Multi-method approach to dating glass: The case of Basiluzzo Islet (Aeolian archipelago, Italy).

Abstract: A rhyolitic lava flow from Basiluzzo islet (Aeolian Islands), has been analysed with the Fission tracks (FT) and Ar40-Ar39 methods on glass, and with the U/Th method on whole rock to constrain its age and to compare the behaviour of different dating methodologies on glass samples late Quaternary in age. Laser Ar40-Ar39 total fusion analyses were performed on populations of grains. Due to the low yields of radiogenic Ar40 the age data are characterised by very high errors. The weighted average of the ages of the whole population is 55.7 +/- 8.7 ka (MSWD = 0.7). The isochron age calculated on all points is 40.6 +/- 11.4 ka (MSWD = 0.6), with an initial Ar40/Ar36 ratio of 297.8 +/- 1.8; the isochron is characterised by very little spread among points. The procedure named 'point-counting technique' was adopted in FT dating. Spontaneous track mean size resulted reduced by around 20% compared to induced tracks, which indicates that the determined FT age, 28.6 +/- 3.6 ka, is a reduced age, due to a certain amount of track annealing. For this reason the plateau technique for correcting thermally lowered ages was applied. We determined a plateau age (commonly assumed as a reliable estimate of the glass formation age) of 43.4 +/- 7.1 ka. Four sub-samples of whole rock from Basiluzzo lava flow have been analysed using U/Th isochron method. The U238/Th232 and Th230/Th232 activity ratios of sub-samples have been determined by alpha counting and plotted on an isochron diagram. The resulting age is 46+/- 8 ka and the U234/U238 activity ratios are always close to one, demonstrating that no significant processes of alteration have occurred. The relatively high error associated with the age is due to a low fractionation of U/Th ratio in the analysed whole rocks. The ages obtained with different methods, 43.4 +/-7.1 ka (FT plateau age), 40.6 +/- 11.4 ka (Ar40-Ar39 isochron age of all grains), and 46 +/- 8 ka (U/Th isochron) agree at the 1 sigma level, excluding a Holocene age for this sample. This could be valuable information for the Department of Civil Protection because it seems to mitigate the potential risk for present volcanic activity in the area. All ages are affected by very high analytical errors, which are due to the characteristics of the material analysed. Young ages result in low tracks numbers (FT dating) and barely detectable amounts of radiogenic Ar40 in the presence of high atmospheric contamination (Ar40-Ar39 dating). Stratigraphic successions without strict chronologic constraints might however benefit even from age data with low precision.