A test sample for fission-track dating of glass shards

Abstract--Four samples from the Roccastrada rhyolites (Italy) have been analysed with different dating methods. Our purpose was to identify a glass which is adequate for checking different techniques for dating glass shards with the fission-track (FT) method. The FT apparent ages (between 1.74 and 2.01 Ma) are significantly lower than the ages measured with other techniques: Rb-Sr (whole rock-biotite), 2.48 +/- 0.04 and 2.40 +/- 0.04 Ma; K-Ar (glass), 2.49 +/- 0.03 and 2.65 +/- 0.05 Ma; 40Ar-39Ar (sanidine), 2.446 +/- 0.026, 2.453 +/- 0.027, and 2.434 +/- 0.026Ma. The FT size-corrected ages (between 2.44 +/- 0.14 and 2.65 +/- 0.19 Ma) and the FT plateau ages (between 2.50 +/- 0.12 and 2.55 +/- 0.06 Ma) agree with the above ages obtained with other methods. The Roccastrada rhyolitic glass shows very good characteristics for FT dating: (1) it is a very clear glass, without defects which could produce "spurious" tracks; (2) the spontaneous track density is relatively high (about 20,000 cm-2); and (3) the uranium content does not show inhomogeneities. For these reasons, this glass is a good putative "test sample" to check counting strategies and apparent age correction techniques applied to glass shards.