New zircon fission-track ages from New Zealand Quaternary tephra: an interlaboratory experiment and recommendations for the determination of young ages

Abstract

Fission-track dating, of zircons from five Pleistocene tephra in the Wanganui Basin, North Island, New Zealand, has been carried out independently in two different laboratories. The results from each are statistically identical even though there were some differences in laboratory techniques. The resulting ages confirm recent isothermal plateau fission-track (ITPFT) ages on two of these tephra, but with increased precision. The weighted mean ages (± 1σ) are: Potaka Tephra, 0.97 ± 0.04 Ma (4 determinations); Rewa Pumice, 1.00 ± 0.06 Ma (3 determinations); Mangapipi Ash, 1.26 ± 0.08 Ma (2 determinations); Pakihikura Pumice, 1.66 ± 0.07 Ma (4 determinations); and the Vinegar Hill Tephra, 1.73 ± 0.08 Ma (3 determinations). These new results further constrain the time control in this important Pleistocene section which contains many cyclothems reflecting climatic and tectonic influences. This study illustrates well the pitfalls that can occur when dating such young material using single-crystal techniques. The difficulties of determining when etching is optimal, the presence of detrital grains and their subsequent removal from the data set are described, as well as a recommendation that a minimum of 15 crystals, but optimally at least 20, be counted before calculating the final age. It may also be necessary to plot running means of the single-crystal ages to determine when the final age has been approached.