Numerical geochemical modelling of basalt-water interaction under subcritical conditions

A comprehensive hydrothermal-geochemical model was built in TOUGHREACT (TM) to match a detailed laboratory simulation of the interaction between tholeiitic basalt from the Elvarpahraun flow of the Svartsengi volcanic system in the Reykjanes Peninsula, Iceland and distilled de-oxygenated water. The experiment was done under subcritical conditions (350 degrees C and 490 bar), conditions associated with seafloor spreading centres and mid-ocean ridges. This modelling study was used to develop methodologies for building reactive transport models in TOUGH-REACT (TM) with a particular focus on the parameters for reaction thermodynamics and reaction kinetics at high temperature and pressure conditions. The validation and update of geochemical assumptions are also integral to the methodology. Equilibrium constants for mineral and aqueous reactions were estimated using SUPCRTBL by Zimmer et al. (2016). Mineral dissolution rates were primarily adopted from Palandri and Kharaka (2004), while precipitation rates were estimated following the work of Horiuti (1957). Justifications for the modelling decisions made are presented in detail. Based on the model, precipitation rates are generally higher than expected rates from the estimated kinetic parameters and mineral surface area estimates, while there is agreement in terms of dissolution rates. Mineral solid solutions dissolved stoichiometrically, except bytownite, whose albite component was preferentially dissolved. The volume of basalt glass in the sample was higher than experimental estimates, based on the silica concentration trends. Sulfide inclusions in the glass also significantly affected the observed sulfate and sulfide trends while the glass dissolved. The potassium concentration trends in the effluents indicate the presence of K-feldspar. Lastly, based on the model results, the bulk flow rates were observed to affect the overall reaction rates.