Moldavite: an age standard for fission-track dating
Bulletin of Geosciences, volume 75, issue 2;
pages: 105 - 114;
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Moldavite is the only one glass accepted as an age standard for fission-track dating by I.U.G.S. with the recommendation that the standard should be a unique moldavite. This recommendation appears arduous to be respected, since samples are very rare. We have analysed using an absolute approach moldavites from seven deposits of different stratigraphical age, in order to identify the deposit bearing the less disturbed glasses, in relationship with partial annealing of spontaneous fission tracks. Moldavites showed variable amount of annealing, with raw ages, known as 'apparent' ages, varying between 12.7 ± 0.5 Ma and 15.4 ± 0.7 Ma with exception of the sample from Slavice which yielded very low apparent age. Corrected fission-track plateau ages showed a significantly lower dispersion (between 14.4 ± 0.6 Ma and 16.1 ± 0.6 Ma), with a mean of 15.1 Ma, well consistent with K-Ar and 40
Ar determined by various authors. The Middle Miocene deposit of Jankov yielded the less rejuvenated fissiontrack ages (mean apparent age 14.8 Ma, standard deviation, s.d. 0.4 Ma, mean plateau age 15.1 Ma, s.d. 0.3 Ma). Therefore, as there is no general agreement about the exact meaning of plateau ages, we have proposed a group of samples from Jankov as a reasonable substitute of a unique moldavite still to be identified and studied. Using moldavite for a zeta; calibration approach, a mean zeta; factor of 319.5 was computed for the sandwich SRM 612 NIST glass standard - muscovite, slightly lower than the zeta; factor referred to standard apatites (329.5). This might be consequence of a partial annealing of spontaneous fission tracks in standard apatites. Jankov moldavites were also dated using the new calibration system based on the recent EC-JRCIRMM-540 standard glass, which appears to yield slightly lower ages: a mean apparent age of 14.2 Ma with a s.d. of 0.3 Ma and a mean plateau age of 14.7 Ma with a s.d. of 0.2 Ma were obtained. To better evaluate accuracy of calibration systems, a better constrain of the reference age of moldavite is needed.