Neon excess in pumice: volcanological implications

Abstract

We measured the noble gas abundance and Ne and Ar isotopic compositions in pumice samples from Japan and the Eolian Islands, Italy. The goal was to give clues on the mechanism of air incorporation into pumice and its relationship with eruption processes. The samples contain large excesses of atmospheric Ne over Ar, with 20 Ne/ 36 Ar ratios up to 600 times the atmospheric ratio. This excess is produced by the preferential diffusion of Ne over Ar in pumice vesicles during volcanic eruptions. The 20 Ne/ 36 Ar ratios measured in pumice reflect the initial quenching temperature, the cooling rate of the eruption column and the bubble's wall thickness. The final Ne amount is likely to be proportional to the volume of preserved vesicles in pumice. Calculations indicate that the fraction of isolated vesicles are less than 10% of the total, suggesting that a few amount of gas is preserved in pumice after deposition.