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 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 and a plateau age of .A ⁴⁰Ar–³⁹Ar isochron age on biotite of 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 , an isochron age of and an integrated age of for LTF analyses, and a w.a. of , an iso.a. of and an int.a. of for LSH analyses (all age errors are 2σ).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.     

Cosmic history of the Bragin pallasite: evidence from the fission-track analysis

Chronology is rather a weak point in the investigation of pallasites, the stony-iron meteorites. No chronological data are known for the Bragin pallasite. Our attempt to reconstruct its cosmic history was based on the interpretation of fission-track analysis data. To apply this method only uranium-rich phosphates can be used. Extremely rare grains of stanfieldite were extracted from the silicate sawing residue and from the pallasite sample directly.

The researches pursued by us made it possible to find two populations of fossil tracks in stanfieldite grains. The tracks of these populations strongly differed both in size, shape and character of distribution. The first population, consisting of short (L2–6 μm instead of L8–12 μm for induced fission tracks), round-shaped tracks irregularly distributed, as we suppose, suffered an intense heating process, which caused a significant amount of partial annealing. The second population, consisting of longer (L8–12 μm), rhombic-shaped tracks homogeneously distributed, occurred after this thermal event. Only the second population track density was used for the fission-track age calculation.

After correction of the fossil track density, consisting of the second population tracks, for other possible track sources, the revealed tracks were unequivocally identified as those due to the spontaneous fission of ²⁴⁴Pu and ²³⁸U. The largest part of them was attributed to the spontaneous fission of ²⁴⁴Pu; ρ_Pu/ρ_U≈3. The model fission-track age of the studied pallasite turned out to be 4.20 Gyr. This value fix the time of the last shock/thermal event in the cosmic history of the Bragin pallasite, which had caused the partial annealing of tracks presented to that time and “fission-track clock” reset.     

Mössbauer,X-ray fluorescence and paleomagnetic studies of deep-sea sediments from the Peru Basin: two million years of sedimentation history

Summary Sediment cores with different sub-bottom depths (I: 45 cm and II: 700 cm) from the Peru Basin have been investigated. From the depth profile of the relative amount of Fe(II) a redox zone is obtained which correlates with the organic carbon flux into the sediment (core I). M?ssbauer parameters suggest that the iron in the sediments is mainly contained in clay minerals and to varying extent also in goethite. Paper presented at ICAME-95, Rimini, 10–16 September 1995.