Many exposed high-pressure meta-serpentinites comprise a channelized network of olivine-rich veins which formed during dehydration at depth and served as pathway for fluid escape. Previous studies showed that the formation of an olivine enriched vein-like interconnected porosity network on the µm-scale is controlled by chemical heterogeneities in the rock. However, the evolution towards larger scale and nearly pure olivine veins is not yet well understood. Here we study the effects of reactive fluid flow on a developing vein system during dehydration. We use thermodynamic equilibrium calculations to investigate the effects of bulk silica content variations in serpentinites on the dehydration reaction of antigorite + brucite = olivine + free fluid and silica content of this fluid phase. We develop a numerical model combining the effects of intrinsic chemical heterogeneities with reactive silica transport. Increasing temperatures lead to local fluid overpressure and the liberation of a silica-poor fluid in a subdomain with initially increased bulk iron and decreased silica content. The fluid overpressure drives fluid flow into other subdomains where the fluid enhances dehydration and leads to olivine enrichment in an iron-enriched vein. Our model shows how reactive silica transport can lead to vein widening and olivine enrichment within the veins as observed in the Erro Tobbio meta-serpentinites. Thus, reactive fluid flow is a critical step in the evolution towards a larger scale vein system and a dynamic porosity evolution by accounting for a chemical feedback between the dehydrating rock and the liberated fluid.

We developed a numerical thermodynamics laboratory called “Thermolab” to study the effects of the thermodynamic behavior of non-ideal solution models on reactive transport processes in open systems. The equations of state of internally consistent thermodynamic datasets are implemented in MATLAB functions and form the basis for calculating Gibbs energy. A linear algebraic approach is used in Thermolab to compute Gibbs energy of mixing for multi-component phases to study the impact of the non-ideality of solution models on transport processes. The Gibbs energies are benchmarked with experimental data, phase diagrams and other thermodynamic software. Constrained Gibbs minimization is exemplified with MATLAB codes and iterative refinement of composition of mixtures may be used to increase precision and accuracy. All needed transport variables such as densities, phase compositions, and chemical potentials are obtained from Gibbs energy of the stable phases after the minimization in Thermolab. We demonstrate the use of precomputed local equilibrium data obtained with Thermolab in reactive transport models. In reactive fluid flow the shape and the velocity of the reaction front vary depending on the non-linearity of the partitioning of a component in fluid and solid. We argue that non-ideality of solution models has to be taken into account and further explored in reactive transport models. Thermolab Gibbs energies can be used in Cahn-Hilliard models for non-linear diffusion and phase growth. This presents a transient process towards equilibrium and avoids computational problems arising during precomputing of equilibrium data.