FISSION-TRACK DATING OF A TEPHRA LAYER IN THE ALAT FORMATION OF THE DANDIERO GROUP (DANAKIL DEPRESSION, ERITREA)

Attempts to date a biotite separate from a tephra layer recognized near Buia (Danakil Depression, Eritrea) in the liwer part of the Homo remains – bearing Dandiero group (formerly attributed to the Danakil Formation) using the 39Ar/40Ar method failed because of xenocrystic contamination. For this reason it was applied the fission-track method on glass, since no other phases datable with this technique were present. The quality of glass was very poor for fission-track dating, because of the small size of grains. In addition, after polishing only few glass shards showed useful surfaces for track counting and only 25 spontaneous tracks were counted. The determined fission-track age - 0.75 +/- 0.16 Ma - is a rejuvenated age due to the presence of a certain amount of annealing of spontaneous tracks. An attempt to apply the plateau method for correcting this apparent age failed. A corrected age of 1.3 +/- 0.3 Ma was computed using the size-correction method. In spite of its low precision, this fission-track age represents a significant result, since it corroborates the attribution to Jaramillo Subchron of the normal magnetozone near the base of which the tephra is located.

Provenance studies of obsidian artifacts in Anatolia: new data from Agean costal sites

Baklatepe and Limantepe are settlements located along the Aegean coast of Anatolia in the Izmir region. The excavations revealed that these sites have been continuously inhabited since the Late Chalcolit hic up to the Early Byzant ine periods (Baklatepe) and since Neolithic t imes up to the present day (Limantepe) . In this study, 40 obsidian and 6 flint artifacts from the Chalcolithic and Early Bronze Age levels of these sites were investigated with an interdisciplinary approach using fission-track (FT)dating, and Instrumental Neutron Activation Analysis (INAA) techniques. Based upon thei r FTages the obsidian artifacts were classified in two groups. The main group of 37 samples is characterized by FT ages varying between 148 ± 0.47 and 1.80 ± 0.20 Ma,with a mean value of 1.65 ± 0.05 Ma, and low U content (around 3 ppm) These data allow attributing this group to the Aegean Melos Island obsidian sources. The second group is represented by only 3 samples, characterized by a lower FT age (around 1 Ma) and a significantly higher U content (around 9 ppm). This group was identi fied as origina t ing from the volcaniccomplex (Central Anatolia). INAA studies have been carried out on 33 obsidian artifacts at the TRIGA Mark II research reactor of the University of Pavia following the techniques which have been routinely used by the Radiochemist ry Group. Results fully agree with those obtained with FT dating: most artifa cts originated from the Melos -Demenegaki flow, and only 3 samples from Central Anatolia . INAA analyses of the f lint artifacts revealed that these samples, unsuitable for application of FT dating, can be easily discriminated from the obsidian artifacts based on their chemical prop erties, especially considering the trace elements contents. Compa rison with data obtained on samples from other settlements of Anato lia and Italy proved that INAA is an efficient technique for discriminating flint artifacts originat ing from different sources

New constraints on ages of glasses proposed as reference materials for fission-track dating

New analyses have been performed in order to enhance the data-set on the independent ages of four glasses that have been proposed as reference materials for fission-track dating. The results are as follows. Moldavite - repeated (40)Ar/(39)Ar age determinations on samples from deposits from Bohemia and Moravia yielded an average of 14.34 +/- 0.08 Ma. This datum agrees with other recent determinations and is significantly younger than the (40)Ar/(39)Ar age of 15.21 +/- 0.15 Ma determined in the early 1980s. Macusanite (Peru) -four K-Ar ages ranging from 5.44 +/- 0.06 to 5.72 +/- 0.12 Ma have been published previously. New (40)Ar/(39)Ar ages gave an average of 5.12 +/- 0.04 Ma. Plateau fission-track ages determined using the IRMM-540 certified glass and U and Th thin films for neutron fluence measurements agree better with these new (40)Ar/(39)Ar ages than the previously published ages. Roccastrada glass (Italy) - a new (40)Ar/(39)Ar age, 2.45 +/- 0.04 Ma, is consistent with previous determinations. The Quiron obsidian (Argentina) is a recently discovered glass that has been proposed as an additional reference material for its high spontaneous track density (around 100 000 cm(-2)). Defects that might produce spurious tracks are virtually absent. An independent (40)Ar/(39)Ar age of 8.77 +/- 0.09 Ma was determined and is recommended for this glass. We believe that these materials, which will be distributed upon request to fission-track groups, will be very useful for testing system calibrations and experimental procedures