Earth and Planetary Science Letters 377–378 (2013) 180–190 Contents lists available at SciVerse ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Distinguishing contributions to diffuse CO 2 emissions in volcanic areas from magmatic degassing and thermal decarbonation using soil gas 222 Rn–δ 13 C systematics: Application to Santorini volcano, Greece Michelle M. Parks a,∗ , Stefano Caliro b , Giovanni Chiodini b , David M. Pyle a , Tamsin A. Mather a , Kim Berlo a , Marie Edmonds c , Juliet Biggs d , Paraskevi Nomikou e , Costas Raptakis f a COMET+, Department of Earth Sciences, Oxford, OX1 3AN, UK b Istituto Nazionale di Geofisica e Vulcanologia, sezione di Napoli, Osservatorio Vesuviano, Naples, Italy c COMET+, Department of Earth Sciences, Cambridge, CB2 3EQ, UK d COMET+, School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK e Department of Geology and Geoenvironment, University of Athens, Athens, Greece f Higher Geodesy Laboratory, National Technical University, Athens, Greece article info abstract Article history: Received 16 July 2012 Received in revised form 21 June 2013 Accepted 28 June 2013 Available online 29 July 2013 Editor: T. Elliot Keywords: volcanic unrest soil gas measurements carbon isotopic analysis magmatic degassing decarbonation Between January 2011 and April 2012, Santorini volcano (Greece) experienced a period of unrest characterised by the onset of detectable seismicity and caldera-wide uplift. This episode of inflation represented the first sizeable intrusion of magma beneath Santorini in the past 50 years. We employ a new approach using 222 Rn–δ 13 C systematics to identify and quantify the source of diffuse degassing at Santorini during the period of renewed activity. Soil CO 2 flux measurements were made across a network of sites on Nea Kameni between September 2010 and January 2012. Gas samples were collected in April and September 2011 for isotopic analysis of CO 2 (δ 13 C), and radon detectors were deployed during September 2011 to measure ( 222 Rn). Our results reveal a change in the pattern of degassing from the summit of the volcano (Nea Kameni) and suggest an increase in diffuse CO 2 emissions between September 2010 and January 2012. High-CO 2 -flux soil gas samples have δ 13 C ∼ 0❤. Using this value and other evidence from the literature we conclude that these CO 2 emissions from Santorini were a mixture between CO 2 sourced from magma, and CO 2 released by the thermal or metamorphic breakdown of crustal limestone. We suggest that this mixing of magmatic and crustal carbonate sources may account more broadly for the typical range of δ 13 C values of CO 2 (from ∼−4❤ to ∼+1❤) in diffuse volcanic and fumarole gas emissions around the Mediterranean, without the need to invoke unusual mantle source compositions. At Santorini a mixing model involving magmatic CO 2 (with δ 13 C of −3 ± 2❤ and elevated ( 222 Rn)/CO 2 ratios ∼ 10 5 –10 6 Bq kg −1 ) and CO 2 released from decarbonation of crustal limestone (with ( 222 Rn)/CO 2 ∼ 30–300 Bq kg −1 , and δ 13 C of +5❤) can account for the δ 13 C and ( 222 Rn)/CO 2 characteristics of the ‘high flux’ gas source. This model suggests ∼ 60% of the carbon in the high flux deep CO 2 end member is of magmatic origin. This combination of δ 13 C and ( 222 Rn) measurements has potential to quantify magmatic and crustal contributions to the diffuse outgassing of CO 2 in volcanic areas, especially those where breakdown of crustal limestone is likely to contribute significantly to the CO 2 flux.  2013 Elsevier B.V. All rights reserved. 1. Introduction Santorini is an active caldera system in the southern Aegean Volcanic Arc (Druitt et al., 1999; Vougioukalakis and Fytikas, 2005). The caldera lies close to an active normal fault system associated with the Santorini–Amorgos ridge. Historic volcanic centres in the * Corresponding author. E-mail address: michelle.parks@earth.ox.ac.uk (M.M. Parks). region include the submarine Kolumbo volcano, 7 km north-east of Santorini (Nomikou et al., 2012; Nomikou et al., 2013), and the intra-caldera Kameni Islands (Fig. 1) which have been the site of the most recent eruptions in the region. The Kameni islands are formed of dacite lavas and domes, and probably began forming as a submarine shield shortly after the Minoan eruption ∼ 3600 yr ago. The first reported eruption associated with the Kameni islands was in 199–197 BC; and there have been 10 effusive eruptions reported during the historical period (Pyle and Elliott, 2006), the 0012-821X/$ – see front matter  2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.06.046