LASER 40AR-39AR DATING OF MICROGRAM-SIZE ILLITE SAMPLES AND IMPLICATIONS FOR THIN SECTION DATING

Abstract

Laser 40Ar-39Ar ages were obtained from a series of replicate illite samples ranging in mass from 14 to 77 μg using vacuum-encapsulated irradiation, and the results were compared with earlier analyses of the same material (Dong et al., 1995). This test confirmed that measured argon ages, in particular argon retention ages, are independent of sample size. Since the argon retention age does not require the 39Ar recoil gas fraction, it is clear that retention ages can be measured on illite within thin sections. Laser spot ages were obtained from thin sections of the same Welsh Basin shale analyzed as illite separates. Early results on untreated thin sections yielded a wide range of ages, some much younger than those measured previously. However, apparent age was negatively correlated with measured Cl/K ratios, suggesting the presence of a young, water soluble component which had been removed during mineral separation. Prolonged water rinsing of the thin sections reduced or virtually eliminated the young phase, yielding age results consistent with earlier work on illite separates. This indicates that dating of clays in thin section has the potential of being a powerful tool in unraveling complex histories of diagenesis and metamorphism.