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ARTICLE IN PRESS VOLGEO-04326; No of Pages 11 Journal of Volcanology and Geothermal Research xxx (2009) xxx–xxx Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores 40 39 14 1 Ar/ Ar and C geochronology of the Albano maar deposits: Implications for 2 defining the age and eruptive style of the most recent explosive activity at Colli 3 Albani Volcanic District, Central Italy 4 B. Giaccio a,⁎, F. Marra b, I. Hajdas c, D.B. Karner d, P.R. Renne e, A. Sposato a a F 5 Istituto di Geologia Ambientale e Geoingegneria, CNR, Area della Ricerca RM1-Montelibretti, Via Salaria km 29,300, 00016 Monterotondo Stazione (Roma), Italy 6 b Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy 7 c Ion Beam Physics, ETH/PSI, Schafmattstrasse 20, CH-8093 Zürich, CH, Switzerland 8 d Department of Geology, Sonoma State University, 1801 East Cotati Avenue, Rohnert Park, CA 94985, U^S^A^ 9 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, U^S^A^ 10 article info abstract 11 12 Article history: New 40Ar/39Ar and 14C ages have been found for the Albano multiple maar pyroclastic units and underlying 25 13 Received 1 December 2008 paleosols to document the most recent explosive activity in the Colli Albani Volcanic District (CAVD) near 26 14 Accepted 20 May 2009 27 Rome, Italy, consisting of seven eruptions (Albano 1^=^oldest).PROO Both dating methodologies have been applied 15 Available online xxxx on several proximal units and on four mid-distal fall/surge deposits, the latter correlated, according to two 28 181617 current different views, to either the Albano or the Campi di Annibale hydromagmatic center. The 40Ar/39Ar 29 19 Keywords: 40 39 ages on leucite phenocrysts from the mid-distal units yielded ages of ca. 72 ka, 73 ka, 41 ka and 36 ka BP, 30 20 Ar/ Ar 40 39 31 21 14C which are indistinguishable from the previously determined Ar/ Ar ages of the proximal Albano units 1, 2, 22 geochronology 5 and 7, thus confirming their stratigraphic correspondence. 32 23 Albano maar Twenty-one 14C ages of the paleosols beneath Albano units 3, 5, 6 and 7 were found for samples collected 33 Central Italy from 13 proximal and distal sections, some of which were the same sections sampled for 40Ar/39Ar 3424 measurements. The 14C ages were found to be stratigraphically inconsistent and highly scattered, and were 35 40 39 fi 36 systematically younger than the Ar/ Ar ages, ranging ^from 35 ka ^to 3 ka. Considering the signi cant consistence of the 40Ar/39Ar chronological framework, we interpret the scattered and contradictory 14C ages 37 to be the result of a variable contamination of the paleosols by younger organic carbon deriving from the 38 superficial soil horizons. CTED 39 These results suggest that multiple isotopic systems anchored to a robust stratigraphic framework may need 40 to be employed to determine accurately the geochronology of the CAVD as well as other volcanic districts. 41 © 2009 Published by Elsevier B.V. 42 44 43 46 45 47 1. Introduction and general background Freda et al., 2006; De Benedetti et al., 2008), is therefore fundamental 60 to evaluate the time elapsed since the last eruptive event as well as its 61 48 Hazard evaluation in active volcanic areas requires documentation activity status. This has a relevant implication on the assessment of the 62 49 on the style, magnitude, source area and frequency of the eruptions potential hazard for the city of Rome, whose southeastern suburb 63 50 and other dangerous volcanic phenomena (e.g. gas emission, sin- or reaches close to the slopes of the Albano crater. 64 51 post-eruptive lahars, etc.). However, based on the strict definition Though many radioisotopic age determinations were made near 65 52 proposed in the Smithsonian Institution'sCORRE catalogue of active the end of the last Century for the Colli Albani Volcanic District (for a 66 53 volcanoes, only volcanoes that have erupted in the last 10,000 years review see Voltaggio and Barbieri, 1995), these were not the result of 67 54 should be considered active. Alternatively, it is considered that a systematic study. Rather, they were a collection of data from many 68 55 volcano is dormant when the time elapsed since its last eruption does different sources, which often yielded contrasting results depending 69 56 not exceed the average recurrence period of its past activity. on the applied dating methodology. Therefore, uncertainty persisted 70 57 Establishing the age of the most recent activity that occurred at the regarding the age of the latest volcanic activity (e.g. De Rita et al., 71 58 UN 40 39 72 Colli Albani Volcanic District, central Italy, documented at the Albano 1995a). Karner et al. (2001) carried out a Ar/ Ar age study of the 59 multiple maar (De Rita et al., 1995a,b; Funiciello et al., 2002, 2003; Monti Sabatini and Colli Albani volcanic districts in order to provide 73 sound radioisotopic ages for the eruptive history and to provide the 74 required groundwork to assess the volcanic hazard for Rome (Fig. 1). 75 76 Q2 ⁎ Corresponding author. Later on, Marra et al. (2003) expanded the geochronological picture 40 39 E-mail address: [email protected] (B. Giaccio). for the Colli Albani, showing inconsistency between Ar/ Ar ages 77 0377-0273/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jvolgeores.2009.05.011 Please cite this article as: Giaccio, B., et al., 40Ar/39Ar and 14C geochronology of the Albano maar deposits: Implications for defining the age and eruptive style of the most recent..., Journal of Volcanology and Geothermal Research (2009), doi:10.1016/j.jvolgeores.2009.05.011 ARTICLE IN PRESS 2 B. Giaccio et al. / Journal of Volcanology and Geothermal Research xxx (2009) xxx–xxx F PROO Fig. 1. Reference map of the investigated sections of the Albano maar products distributed in the three main depositional domains of the (i) proximal pyroclastic density current, D 40 39 14 (ii) distal fallout (Albano DUn of Giaccio et al., 2007) and (iii) distal channeled deposits. White circles: sampled sections for Ar/ Ar dating; white triangles: sampled sections for C age determinations; black dots: other investigated sections. Numbers refer to sections mentioned in the text. 78 and previously published U/Th measurements that provided very young Geofisica e Vulcanologia (National Institute of Geophysics and 109 79 ages (Voltaggio and Barbieri, 1995). These studies enabled Marra et al. Volcanology), a study was initiated to establish the age and eruptive 110 80 (2004) to estimate an average recurrence interval of approximately style of the last eruptive activity or other volcanic manifestations in 111 81 112 ^45 kyr for the overall activity at the Colli Albani Volcanic District. Ensuing the Colli Albani Volcanic District. The study was based on an extensive 82 studies were focused on the youngest volcanic deposit of Colli Albani field and laboratory investigations that allowed a refinement of the 113 83 “ fl ” 114 Volcanic District, called^ Peperino Albano pyroclastic ow^ and erupted stratigraphic framework of the Albano maar suite and a substantial 84 from the Albano multiple maar (De Rita et al.,1995a,b). Leucite from this reassessment of the actual extension of the distribution area of 115 85 unit yielded a 40Ar/39Ar age of 36±1 ka (Freda et al., 2006;seealso these deposits (Giaccio et al., 2007). Based on this study and some 116 86 Marra and Karner, 2005 for a discussion), which is in good agreement additional field investigations, we selected the best type-sections of 117 87 with the previously 14C age determinations performed on unburned the Albano multiple maar pyroclastic units, occurring both in proximal 118 88 wood fragments embedded in the Peperiono Albano deposits (Fornaseri and distal area, for 14C age determinations on paleosols separating the 119 89 et al.,1963; Fornaseri and Cortesi,1989). Based on theseRRECTE recent studies, it primary deposits of the Albano activity and direct 40Ar/39Ar dating of 120 90 121 becomes apparent that the time elapsed (^36 kyr) since the last eruption potassium-bearing mineral separate from these volcanic deposits. 91 is shorter than the estimated recurrence interval (45 kyr) for the Colli The aim of the present work is thus twofold: (i) to verify, through 122 ^ 40 39 92 Albani Volcanic District. Therefore, the Colli Albani may conservatively Ar/ Ar measurements, the soundness of the lithostratigraphic- and 123 93 be regarded as a dormant rather than extinct volcano (Marra and Karner, geochemical-based correlation between the widespread distal depos- 124 94 2005). However, the existence of volcanic activity occurring at the its (Giaccio et al., 2007) and their proximal equivalents, previously 125 95 Albano maar after this eruption, and extending to Holocene times, has dated by Freda et al. (2006);(ii) to assess the applicability and 126 96 been suggested (Funiciello et al., 2002, 2003; De Benedetti et al., 2008), reliability of the 14C dating on paleosols of the Colli Albani area. Results 127 97 based on the stratigraphic and geochronological evidences deriving of this study are presented and discussed in this paper. 128 98 from the study of relatively distal sections of chanelled Albano deposits 99 (GRA and the LR sections in Fig.1). In particular, these works interpreted 2. Geological setting and stratigraphic framework 129 100 the volcaniclastic deposits atUNCO the top of these successions to be derived 101 from lahar emplacements, triggered by repeated and catastrophic The Albano crater (Fig.