NEW ZIRCON FISSION-TRACK AGES FROM NEW-ZEALAND QUATERNARY TEPHRA - ANINTERLABORATORY EXPERIMENT AND RECOMMENDATIONS FOR THE DETERMINATION OF YOUNG AGES

Fission-track dating of zircons from five Pleistocene tephra in the Wa nganui Basin, North Island, New Zealand, has been carried out independ ently in two different laboratories. The results from each are statist ically identical even though then were some differences in laboratory techniques. The resulting ages confirm recent isothermal plateau fissi on-track (ITPFT) ages on two of these tephra, but with increased preci sion. The weighted mean ages (+/- 1 sigma) are: Potaka Tephra, 0.97 +/ - 0.04 Ma (4 determinations); Rewa Pumice, 1.00 +/- 0.06 Ma (3 determi nations); Mangapipi Ash, 1.26 +/- 0.08 Ma (2 determinations);; Pakihik ura 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 whic h 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 grain s and their subsequent removal from the data set are described, as wel l 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 determin e when the final age has been approached. (C) 1997 Elsevier Science B. V.