The obsidian from Quiron (Salta province, Argentina): a new reference glass for fission-track dating.

In the course of a geochronological study of the volcanic activity in the Andean Cordillera in northern Argentina, we have found in the El Quevar volcanic complex (24º19’S/66º43’W,6180m) a phenocryst poor obsidian (Quiron obsidian) showing an unusually high spontaneous track density. Defects which might produce “spurious” tracks are virtually absent. Application of fission-track dating using an absolute approach, based on the IRMM-540 standard glass for neutron fluence measurements, yielded an apparent age of 7.27±0.29 Ma(1 sigma) and a plateau age of 8.99±0.31 Ma(1 sigma). A 40Ar–-39Ar isochron age on biotite of 8.61±0.04 Ma(1 sigma) was already available for the Quiron rhyolite. We determined further 40Ar–-39Ar ages on several chips of the glass itself using two analytical approaches: total fusion with a focussed laser beam (LTFA) and a step-heating approach using a de-focussed laser beam (LSHA). We have obtained a weighted average of 8.77±0.09 Ma, an isochron age of 8.71±0.12 Ma and an integrated age of 8.77±0.09 Ma for LTF analyses, and a w.a. of 8.75±0.09 Ma, an iso.a. of 8.77±0.09 Ma and an int.a. of 8.77±0.09 Ma for LSH analyses (all age errors are 2 sigma ). The Quiron obsidian is very easy to analyse for its high spontaneous track density and because microlites which might produce spurious tracks are very rare. Independent reference 40Ar–-39Ar ages determined in different laboratories are available. For these reasons we believe that this glass may be very useful for testing fission-track system calibrations and apparent age correction procedures. Splits of obsidian Quiron will be distributed upon request to colleagues who intend to test it.