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

G.BIGAZZI, M.A. LAURENZI & J.G.VIRAMONTE

RADIATION MEASUREMENTS

Elsevier

Lugar: Holanda; Año: 2005 p. 613 - 616

1350-4487

Abstract   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 (24°19´S/66°43´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 .uence measurements, yielded an apparent age of 7.27 ± 0.29Ma (1o) and a plateau age of 8.99 ± 0.31Ma (1o). A 40Ar–39Ar isochron age on biotite of 8.61±0.04Ma (1o) 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 ± 0.09Ma, an isochron age of 8.71 ± 0.12Ma and an integrated age of 8.77 ± 0.09Ma for LTF analyses, and a w.a. of 8.75 ± 0.09Ma, an iso.a. of 8.77 ± 0.09Ma and an int.a. of 8.77 ± 0.09Ma for LSH analyses (all age errors are 2o). 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.