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TOWARDS AN ISOTOPIC MODELING OF THE ILLITIZATION PROCESS BASED ON DATA OF ILLITE-TYPE FUNDAMENTAL PARTICLES FROM MIXED-LAYER ILLITE-SMECTITE

Centre de Géochimie de la Surface (CNRS/ULP), 1 rue Blessig, 67084 Strasbourg, France

^* E-mail address of corresponding author: nclauer{at}illite.u-strasbg.fr

Burial-induced and hydrothermal-related illitization in bentonites and in sandstones can be modeled on the basis of isotopic studies of fundamental particles separated from mixed-layer illite-smectite. The model envisages different reaction rates and durations relative to the varied impacts of temperature, considering that the water:rock ratio also has an influence. The different pathways for illitization are suggested on the basis of the K-Ar, Rb-Sr and ¹⁸O compositions of previously studied materials.

New information is provided on why fundamental particles separated from mixed-layer illite-smectite in shales yield K-Ar age data that are systematically greater than the ages of the fundamental particles from associated bentonites and/or sandstones, and greater than the reported stratigraphic ages. The study of pure authigenic, recent to present-day smectite from Pacific sediments shows that (1) those collected from active hydrothermal vents have ⁴⁰Ar/³⁶Ar ratios identical to that of the atmosphere, and (2) those of mud sediments have ⁴⁰Ar/³⁶Ar ratios above the atmospheric value, indicating addition of ⁴⁰Ar not generated in situ by radioactive decay. A preliminary but detailed analysis of the noble-gas (Ar, Xe, Kr) contents of authigenic smectite-rich size fractions from Pacific deep-sea red clays suggests trapping of these gases by smectite. Therefore, the results point to the fact that fundamental particles can incorporate excess ⁴⁰Ar into their structure when nucleating in restricted to closed systems, such as shales. This excess ⁴⁰Ar, which represents radiogenic ⁴⁰Ar released from nearby altered silicates, might be temporarily adsorbed at the surface of the rock pore spaces and is therefore available for incorporation in nucleating and growing particles.

Key Words: Burial Diagenesis • Crystal Growth • Fundamental Particles • Hydrothermal Activity Illitization • K-Ar • Mixed-layer Illite-smectite • Noble Gases • Nucleation • ¹⁸O • Rb-Sr