Application and limitation of fission-track geochronology to quarternary tephras

Abstract

Zircon and glass are the principal materials used for fission-track dating of Quaternary tephras. This is because zircon is a common accessory mineral in rhyolitic tephras, and it usually contains sufficient uranium to produce detectable quantities of spontaneous fission tracks over the time span of the Quaternary. Volcanic glass, though uranium-poor, is typically the dominant component of rhyolitic tephras, comprising up to 99% of some distal tephra-fall deposits. Low uranium contents, however, result in low spontaneous track densities that make the age measurement difficult and laborious, a condition exacerbated by very young glasses. In addition, fission tracks in glass shards are unstable and can partially anneal at ambient temperatures. Therefore, unless the glass age is corrected by plateau or isothermal heating experiments, partial annealing causes a reduction in the measured age compared to the true age of the sample. Fission-track dating has a large subjective component that can contribute systematic errors to the analysis; microlite inclusions and some structural defects resemble fission tracks, and if misidentified and included in the count of spontaneous tracks the calculated age would be too old. The most prevalent source of systematic error is contamination in the natural environment, for many tephras in the stratigraphic record are reworked tuffaceous sediments. These deposits may contain detrital components that could mask the primary age of the tephra. However, by using the external detector fission-track method it is possible to calculate ages of single mineral grains, and to distinguish contaminants from primary phases on the basis of their age distribution. This ‘grain-discrete’ capability is the major advantage of the fission-track method over ‘bulk’ geochronological systems.