New fission-track ages of Australasian tektites define two age groups: Discriminating between formation and reset ages

In this study tektites from across the Australasian strewn field (specifically, mainland Australia, Tasmania, Java, Malaysia) have been dated by the fission-track method. Twelve tektites from 9 localities define two age groups: one has a weighted mean age of 822 ± 20 ka based on 12 age determinations, and the other is 655 ± 20 ka based on 10 age determinations. The older group represents formation age of the tektites, and, given its error, is close to the precise ⁴⁰Ar/³⁹Ar age of the Australasian tektites at 788 ± 3 ka. Tektites of the younger group have a widespread distribution, occurring in Australia and southeast Asia. Their major- and trace-element composition is similar to the older tektites and their young age is thought to be due to heating subsequent to their original fall position, probably by wildfires that totally reset the fission-track clock. Soil surface temperatures, in the range of 500–800 °C, have been reported during experimental studies and would anneal all fission tracks in tektites within a few hours. The fission-track age record with its ~10% associated error on individual age determinations cannot precisely link thermal resetting to a particular elevated phase of wildfires. This task must await studies on high-resolution sedimentary sequences deposited around 650 ka. The view that these younger tektites were formed by a second impact event is discounted.