TY - JOUR
T1 - Laser 40Ar/39Ar microprobe analyses of fine-grained illite
AU - Onstott, Tullis C.
AU - Mueller, C.
AU - Vrolijk, P. J.
AU - Pevear, D. R.
N1 - Funding Information:
Acknowledgments--Partial support for TCO was provided by grant DE-AC06-76RL01830 from the Subsurface Science Program of the U.S. Department of Energy. The authors thank Chris Hall and Jim Aronson for their helpful reviews.
PY - 1997/9
Y1 - 1997/9
N2 - 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.
AB - 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.
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U2 - 10.1016/s0016-7037(97)00288-3
DO - 10.1016/s0016-7037(97)00288-3
M3 - Article
AN - SCOPUS:0031422229
SN - 0016-7037
VL - 61
SP - 3851
EP - 3861
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 18
ER -