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Bull Volcanol (2013) 75:711 DOI 10.1007/s00445-013-0711-8 RESEARCH ARTICLE Geochemical and isotopic changes in the fumarolic and submerged gas discharges during the 2011–2012 unrest at Santorini caldera (Greece) F. Tassi & O. Vaselli & C. B. Papazachos & L. Giannini & G. Chiodini & G. E. Vougioukalakis & E. Karagianni & D. Vamvakaris & D. Panagiotopoulos Received: 26 November 2012 /Accepted: 5 March 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract A geochemical survey of fumarolic and submerged increase of H2 concentrations, when values up to 158 mmol/ 13 gases from fluid discharges located in the Nea Kameni and mol were measured, the δ C–CO2 values, which prior to Palea Kameni islets (Santorini Island, Greece) was carried out January 2011 were consistent with a dominant CO2 before, during, and after the unrest related to the anomalously thermometamorphic source, have shown a significant de- high seismic and ground deformation activity that affected this crease, suggesting an increase of mantle CO2 contribution. volcanic system since January 2011. Our data show that from Light hydrocarbons, including CH4, which are controlled by May 2011 to February 2012, the Nea Kameni fumaroles chemical reactions kinetically slower than H2 production from showed a significant increase of H2 concentrations. After this H2O dissociation, displayed a sharp increase in March 2012, period, an abrupt decrease in the H2 contents, accompanied by under enhanced reducing conditions caused by the high H2 decreasing seismic events, was recorded. A similar temporal concentrations of May 2011–February 2012. The general − − 2− + pattern was shown by the F ,Cl,SO4 ,andNH4 concen- increase in light hydrocarbons continued up to July 2012, trations in the fumarolic condensates. During the sharp notwithstanding the contemporaneous H2 decrease. The tem- poral patterns of CO2 concentrations and N2/Ar ratios in- creasedsimilarlytothatofH2, possibly due to sealing Editorial responsibility: G. Giordano : : processes in the fumarolic conduits that diminished the con- F. Tassi O. Vaselli L. Giannini tamination related to the entrance of atmospheric gases in the Department of Earth Sciences, University of Florence, fumarolic conduits. The compositional evolution of the Nea Via G. La Pira 4, 50121 Florence, Italy Kameni fumaroles can be explained by a convective heat : pulse from depth associated with the seismic activation F. Tassi (*) O. Vaselli of the NE–SW-oriented Kameni tectonic lineament, pos- CNR-IGG Institute of Geosciences and Earth Resources, sibly triggered by either injection of new magma below Via G. La Pira 4, 50121 Florence, Italy Nea Kameni island, as apparently suggested by the e-mail: [email protected] evolution of the seismic and ground deformation activ- : : : ity, or increased permeability of the volcanic plumbing C. B. Papazachos E. Karagianni D. Vamvakaris system resulting from the tectonic movements affecting D. Panagiotopoulos Geophysical Laboratory, Aristotle University of Thessaloniki, thearea.Theresultsofthepresentstudydemonstrate 54124 Thessaloniki, Greece that the geophysical and geochemical signals at Santorini are interrelated and may be precursory signals G. Chiodini of renewed volcanic activity and encourage the devel- Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Via Diocleziano 328, opment of interdisciplinary monitoring program to mit- Naples, Italy igate the volcanic risk in the most tourist-visited island of the Mediterranean Sea. G. E. Vougioukalakis — IGME Institute for Geology and Mineral Exploration, . 3rd Exit Olympic Village, Keywords Santorini Island Fluid geochemistry 13677 Aharne, Athens, Greece Geochemical monitoring . Seismic crisis 711, Page 2 of 15 Bull Volcanol (2013) 75:711 Introduction 4 km, north of the Nea Kameni island, via GPS and IsSAR satellite data by modeling it as a spherical Mogi source (Fig. 1) Santorini is the most active volcanic system of the South with an estimated volume of ~1.5×107m3 (Newman et al. Aegean Active Volcanic Arc (SAAVA) (southern Aegean 2012; Parks et al. 2012). The early deviation of both seismic Sea), which extends from Methana peninsula to the west to and geodetic signals from the typical background activity in Nysiros Island to the east. Santorini presently consists of a this area was interpreted as potential precursory signals of small archipelago of five islands (Fig. 1): Thera, Thirasia, and volcanic unrest (Papazachos et al. 2012). In response to this Aspronisi, which constitute a ring structure bordering the volcanic crisis, the Institute for the Study and Monitoring of Santorini caldera, and Palea Kameni and Nea Kameni which the Santorini Volcano (ISMOSAV), in collaboration with re- emerge in the caldera center. Two volcanic systems occur searchers from various European universities and scientific along a NE–SW trending tectonic line produced by a NNW– institutions, intensified the geophysical and geochemical SSE extensional stress regime (Vougioukalakis and Fytikas monitoring of this system. Geochemical monitoring of fluid 2005; Sakellariou et al. 2010; Dimitriadis et al. 2009), namely, discharges is widely accepted as a useful method for investi- Christiana Islands, 20 km SW of Santorini caldera, and gating volcanic systems especially during periods of possible Coloumbo submarine volcano, an oval-shaped crater ongoing unrest since the chemical and isotopic features of (1,700 m in diameter and up to 500 m in depth) located volcanic gases may act as precursory signals for renewed 8 km NW of Thera, where more than 20 volcanic cones were volcanic activity (Giggenbach et al. 1990; Chiodini et al. recognized (Alexandri et al. 2003; Nomikou et al. 2012a). 2002; Capasso et al. 2005; Rouwet et al. 2009; Vaselli et al. Since January 2011, several (up to 50 per day) earth- 2010). quakes (M<3.5) have been recorded beneath the Santorini In the present study, we report the chemical and isotopic 13 12 caldera, and significant intra-caldera ground deformation ( C/ CinCO2) investigations of fluids discharged from (1) (up to 15–20 cm of radial extension and vertical uplift) has fumarolic vents located at the summit craters of Nea Kameni affected the Santorini islands (Newman et al. 2012; and (2) submerged hot springs off Nea Kameni and Palea Papazachos et al. 2012; Parks et al. 2012). Most earthquakes Kameni islets. Six geochemical surveys were carried out from were located along the NE–SW-oriented Kameni tectonic July 2010 to July 2012. The main goals of this study are to (1) line that crosses the Palea and Nea Kameni islets, corre- describe the temporal evolution of the chemical and isotopic sponding to the preferential vent location of the historical compositions of gas discharges from the Santorini hydrother- Kameni eruptions (e.g., Druitt et al. 1989). The main magma mal–magmatic system during the 2011–2012 unrest and (2) pressure source, however, has been identified at a depth of define the geochemical parameters which were more Fig. 1 Location of Santorini in Southern Aegean and tectonic regime on the broader Santorini area presenting faults (solid lines) and probable faults (dashed lines) (modified from Dimitriadis et al. (2010)). The inset map shows the five islets of Santorini (Thera, Thirasia, Aspronisi, Nea Kameni, and Palea Kameni) as well as the main hot springs of the area, including the sampled sites of Agios Georgios (AG) and Agios Nikolaos (AN). The Coloumbo and Kameni lines (see text for explanation) and the proposed location of the 2011–2012 Mogi source of the observed deformation according to Newman et al. (2012) are also depicted Bull Volcanol (2013) 75:711 Page 3 of 15, 711 sensitive to the changed physical conditions and thus more (Akrotiri volcanism in Middle Pleistocene) and acting suitable for monitoring purposes. as preferential pathways for magma uprising (Druitt et al. 1989). In the mid-Quaternary, strong NE–SW trending normal faulting resulted in the formation of Geodynamic setting and volcanic activity the neighboring Anydros and Amorgos Basins and the Santorini–Amorgos Fault Ridge (Perissoratis 1995;Piper SAAVA has formed in response to the subduction of the and Perissoratis 2003; Piper et al. 2004). The fault ridge shows African plate beneath the Aegean microplate (Papazachos characteristics of a continuous zone (e.g., similar seismicity and Comninakis 1971; Mckenzie 1972; Le Pichon and levels), although its faults have dip angle change which occurs Angelier 1979; Francalanci et al. 2005). Over a hundred at its western termination (Fig. 1). Dimitriadis et al. (2009) explosive eruptions and at least four caldera collapses suggest that in the area between