Thermal 40Ar/39Ar separation of diagenetic from detrital illitic clays in Gulf Coast shales

The 40Ar/39Ar age spectra for illite-rich, mixed-layered illite/smectite (I/S) from Texas Gulf Coast bentonites contain only a diagenetic component and are characteristically ‘staircase’-shaped. This spectrum shape contrasts with patterns seen for anchizonal- and epizonal-grade bentonites that are flatter. This may be due to the structural and compositional heterogeneity (mixed layering) typical of low-grade clays, especially with respect to K. On the other hand, shales from depths exceeding those of the smectite-to-illite transformation have both detrital and diagenetic components. Although diagenetic illite-rich I/S dominates shale fine fraction, detrital mica is observed even in the <0.2 μm size fraction. Age spectra of shales consist of two portions: (1) a staircase-shaped low temperature section, corresponding to degassing of the diagenetic illite-rich I/S component, and (2) a peak in the high temperature section, corresponding to degassing of well-crystallized detrital mica. The data obtained from 40Ar/39Ar stepwise analysis of these mixtures of diagenetic and detrital components permit at least partial resolution of the diagenetic from the detrital ages. We have developed techniques for quantifying the separate diagenetic and detrital components based on subtraction of the detrital contribution from the age spectrum of mixture, giving rise to an age corrected for the detrital contamination. The resulting corrected 40Ar/39Ar ages are closer to the diagenetic ages. This is in sharp contrast to the old K/Ar ages of the same samples. Although still in the developmental state, these techniques appear to represent a solution to the problem of contamination of diagenetic illitic clays by detrital illitic phyllosilicates.