural analogue for reactive fluid flow between the downgoing hydrated lithospheric mantle and the overlying mafic crust. The reaction zone features distinct mineralogical changes: a chlorite- and amphibole-rich assemblage near the lithological contact and an epidote-rich assemblage further away.

Evidence for Mg metasomatism includes a continuous MgO gradient, transitioning from serpentinite (~40 wt.%) to metagabbro (~5 wt.%). Isotopic analysis reveals significant fractionation along the transect, with δ²⁶Mg values ranging 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 the duration of the process. By integrating bulk rock major element and Mg isotope geochemistry with fully coupled Thermo-Hydro-Mechanical-Chemical (THMC) modeling 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.