Reference materials for Fission-Track Datig od natural glasses

The mutual understanding between geochronologists and users might be improved if the latter are enabled to estimate the accuracy of a given dating methodology. One of the grounds of accurate dating system calibrations is the availability of age standards. Only standards allow comparing data produced by different laboratories and/or with different dating methods. During last years much effort has been made to make available a set of reference materials for fission-track (FT) dating of glasses, representing the various experimental conditions one can meet. New analyses have been performed to improve the reference age data-set of these proposed reference glasses. Moldavite - Repeated 40Ar/39Ar age determinations on samples from Bohemian and Moravian deposits yielded a weighted average of 14.34 ± 0.08 Ma. This datum is significantly lower than the 40Ar/39Ar plateau age of 15.21 ± 0.15 Ma, which was considered for a long time the reference aae for this glass. FT age determinations using the recent IRMM - 540 standards glass and U and Th thin films for neutron fluence measurements yielded mean apparent ages of 14.03 ± 0.15 and 13.38 ± 0.66 Ma and mean plateau-corrected ages of 14.38 ± 0.16 and 13.98 ± 0.58 Ma, respectively. These plateau ages better agree with the new 40Ar/39Ar ages than with the previous one cited above. Macusanite (obsidian from the Macusani volcanic field, SE Peru) - New 40Ar/MAr ages have an average of 5.12 ± 0.02 Ma, significantly lower than the previously available K-Ar age (average 5.53 ± 0.06 Ma). Apparent and plateau ages of Macusanites using the same neutron dosimetres as for Moldavite have averages of 4.15 ± 0.09, 3.52 ± 0.07 Ma and 5.20 ±0.11 and 4.64 ± 0.17 Ma, respectively. Also for Macusanite these plateau ages better agree with the new 40Ar/39Ar ages than with previous ages. Roccastrada (population of glass shards separated from the Roccastrada rhyolite, Tuscany, Italy) - a new 40Ar/xAr age - 2.51 ± 0.02 Ma (average of 2 measures) - well consistent with the previous determination on sanidine (2.48 ± 0.05 Ma, recalculated with a new value for the FCT bt standard, is also available for this glass. Quiron (obsidian from the El Quevar volcanic complex, Salta Province, Argentina) - Quiron is a recently discovered phenocryst poor glass which has been prop6sed as an additional reference material for its high spontaneous track density. 40Ar/39Ar ages of 8.75 ± 0.09 (step-heating analysis) and 8.77 ± 0.09 Ma (laser total fusion analysis) were determined. Quiron glass yielded FT apparent and plateau ages of 7.27 ± 0.29 and 8.99 ± 0.31 Ma, respectively. All 40Ar/39Ar ages are given ±20 whereas FT ages are given ± 1. We believe that these glasses we distribute upon request may turn very useful for testing FT system calibrations and experimental procedures. It should be a good practice for analists to inform users on the performance of their system calibration on reference materials.