Laser ⁴⁰Ar/³⁹Ar microprobe analyses of fine-grained illite

Fine-grained (<0.02 μm) to coarse-grained (2.0-0.2 μm) illite separates and finely powdered muscovite standards were analyzed with a microencapsulation technique and an ⁴⁰Ar/³⁹Ar laser microprobe. The integrated ages of the illite agreed within error with conventional K/Ar analyses, even though the sample sizes, 1-100 micrograms, were at least a 10,000-fold less. Incremental laser heating of an artificial mixture of illite and muscovite of two different ages yielded a stair step profile, where the youngest and oldest incremental ages approximately coincided with their K/Ar ages. The thermally activated argon release rate from illite was distinct from that of the muscovite and may result from differences in grain thickness, lower K concentration, and the presence of cis vs. trans-sited vacancies. Incremental heating, therefore, may prove capable of delineating detrital from authigenic components in illite extracted from shale and sandstone. Microencapsulation and laser ⁴⁰Ar/³⁹Ar analyses, when combined with sophisticated techniques for separating clays, will permit dating of samples where clay is a minor constituent, such as sandstones and meteorites, and will enhance identification of endmember ages in naturally occurring clay.