Propagation of high-alkaline fluids in an argillaceous formation: Case study of the Khushaym Matruk natural analogue (Central Jordan)

The Khushaym Matruk site in Central Jordan is investigated as a natural analogue of a deep geological repository for intermediate to high-level radioactive wastes to discuss the impact of high-alkaline solutions in an argillaceous formation. The studied site is characterised by Maestrichtian bituminous-rich marls, consisting of 66–83% of carbonates and 10–15% of clays of mixed-layers illite–smectite type (I–S), overlapped by a marble/cement unit formed by spontaneous in situ combustion of the sedimentary organic matter in sub-surface conditions. This high-temperature event produced marble and cementitious phases typical to those observed in industrial cements whose alteration generates high-alkaline solutions. Evidences of fluid circulations in Khushaym Matruk site are underlined by secondary minerals, mainly calcite and gypsum, neoformed in cracks and micro-cracks crosscutting the bituminous-rich marls, and in travertines capping them. The chemical and Sr isotopic data of these phases are characteristics of high-alkaline fluids generated by alteration of the cementitious phases and having percolated with partial equilibration in the underlying bituminous-rich marls. The propagation of these alkaline solutions by structural discontinuity paths in the clayey formation is underlined over distances greater than 5 m. Also, the mineralogical and petrographic characterisation of the bituminous-rich marls, far and close to the marble/cement unit, emphasizes slight variations in mineral contents, a few metres close to the marble/cement contact, with notably a decrease of I–S crystallinity, a change from beideillite to montmorillonite type and, calcite and zeolite recrystallisation. Together, textural disturbances of these rocks, underlined by micro-fissuration of the sedimentary matrix, are observed. A pervasive circulation of high-alkaline fluids in the argillaceous matrix is proposed to explain these changes and is underlined over a distance of a few metres under the contact with the marble/cement unit. Cathodoluminescence observations and U–Th radioactive disequilibrium dating applied on infilling calcite cracks indicate that these circulations were not continuous but occurred in several steps with a mean age of 110,000–130,000 years.