Research Paper 40 Ar/ 39 Ar geochronology of Holocene basalts; examples from Stromboli, Italy Jan Wijbrans a, * , Björn Schneider a , Klaudia Kuiper a , Sonia Calvari b , Stefano Branca b , Emanuela De Beni b , Gianluca Norini c, d , Rosa Anna Corsaro b , Lucia Miraglia b a Department of Earth Sciences, Faculty of Earth and Life Sciences, VU University, de Boelelaan 1085, NL-1081HV, Amsterdam, The Netherlands b Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Catania, Italy c Computational Geodynamics Laboratory, Centro de Geociencias, Universidad Nacional Autónoma de México, Querétaro, Mexico d Dipartimento di Scienze Geologiche e Geotecnologie, Università degli Studi di Milano-Bicocca, P.za della Scienza 4, 20126 Milano, Italy article info Article history: Received 29 July 2010 Received in revised form 21 September 2010 Accepted 10 October 2010 Available online 27 October 2010 abstract Absolute chronologies of active volcanoes and consequently timescales for eruptive behaviour and magma production form a quantitative basis for understanding the risk of volcanoes. Surprisingly, the youngest records in the geological timescale often prove to be the most elusive when it comes to isotopic dating. Absolute Holocene volcanic records almost exclusively rely on 14 C ages measured on fossil wood or other forms of biogenic carbon. However, on volcanic flanks, fossil carbon is often not preserved, and of uncertain origin when present in paleosols. Also, low 14 C-volcanic CO 2 may have mixed with atmo- spheric and soil 14 C-CO 2 , potentially causing biased ages. Even when reliable data are available, it is important to have independent corroboration of inferred chronologies as can be obtained in principle using the 40 K/ 40 Ar decay system. Here we present results of a 40 Ar/ 39 Ar dating study of basaltic groundmass in the products from the Pleistocene e Holocene boundary until the beginning of the historic era for the north-northeastern flank of Stromboli, Aeolian Islands, Italy, identifying a short phase of intensified flank effusive activity 7500 Æ 500 a ago, and a maximum age of 4000 Æ 900 a for the last flank collapse event that might have caused the formation of the Sciara del Fuoco depression. We expect that under optimum conditions 40 Ar/ 39 Ar dating of basaltic groundmass samples can be used more widely for dating Holocene volcanic events. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Definitive isotopic dating of Holocene volcanic strata often proves elusive, as such chronologies must be measured by 14 C on sparsely preserved fossil carbon or by the 40 Ar/ 39 Ar technique on sanidine which may contain up to 16% K 2 O (e.g. Renne et al., 1997). 40 Ar/ 39 Ar dating of Holocene basalts that commonly contain much less K 2 O when compared with sanidine is difficult because of the extremely low enrichments in radiogenic 40 Ar, fundamentally limiting the range of the method (Bacon and Lanphere, 2006; Renne et al., 1997; Hora et al., 2007, 2007; Singer et al., 2008; Carracedo et al., 2007; Gillot and Keller, 1993; Singer et al., 2008). We have applied a 40 Ar/ 39 Ar laser incremental heating technique to groundmass separates from high K-calcalkaline basalts to shosho- nitic basalts (K 2 O contents in the range 1.8e5.0%) from the lower northeast flank of Stromboli volcano and obtained plateau ages that consist in most cases of at least 6 steps and more than 95% of the total gas release. To precisely measure the radiogenic 40 Ar content of young basalts we have optimized signal intensities by applying sample pans that allow even heating of >100 mg basalt ground- mass and we use a new laser beam delivery system that allows more even heating of the sample. In addition, for discrimination, we use correction a 38 Ar-spiked air argon pipette system where the 40 Ar/ 38 Ar ratio is ca 2, which allows us to monitor variations in discrimination more precisely than it is possible with conventional air pipette systems (a more detailed account of our discrimination determination protocol is given in the Appendix/electronic supplement). With our laser step heating technique we can ach- ieve extremely small blanks, with blank proportions on the most important 40 Ar and 36 Ar beams commonly in the 0.1-permille range. Stromboli’s volcanic history (Hornig-Kjarsgaard et al., 1993; Keller et al., 1993; Pasquaré et al., 1993; Tanner and Calvari, 2004) consists of several cycles of topography build-up and collapse over the last ca 100 ka: short periods of rapid topography build-up were documented at 62 Æ 1 ka and 40 Æ 2 ka during the Paleostromboli phase (Quidelleur et al., 2005; Cortés et al., 2005), resulting in collapse of the volcano at ca 35 ka (Gillot and Keller, 1993). The * Corresponding author. Tel.: þ31 205987296. E-mail address: Jan.Wijbrans@falw.vu.nl (J. Wijbrans). Contents lists available at ScienceDirect Quaternary Geochronology journal homepage: www.elsevier.com/locate/quageo 1871-1014/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2010.10.003 Quaternary Geochronology 6 (2011) 223e232