40 Ar/ 39 Ar and 14 C geochronology of the Albano maar deposits: Implications for defining the age and eruptive style of the most recent explosive activity at Colli Albani Volcanic District, Central Italy B. Giaccio a, ⁎, F. Marra b , I. Hajdas c , D.B. Karner d , P.R. Renne e , A. Sposato a a Istituto di Geologia Ambientale e Geoingegneria, CNR, Area della Ricerca RM1-Montelibretti, Via Salaria km 29,300, 00016 Monterotondo Stazione (Roma), Italy b Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy c Ion Beam Physics, ETH, Schafmattstrasse 20, CH-8093 Zürich, CH, Switzerland d Department of Geology, Sonoma State University,1801 East Cotati Avenue, Rohnert Park, CA 94985, USA e Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, U.S.A. and Department of Earth and Planetary Sciences, University of California, Berkeley, CA 94720, USA abstract article info Article history: Received 1 December 2008 Accepted 20 May 2009 Available online 2 June 2009 Keywords: 40 Ar/ 39 Ar 14 C geochronology Albano maar Central Italy New 40 Ar/ 39 Ar and 14 C ages have been found for the Albano multiple maar pyroclastic units and underlying paleosols to document the most recent explosive activity in the Colli Albani Volcanic District (CAVD) near Rome, Italy, consisting of seven eruptions (Albano 1 = oldest). Both dating methodologies have been applied on several proximal units and on four mid-distal fall/surge deposits, the latter correlated, according to two current different views, to either the Albano or the Campi di Annibale hydromagmatic center. The 40 Ar/ 39 Ar ages on leucite phenocrysts from the mid-distal units yielded ages of ca. 72 ka, 73 ka, 41 ka and 36 ka BP, which are indistinguishable from the previously determined 40 Ar/ 39 Ar ages of the proximal Albano units 1, 2, 5 and 7, thus confirming their stratigraphic correspondence. Twenty-one 14 C ages of the paleosols beneath Albano units 3, 5, 6 and 7 were found for samples collected from 13 proximal and distal sections, some of which were the same sections sampled for 40 Ar/ 39 Ar measurements. The 14 C ages were found to be stratigraphically inconsistent and highly scattered, and were systematically younger than the 40 Ar/ 39 Ar ages, ranging from 35 ka to 3 ka. Considering the significant consistence of the 40 Ar/ 39 Ar chronological framework, we interpret the scattered and contradictory 14 C ages to be the result of a variable contamination of the paleosols by younger organic carbon deriving from the superficial soil horizons. These results suggest that multiple isotopic systems anchored to a robust stratigraphic framework may need to be employed to determine accurately the geochronology of the CAVD as well as other volcanic districts. © 2009 Elsevier B.V. All rights reserved. 1. Introduction and general background Hazard evaluation in active volcanic areas requires documentation on the style, magnitude, source area and frequency of the eruptions and other dangerous volcanic phenomena (e.g. gas emission, sin- or post-eruptive lahars, etc.). However, based on the strict definition proposed in the Smithsonian Institution's catalogue of active volcanoes, only volcanoes that have erupted in the last 10,000 years should be considered active. Alternatively, it is considered that a volcano is dormant when the time elapsed since its last eruption does not exceed the average recurrence period of its past activity. Establishing the age of the most recent activity that occurred at the Colli Albani Volcanic District, central Italy, documented at the Albano multiple maar (De Rita et al., 1995a,b; Funiciello et al., 2002, 2003; Freda et al., 2006; De Benedetti et al., 2008), is therefore fundamental to evaluate the time elapsed since the last eruptive event as well as its activity status. This has a relevant implication on the assessment of the potential hazard for the city of Rome, whose southeastern suburb reaches close to the slopes of the Albano crater. Though many radioisotopic age determinations were made near the end of the last Century for the Colli Albani Volcanic District (for a review see Voltaggio and Barbieri, 1995), these were not the result of systematic study. Rather, they were a collection of data from many different sources, which often yielded contrasting results depending on the applied dating methodology. Therefore, uncertainty persisted regarding the age of the latest volcanic activity (e.g. De Rita et al., 1995a). Karner et al. (2001) carried out a 40 Ar/ 39 Ar age study of the Monti Sabatini and Colli Albani volcanic districts in order to provide sound radioisotopic ages for the eruptive history and to provide the required groundwork to assess the volcanic hazard for Rome (Fig. 1). Later on, Marra et al. (2003) expanded the geochronological picture for the Colli Albani, showing inconsistency between 40 Ar/ 39 Ar ages Journal of Volcanology and Geothermal Research 185 (2009) 203–213 ⁎ Corresponding author. Tel.: +39 0690672732; fax: +39 069067233. E-mail address: biagio.giaccio@igag.cnr.it (B. Giaccio). 0377-0273/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2009.05.011 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores