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

摘要

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.