The dependence of K-Ar age on the mineralogy of various particle size ranges in a shale

Details The dependence of K-Ar age on the mineralogy of various particle size ranges in a shale The dependence of K-Ar age on the mineralogy of various particle size ranges in a shale

May 1963

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1963gecoa..27..405h&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 27, Issue 5, pp.405-410

Computer Science

11

Scientific paper

The mineralogy and K-Ar age of the whole rock and six size fractions, ranging from <0.08-62 , have been determined for the Upper Ordovician Sylvan shale. The whole rock gave an age value of 450 m.y., which is an approximate agreement with the estimated true age of sedimentation. However, this agreement is fortuitous, as the different size fractions show a spread in age values from 350 m.y. for the finest to 540 m.y. for the coarsest. The shale contains the following minerals, in approximate order of abundance: 10 Å layer silicate plus mixed-layer illite/montmorillonite, quartz, chlorite, kaolinite, and very small amounts of dolomite and feldspar. The 10 Å plus mixed-layer material is concentrated in the fine sizes and is the exclusive material in the finest size range. At least a portion of the 10 Å structure in the coarse sizes is of the 2 M muscovite polytype--it is evidently detrital muscovite carrying inherited radiogenic argon, which accounts for the high ages of the coarse size material. The 10 Å material in the finer sizes has the disordered IMd muscovite structure. The IMd material could be diagenetic and its low age caused by its formation much later than the time of sedimentation, or because it leaks argon. It could also be detritus disordered by degradation and its low age caused by argon leakage which overcompensates for inherited radiogenic argon. Whatever the cause of the low age of the IMd material, it is obvious that the whole rock age is not directly indicative of the age of sedimentation, as it is composed of a mixture of ages in the different size fractions of the shale.

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