AGE AND URANIUM CONTENT OF SOIL MICAS FROM ANTARCTICA BY THE FISSION PARTICLE TRACK REPLICA METHOD

Abstract

A replica method was developed for fission particle track age dating and determination of uranium content of micas in a Wright Valley, Antarctica, soil and parent rock. The mica particles in the soil were relatively small, fragile, and easily dissolved by chemical etchings. Muscovite mica flakes were therefore used as a replicating substrate to register induced fission tracks of the soil mica. The micas from three soil horizons (0–10, 10–30, and 30–50 cm depths) and parent rock, in four size fractions (100–500,2–5, 0.2–2, and < 0.2μm) were examined to illustrate use of the method. The fission track age of the soil mica particles was 4.1±0.2 million years (MY), slightly older than the minimum landscape (glacial bench) age of 3.7 ± 0.2 MY obtained previously by K-Ar dating of a volcanic cone formed after the glacial valley was formed. The ages of the 2–5,0.2–2, and < 0.2μm fractions were the same as for the 100–500 μm flakes, when the spontaneous track density was calculated from that of the latter, in proportion to the U content. The fission track age of parent rock mica was 151 million years. The U content of the soil mica was 0.5 to 1.6 ppm which is 20 to 50 times higher than that of mica from the parent rock (0.03 ppm). The mica in the soil appears from age and petrographie thin-section results to be of pedogenic origin, formed by replacement in feldspars after the bench was carved. The fission particle track replica method is useful for age-dating of micas in soils, for determining trace amounts of uranium in micas, and for study of soil genesis in the Antarctic cold desert.