The leakage of radiogenic argon from sanidine

Abstract

The presence of unaltered sanidine in stratigraphically well-defined bentonite clay horizons offers an opportunity to date a sedimentary horizon by the Ar 40 K 40 method. It has been demonstrated that radiogenic argon losses of from 15–40 per cent may take place from microcline and orthoclase, and an evaluation of the argon retentivity of sanidine is necessary. The effects of variable temperature and grain-size in short-term (days) tests of argon leakage from pure sanidines were investigated. The results indicate that the radiogenic argon is quantitatively retained at temperatures below 400°C except, possibly, for the finest grain-sizes (− 325 mesh) and for sanidines with lattice irregularities. Argon loss above 400°C and below the melting temperature appears to involve the loss of at least two separate argon components for structurally inhomogeneous sanidine. Bentonitic sanidine (of homogeneous structure) showed only simple diffusion loss of argon. Argon loss from feldspars is discussed in terms of diffusion and lattice discontinuities. To test possible long term (millions of years) low-temperature losses of radiogenic argon from bentonitic sanidine, seven biotite-sanidine pairs, taken from bentonite clay horizons and ranging in age from ~65 to ~450 million years, were sampled and analysed. The sets of Ar 40 K 40 values obtained are the same within analytical error (± 5 per cent) over the entire range of ages and show that bentonitic sanidine retains radiogenic argon as well as comparable biotite. The data appear to indicate that fresh sanidine obtained from volcanic bentonite horizons retains radiogenic argon sufficiently well to yield reliable Ar 40 K 40 ages.