Long-term modeling of alteration-transport coupling: Application to a fractured Roman glass Aure ´lie Verney-Carron a,b, * , Ste ´phane Gin a , Pierre Frugier a , Guy Libourel b,c a Commissariat a ` l’E ´ nergie Atomique, Marcoule, DEN/SECM/LCLT, BP 17171, 30207 Bagnols-sur-Ce `ze Cedex, France b Centre de Recherches Pe ´trographiques et Ge ´ochimiques, Nancy-Universite ´, CNRS/UPR 2300, BP 20, 54501 Vandoeuvre-le `s-Nancy, France c Ecole Nationale Supe ´rieure de Ge ´ologie, Nancy-Universite ´, INPL, BP 40, 54501 Vandoeuvre-le `s-Nancy, France Received 18 March 2009; accepted in revised form 4 January 2010; available online 11 January 2010 Abstract To improve confidence in glass alteration models, as used in nuclear and natural applications, their long-term predictive capacity has to be validated. For this purpose, we develop a new model that couples geochemical reactions with transport and use a fractured archaeological glass block that has been altered for 1800 years under well-constrained conditions in order to test the capacity of the model. The chemical model considers three steps in the alteration process: (1) formation of a hydrated glass by interdiffusion, whose kinetics are controlled by a pH and temperature dependent diffusion coefficient; (2) the dissolution of the hydrated glass, whose kinetics are based on an affinity law; (3) the precipitation of secondary phases if thermodynamic saturation is reached. All kinetic parameters were determined from experiments. The model was initially tested on alteration experiments in different solutions (pure water, Tris, seawater). It was then coupled with diffusive transport in solution to simulate alter- ation in cracks within the glass. Results of the simulations run over 1800 years are in good agreement with archaeological glass block observations concerning the nature of alteration products (hydrated glass, smectites, and carbonates) and crack alteration thicknesses. External cracks in direct contact with renewed seawater were altered at the forward dissolution rate and are filled with smectites (400500 lm). Internal cracks are less altered (by 1 or 2 orders of magnitude) because of the strong coupling between alteration chemistry and transport. The initial crack aperture, the distance to the surface, and sealing by secondary phases account for these low alteration thicknesses. The agreement between simulations and observations thus val- idates the predictive capacity of this coupled geochemical model and increases more generally the robustness and confidence in glass alteration models to predict long-term behavior of nuclear waste in geological disposal or natural glass in the environment. Ó 2010 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Geochemical modeling of glass and silicate mineral alteration is a major challenge for understanding natural processes in the earth sciences (e.g. chemical compositions of natural waters and oceans, global cycles of elements...) and also for industrial application such as prediction of the long-term behavior of vitrified nuclear waste in a geo- logical repository. Different mechanisms have been investigated to explain the interactions between solution and glass and the release of elements in solution. (1) First an ion exchange or inter- diffusion mechanism occurs. The glass modifier cations, such as alkalis metals, are replaced by hydrogen species (H 3 O + ,H 2 O) in solution resulting in a selective leaching and an increase in pH. This mechanism has been shown to work particularly in alkali silicate glass (Rana and Douglas, 1961a,b; Doremus, 1975; Lanford et al., 1979; 0016-7037/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.01.001 * Corresponding author. Address: Commissariat a ` l’E ´ nergie Atomique, Marcoule, DEN/SECM/LCLT, BP 17171, 30207 Bag- nols-sur-Ce `ze Cedex, France. Tel.: +33 3 83 59 42 11; fax: +33 3 83 51 17 98. E-mail address: verney@crpg.cnrs-nancy.fr (A. Verney-Carron). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 74 (2010) 2291–2315