Národní geovědní bibliografie - GeoLib: Podrobnosti záznamu

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 moldaite. 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, know 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 - 39 Ar determined by various authors.

The Middle Miocene deposit of Jankov yielded the less rejuvenated fission-track 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 calibration approach, a mean factor of 319.5 was computed for the sandwich SRM 612 NIST glass standard - muscovite, slightly lower than the factor referred to standard apatites (329.5). This might be consequence of a partial annealing of spontaneous fission 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. The better evaluate accuracy of calibration systems, a better constrain of the reference age of moldavite is needed

Přispěvatel

Česká geologická služba

Kód přispěvatele

ČGS (UNM)

Zdrojový formát

Datum vložení

11. 6. 2007

Datum importu

8. 8. 2012