NEW U-SERIES DATING OF A CARBONATE LEVEL UNDERLYING THE PEPERINO ALBANO PHREATOMAGMATIC IGNIMBRITE (COLLI ALBANI, ITALY)

Abstract

Increasing attention has been recently devoted to the last phases of the Colli Albani activity (Rome, Italy), in consideration of new stratigraphic data suggesting a possible Holocene activity of the volcanic complex located few kilometres to the south of the city of Rome. In particular the focus has been directed to the activity of the Albano maar and related pyroclastic deposits, the youngest eruptive centre of the Colli Albani volcano.
The most recent dated product from the Albano maar is known as Peperino Albano (PA) ignimbrite, which has been dated with several
different techniques. The age of the PA spans between 51 and 19 ka. A recent study has shown that PA is overlain by at least other
two pyroclastic deposits and several lahar deposits, all separated by paleosoils, making the recent eruptive history of the Albano maar
rather younger than previously thought. The exact age determination of the recent succession is therefore a key issue to be tackled in
order to evaluate the volcanic hazard in the Roman region.
The aim of this paper is to improve the chronology of the last eruptive episodes of the Albano maar, by dating a carbonate layer interbedded within the recent deposits using U-series disequilibria. The idea came out from the discordant results obtained by applying different dating methods to PA ignimbrite as a consequence of the difficulties connected with the application of classical isotope chronological methods to young volcanic products.
In order to date the carbonate layer, the total sample dissolution technique (TSD) has been preferred to using leachates alone because
the leaching method gives reliable results only in the case of simple dissolution of the carbonate fraction, without any removal of U and Th isotopes from the detritic component. Three coeval sub-samples from the carbonate level underlying PA have been analysed, giving an age of 22.9 ± 6.7 ka, younger than other previous chronological data. The age limit of about 23 ka found for PA supports the
hypothesis of a possible very recent activity of the Albano maar, justifing the increasing efforts devoted to improving the chronology of
the last phreatomagmatic phases of Colli Albani volcano.