Constraining timescales of fluid-driven metamorphic rock transformation at a subduction interface using THMC modeling combined with Mg isotope diffusion chronometry

The plate interface at subduction zones represents one of Earth’s most dynamic environments in terms of deformation, metamorphism, and chemical exchange. The efficiency with which these processes affect rocks at the interface is strongly controlled by the presence of fluids. Accordingly, quantifying the rates of fluid–rock interaction is essential for understanding the pressure–temperature–deformation (P–T–d) evolution of rocks at plate interfaces.

In this study, we investigate a metasomatic reaction zone developed along the tectonic contact between serpentinite and metagabbro in the Voltri Massif (Ligurian Alps, Italy) under high-pressure conditions. The hydrated mantle-derived rocks were juxtaposed with the mafic oceanic crust at lower temperatures prior to the metasomatic process. A temperature increase led to dehydration of both lithologies, the serpentinite and the metagabbro, both of which liberated different amounts of aqueous fluids with very distinct fluid chemistries. This setting enabled Mg-rich fluids derived from serpentinite to infiltrate the adjacent mafic crust, triggering extensive metasomatic transformation. The aim of this study is to constrain the timescale of rock transformation and to explore the evolution of porosity and permeability within the modified system.

Our approach integrates a fully coupled Thermo–Hydro–Mechanical–Chemical (THMC) reactive transport model with thermodynamic phase equilibria calculations and diffusion chronometry. Phase equilibria calculations, validated by observed mineral assemblages, modal abundances, and mineral chemistry, are used to constrain the pressure–temperature conditions of the reaction zone formation. The estimated conditions correspond to pressures of 1.6 ±0.1 GPa and temperatures of 600 ±20 °C, with the maximum temperature being constrained by the serpentinite stability field. The profile across the reaction zone displays a continuous gradient in bulk MgO concentration from serpentinite (~40 wt.%) to metagabbro (~5 wt.%). This gradient is accompanied by a systematic Mg isotope fractionation, with δ²⁶Mg values decreasing from +0.09‰ in serpentinite to −1.1‰ within the reaction zone and an increase to −0.1‰ towards the least affected metagabbro. Such an Mg isotope profile indicates kinetic fractionation during Mg diffusion and provides the basis for Mg isotope diffusion chronometry.

Our THMC model results reproduce the observed major-element and isotopic profiles and suggest transient porosity generation localized at the reaction front. Calculated Peclet numbers (~0.01–0.1) indicate diffusion-dominated mass transport, with a minor advective component. The chronometric results of the modeling constrain the duration of metasomatic transformation to 10³–10⁴ years, highlighting the rapid nature of fluid-mediated processes at the subduction interface. This study shows how integrating diffusion chronometry with phase equilibria and reactive transport modeling helps bridge small-scale metamorphic processes and larger-scale subduction dynamics.