Study Of Volcanic Risk Through 40Ar/39Ar Elevated Precision Datings Of Neapolitan And Phlegraean Volcanic Deposits

The Neapolitan area extends, virtually unbroken, from the Bay of Naples to the Bay of Pozzuoli hosting some of the most densely populated districts in Europe. The geological nature of this volcanic territory, and the presence of three active volcanic zones (Vesuvius, Campi Flegrei, and Ischia), poses a high risk for all the inhabitants (1.5 millions) and the tourists. The autochthonous volcanism in the Neapolitan urban area has been investigated to fully evaluate the volcanic risk in this urban environment. That has been made through 40Ar/39Ar elevated precision dating on samples obtained from eight volcanic deposits cropping out in the Neapolitan area. By using this dating technique, it is possible to obtain radiometric ages with a 2σ less than 1000 years. Instead, through stratigraphic hypothesis has been only stated that the volcanism in the Neapolitan urban area started before the Campanian Ignimbrite eruption (39 ka) and possibly ended about few thousand years ago with minor eruptions building small tuff cones. Therefore, there have been measured, with the high sensitivity MAP216 mass spectrometer, argon isotopes extracted stepwise thermally from those samples previously irradiated. Then, from these measurements, the ages were evaluated in two ways. One way is to plot all the apparent ages vs the cumulative 39Ar released (Age Spectrum), to calculate a Weighted Mean Plateau Age (WMPA) with a 2σ precision. The other way is to plot the 40Ar/36Ar ratio vs the 39Ar/36Ar ratio on an Isotope Correlation (Isochron) Plot, to calculate an Isochron Age (IA) with a 2σ precision. The oldest 40Ar/39Ar predates local tuff cones above 78.43 ± 0.57 ka ago, and the youngest below 15.56 ± 0.36 ka. These results testify the existence of a volcanic field in the Neapolitan area active for a much wider period of time than previously believed