LASER 40AR/39AR MICROPROBE ANALYSES OF FINE-GRAINED ILLITE

Abstract

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 laser microprobe. The integrated ages of the illite agreed within error with conventional 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 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 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.