Alteration of a basaltic glass in an argillaceous medium: The Salagou dikeof the Lodeve Permian Basin (France). Analogy with an underground nuclear waste repository

Volcanic basaltic glasses are commonly considered to be very suitable natur al analogs of vitrified waste packages intended for geological disposal. Th e studied basaltic glass of the Salagou dike (Lodeve Basin, France) intrude d into a Permian argillaceous formation about 1.4 Ma ago, provides a means of assessing the long-term behavior of nuclear waste glass in an argillaceo us repository concept. This study is based on combining chemical, mineralog ical and isotopic investigations. The occurrence of a palagonite film no mo re than 1.2 mm thick characterizes the basaltic glass alteration in contact with the argillaceous host rock. The chemical and structural properties; o f the palagonite suggest constant volume alteration. The alteration rates e stimated from palagonite thickness and age of the glass are comparable to t hose measured on natural glasses altered in nonargillaceous media The occur rence of the studied argillaceous material in contact with the basaltic gla ss did not result in long-term alteration kinetics significantly different from those measured in simple glass/water systems. Mineralogical, chemical, and isotopic data obtained on the basaltic and argillaceous components sug gest that an aqueous fluid flowed primarily at the glass/argillite interfac e and did not propagate in the argillaceous host rock beyond 5 cm from the basaltic dike. The elements released by alteration of the basaltic glass-no tably strontium-did not diffuse into the surrounding clay. This conclusion is important from the perspective of a natural analog for a deep geological repository for nuclear waste, and highlights the major role of the structu ral properties of the clayey barrier. Copyright (C) 2001 Elsevier Science L td.