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

The Ar-40/Ar-39 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 contrast s 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-i llite 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 mu m size fraction. Age spectra of shales consi st of two portions: (1) a staircase-shaped low temperature section, corresp onding to degassing of the diagenetic illite-rich I/S component, and (2) a peak in the high temperature section, corresponding to degassing of well-cr ystallized detrital mica. The data obtained from Ar-40/Ar-39 stepwise analy sis of these mixtures of diagenetic and detrital components permit at least partial resolution of the diagenetic from the detrital ages. We have devel oped techniques for quantifying the separate diagenetic and detrital compon ents based on subtraction of the detrital contribution From the age spectru m of mixture, giving rise to an age corrected for the detrital contaminatio n. The resulting corrected Ar-40/Ar-39 ages are closer to the diagenetic ag es. This is in sharp contrast to the old K/Ar ages of the same samples. Alt hough still in the developmental state, these techniques appear to represen t a solution to the problem of contamination of diagenetic illitic clays by detrital illitic phyllosilicates, (C) 2000 Elsevier Science B.V. All right s reserved.