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RESEARCH ARTICLE

Geochemical and isotopic changes in the fumarolic and submerged gas discharges during the 2011–2012 unrest at ()

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 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 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- , 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 and 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 Columbo and Santorini the occurred at Santorini in the last 400,000 years (Druitt local stress field is characterized by an anti-clockwise rotation et al. 1989; Druitt and Francaviglia 1992), whereas nine of ~30–35°. This explains the different orientation of the major eruptions were recorded in the last 0.6 ky (e.g. “Kameni–Coloumbo Fracture Zone” (NE–SW) with respect Georgalas 1962; Fytikas et al. 1984). The Santorini cal- to the ENE–WSW-oriented normal faults of the Amorgos area. dera formed from the huge Minoan Despite this active seismotectonic setting, the intra-caldera occurred in 17 BC (Bond and Sparks 1976; Heiken and region has exhibited very low seismicity in the last two de- McCoy 1984; Sparks and Wilson 1990;Pyle1990; cades, i.e., since the local seismological network (ISMOSAV) Friedrich 2000), along the so-called Kameni tectonic line began to operate. In this period, the seismic activity was mostly (Fytikas et al. 1990a), whereas Palea Kameni and Nea concentrated along a near-vertical “tube” of earthquakes in the Kameni islets were formed by extrusion of dacitic domes uppermost 5–8 km beneath the Coloumbo Volcano Reef and and flows between 197 BC and 1950 AD (Pichler along the Coloumbo–Anydros–Amorgos lineament and Kussmaul 1980). The last eruptions occurred at Nea (Dimitriadis et al. 2005; Bohnhoff et al. 2006), whereas the Kameni in 1926, 1940, and 1950 (Georgalas 1953). In Kameni line remained seismically inactive. Therefore, 1649–1650 AD, Coloumbo volcano erupted, causing Coloumbo appeared to be the westernmost termination of the strong damage and more than 70 casualties on Thera Amorgos–Santorini line, although the trans-tensional (Fouque 1879; Vougioukalakis et al. 1994, 1995). This neotectonic faulting observed on the NW part of Thera submarine volcano currently shows intense hydrothermal (Mountrakis et al. 1996) appeared to be in excellent agreement activity, emitting fluids at temperature up to 220 °C from with the observed stress field and faulting at Coloumbo vent chimneys up to 4-m high constructed of (Dimitriadis et al. 2009). polymetallic massive sulfides and sulfates (Sigurdsson et In January 2011, a significant increase of seismicity on al. 2006; Carey et al. 2011). the central part of the Kameni line was detected by the local network, which initially consisted of six permanent, mostly analog, stations run by ISMOSAV and the Seismotectonic setting and the 2011–2012 seismic crisis Geophysical Laboratory of the Aristotle University of Thessaloniki, as well as station SANT, jointly operated The Santorini, Sousaki, Methana, , and vol- by GEOFON and the National Observatory of Athens. canic systems that constitute SAAVA are sites of earth- Since March 2011, the entire extent of this NW-oriented quakes whose hypocenters are of shallow to intermediate line, from Palea Kameni islet to Imerovigli and Firostefani depth (5–10 km), forming five seismo-volcanic clusters villages at Thera, has become active. Seismicity increased (Papazachos and Panagiotopoulos 1993; Dimitriadis et al. until January 2012, whereas from February to September 2010). The broader area of the Santorini volcanic system 2012 it significantly decreased (Fig. 2), exhibiting activity is characterized by medium to high seismicity along the levels as low as during the pre-crisis period. All earth- NE–SW-aligned Santorini–Amorgos Ridge (Fig. 1). quakes were initially relocated with HYPOINVERSE Recent studies, using either permanent or local seismological (Klein 1988) using an appropriate local 1D model for networks, all confirm this seismic activity (Panagiotopoulos et the area (Dimitriadis et al. 2010). Even though the errors al. 1996;Bohnhoffetal.2004, 2006; Dimitriadis et al. 2005, associated with hypocenter location have progressively 2009, 2010;Bohnhoffetal.2006), which probably originated reduced during the study period due to network densifi- by NNW–SSE-oriented extensional stress. cation, the larger events presented here have rather con- In the Santorini area, the Late Pliocene to Early stant error values, with typical horizontal and vertical Quaternary faulting was associated with the starting of errors of the order of 0.7 and 1 km, respectively. As volcanism, with a NE-trending strike–slip fault system shown in Fig. 3, seismic events (M≥1.1) have mainly triggering the earlier volcanism observed in the area occurred on the Kameni line (Fig. 1), which probably 711, Page 4 of 15 Bull Volcanol (2013) 75:711

Hydrothermal activity

A permanent weak fumarolic field, with outlet tempera- tures ranging from 60 to 97 °C, is present at the summit of Nea Kameni island, representing the main hydrother- mal manifestations in the Santorini archipelago. Chiodini et al. (1998), on the basis of fumarolic gas samples collected in 1993 and 1995, showed that the chemical composition of the Nea Kameni fumaroles was charac-

terized by dominant CO2, relatively high concentrations of H2 (up to ~7 mmol/mol) and CH4 (< 0.14 mmol/mol), and strong contamination of atmospheric gases. Similar compositional features were also measured by ISMOSAV during a monitoring activity carried out from 1993 to – Fig. 2 Cumulative number of earthquakes (ML≥1.1) in the intra- 2005, although in the last period of that monitoring (2002 caldera area from January 2011 to mid-September 2012 2005) a decrease of the fumarolic flux, interpreted as the result of self-sealing of the fumarolic conduits, was recorded corresponds to a weak ring fracture created during the (Vougioukalakis and Fytikas 2005). Relatively high R/Rair , where earthquake rupturing also prefer- values (up to 3.7; Nagao et al. 1991; Shimizu et al. 2005) entially occurs. indicated a significant contribution of magmatic He. Newman et al. (2012) showed that the observed seis- Several thermal springs characterized by Fe- and Mn-rich micity variations are in excellent agreement with the waters occur along the shorelines of Palea and Nea Kameni temporal evolution of ground deformation evaluated from islands (Varnavas and Cronan (2005) and references there- GPS modeling during 2011, an observation that is also in). Among them, the most important discharges are Agios supported by the independent analysis of InSAR data Nikolaos (AN) and Agios Giorgios (AG), located in the from Papageorgiou et al. (2012), suggesting a single north and eastern part of Palea and Nea Kameni, respective- source of unrest. Recent results from Papoutsis et al. ly (Fig. 1). AG and AN have outlet temperatures of up to 34 (2013) using a combination of GPS and InSar data and 40 °C, respectively (Böstrom and Widenfalk 1984; suggest that the observed deformation has strongly de- Dotsika et al. 2009). These thermal fluids were also recog- creased since the end of February of 2012, in excellent nized in a deep (−201.5 m) exploratory well (GK1) drilled in agreement with the seismicity decrease (Fig. 2). 1987–1988 on a site located 40 m to the northeast of the AN

Fig. 3 Spatial distribution of seismic events (M≥1.1) which occurred inside the Santorini caldera since the beginning of the earthquake crisis from January 2011 to September 2012. The pink line and diamond are used for the cross- sections later presented in Fig. 4, while the yellow circle shows the proposed location of the 2011–2012 Mogi deformation (Newman et al. 2012) Bull Volcanol (2013) 75:711 Page 5 of 15, 711 spring, where a maximum temperature of 29 °C was mea- sured at a depth of 10 m (Arvanitides et al. 1988, 1990). Hydrothermal activity on Thera mainly consists of three hot springs located along the western seashore of the island (Fig. 1): Plaka (34 °C), Anthermi Christou (56 °C), and Vlychada (32 °C) (Arriaga et al. 2008; Dotsika et al. 2009). Geothermometry based on the chemical composition of springs located in the southern part of Thera has suggested the occurrence of a relatively shallow (from 800 to 1,000 m depth) geothermal reservoir with temperatures ranging from 130 to 160 °C (Fytikas et al. 1990b). A soil survey measuring diffuse CO2 carried out in the 1990s (Barberi and Carapezza 1994) showed relatively high fluxes in correspondence of two main NE–SW trending fault sys- tems (Kameni and Coloumbo lines) that have controlled the position of the eruptive vents in the last 600 ky (Vougioukalakis and Fytikas 2005). This implies that the degassing phenomena of deep-originated fluids are intimate- ly associated with the regional tectonic assessment. Recent remotely operating vehicle explorations carried out with an E/V Nautilus (Nomikou et al. 2012b) recognized the presence of submarine gas discharges located NNE of Riva port (Therassia) at 37 m depth. According to local witnesses, in July 2011, a submarine gas blast event has likely occurred in this area. A number of mounds (up to 1-m high), constituted by yellowish bacterial mat at depths rang- ing from 200 m to 350 m in the NE part of the North Basin Fig. 4 Map of Nea Kameni and Palea Kameni islets with location of the sampled fumaroles (NK1, NK3, NK4, NK5, and NK10) and of the within the Santorini caldera, were also recognized. The Agios Nikolaos (AN) and Agios Giorgios (AG) submarine springs location of these bacterial colonies corresponded to those of fluid vents having temperatures up to 17 °C, i.e., slightly above the ambient temperature. Similar bacterial mounds (N2,H2,O2,Ar,Ne,CH4, and light hydrocarbons) filled the were also recognized at shallow depths around the Nea and flask headspace, whereas CO2 and other acidic gases (e.g., Palea Kameni islets. HCl, HF and SO2) dissolved in the alkaline solution, and H2S reacted with Cd2+ to form insoluble CdS (Montegrossi et al. 2001). In selected Nea Kameni fumaroles, i.e., those having Methods relatively high concentrations of atmospheric gases, pre- evacuated 50-mL glass flasks equipped with thorion-valves

Gas sampling were used for the analysis of carbon isotopes in CO2 and for chemical composition. Gas discharges from fumaroles of the Nea Kameni sum- mit craters (NK1, NK3, NK4, NK5, and NK10; Fig. 4) Chemical analyses of gases were convoyed into modified Giggenbach-type flasks (Giggenbach 1975), consisting of pre-weighted and pre- Inorganic incondensable gases were analyzed using a evacuated 50-mL thorion-tapped glass flasks partially Shimadzu 15A gas chromatograph equipped with a 9-m- filled with 20 mL of 0.15 M Cd(OH)2 and 4 N NaOH long stainless steel molecular sieve column and a thermal suspension (Montegrossi et al. 2001) through a titanium conductivity detector (GC–TCD). To allow the complete tube inserted in the fumarolic vent and connected to separation of H2 and Ne peaks, as well as those of Ar and dewared tubes. At each sampling site, an ice-cooled con- O2, the oven temperature was lowered to −10 °C by means denser was used to collect ~50 mL of condensate. Gas of a cryogenic cooler (Shimadzu CRG-15) fed by liquid samples from the AG and AN submarine springs (Fig. 1) CO2. The analysis of CO2 in the Nea Kameni fumaroles were collected into the same flasks used for the fumarolic was carried out by GC–TCD using a 5-m-long Porapack gases and connected to a plastic funnel positioned over stainless steel column. Methane and light hydrocarbons the rising bubbles. During sampling, incondensable gases were analyzed with a Shimadzu 14A gas chromatograph 711, Page 6 of 15 Bull Volcanol (2013) 75:711

equipped with a flame ionization detector using a 10-m-long high concentrations of O2 (from 15.5 to 65 mmol/mol), Ar stainless steel column packed with Chromosorb PAW (from 3.33 to 15 mmol/mol), and Ne (from 0.00017 to 80/100 mesh coated with 23 % SP 1700. 0.00081 mmol/mol). Hydrogen concentrations were in a The solution and the CdS precipitate in the soda flasks were wide range (from 2.11 to 158 mmol/mol), whereas those separated by centrifugation at 15,000 rpm. Carbon dioxide in of CH4,C2H6,andC6H6 were <0.204, <0.0019, and <0. the alkaline solution was analyzed by automatic titration using 0013 mmol/mol, respectively. No acidic gases such as

0.5 N HCl solution, whereas CdS was dissolved and oxidized SO2,H2S, HCl, and HF were recognized. On the whole, 2− with H2O2 and then analyzed as SO4 by ion chromatogra- these chemical features are consistent with those mea- phy (Metrohm Compact 761). suredin1993–2005 by ISMOSAV and Chiodini et al. − − 2− + The main anions (F ,Cl, and SO4 ) and NH4 in the (1998). 2− condensate samples were analyzed by ion chromatography Large variations in concentrations are shown by SO4 (Metrohm Compact 761 and Metrohm Compact 771, re- (from 3.53 to 68.5 mg/L) in the fumarolic condensates spectively). The analytical error for gas chemical analyses (Table 2). Significant content variations were also found for − − + is <5 %. F (0.02 to 0.81 mg/L), Cl (1.58 to 15.3 mg/L), and NH4 (0. 03 to 8.53 mg/L). 13 Analysis of the δ C–CO2 ratios The chemical composition of AG and AN gases (Table 1) was significantly different with respect to that of the sub- 13 12 13 The C/ CratiosofCO2 (expressed as δ C–CO2 ‰ vs. V- aerial fumaroles since they are characterized by dominant PDB) were determined by using a Finningan Delta S mass CO2 (from 854 to 886 and from 978 to 988 mmol/mol, spectrometer after purification of the gas mixture by standard respectively) and low N2 (<142 and <18.1 mmol/mol, procedures (Evans et al. 1998). Internal (Carrara and San respectively), O2 (<28 and <8.99 mmol/mol), and Ar Vincenzo marbles) and international (NBS18 and NBS19) (<2.78 and <0.43 mmol/mol, respectively) concentrations. standards were used in order to estimate external precision. Relatively low concentrations of H2S (up to 0.026 and 0. Analytical uncertainties and reproducibility are ±0.1‰. 415 mmol/mol, respectively) were also measured. HThe hydrogen concentrations of these two springs were sim-

ilar and did not exceed 0.16 mmol/mol, whereas CH4 Results concentrations (up to 0.312 and 0.359 mmol/mol, respective-

ly) were higher than in the fumaroles. Besides C2H6 and C6H6 13 Chemical and isotopic (δ C–CO2) compositions of gases (up to 0.0061 and 0.0044 mmol/mol, respectively), C3H8 and C3H6 were also detected, although their concentrations are as The outlet temperatures of gases of the Nea Kameni fuma- low as <0.0007 and <0.0002 mmol/mol, respectively. 13 roles were from 78 to 97 °C (Table 1), consistent with the The δ C–CO2 ratio of gases from both sub-aerial fuma- temperature range (from 60 to 97 °C) measured at the same roles and submarine springs ranges from 0.03 to −0.90 ‰ sites in the past (Vougioukalakis and Fytikas 2005). The vs. V-PDB (Table 1). temperature of the AN submarine thermal spring did not exceed 36 °C (Table 1), i.e., slightly lower than most (from 36° to 40 °C) reported in literature (Böstrom and Widenfalk Discussion 1984; Dotsika et al. 2009). However, this apparent discrep- ancy is likely due to the fact that (1) the spring(s) is(are) The hydrothermal–magmatic system of Palea and Nea located in the seashore and (2) sea level variations (tidal Kameni islands effect) may temporarily cool down the emerging thermal water. Difficulties were also encountered in measuring the The occurrence of significant mantle He in Nea Kameni outlet temperature of the AG spring because it was impossible fumarolic fluids indicated by the relatively high R/Rair to exactly localize the vent(s) at the sea bottom (~1.5 m depth). values (up to 3.77) measured by Shimizu et al. (2005)is The chemical composition of gases from the sub-aerial the only geochemical evidence of current active magmatic discharges was characterized by CO2 and N2 in comparable degassing beneath Santorini. The Nea Kameni R/Rair ratios amounts, ranging from 332 and 784 and from 168 to are significantly higher than those measured in the hydro- 592 mmol/mol, respectively (Table 1), with the exception of thermal fluids of Sousaki and Methana in the western end gases collected from the NK4 fumarole in April and July 2012, of the Aegean arc (R/Rair <2.3), whereas they are similar whose CO2 concentrations were up to 933 and 948 mmol/mol, to those of gases from Milos Island and significantly respectively. The relatively high N2 concentrations characteriz- lower than those of gas discharges located in Kos and ing the Nea Kameni fumaroles indicate a significant contami- Nisyros islands (R/Rair up to 6.2) in the eastern end of the nation of atmospheric gases, as also suggested by the relatively arc (Shimizu et al. 2005). The cause of the progressive 13 75:711 (2013) Volcanol Bull Table 1 Outlet temperature (in °C), chemical composition (CO2,N2,Ar,O2,H2, Ne, CH4,C2H6,C3H8,C3H6 and C6H6), and δ C–CO2 values (‰V-PDB) of the gas discharges sampled at Nea Kameni (NK1, NK3, NK4, NK5, NK10, and AG) and Palea Kameni (AN) in August 2010, May 2011, and February, March, April, and July 2012 (gas concentrations are in mmol/mol)

13 Sample Date T CO2 H2SN2 CH4 Ar O2 Ne H2 C2H6 C3H8 C3H6 C6H6 δ C-CO2

AG Jul-2010 31 854 0.015 142 0.234 2.78 0.55 0.00015 0.011 0.0012 0.0001 0.0002 0.0007 0.03 AG May-2011 n.d. 864 0.026 105 0.255 2.62 28.0 0.00015 0.076 0.0018 0.0003 0.0001 0.0011 AG Apr-2012 n.d. 869 <0.005 128 0.287 2.56 0.61 0.00013 0.013 0.0031 0.0004 0.0001 0.0015 AG Jul-2012 n.d. 886 <0.005 111 0.312 2.56 0.55 0.00013 0.011 0.0038 0.0006 0.0001 0.0019 AN Jul-2010 35 984 0.258 15.2 0.151 0.17 0.22 0.00001 0.065 0.0015 0.0002 0.0002 0.0009 AN May-2011 35 978 0.361 11.8 0.189 0.13 8.99 0.00001 0.12 0.0021 0.0002 0.0002 0.0009 −0.90 AN Feb-2012 32 988 0.415 10.1 0.216 0.22 0.90 0.00001 0.16 0.0026 0.0003 0.0001 0.0018 AN Mar-2012 33 987 0.155 11.9 0.252 0.17 0.51 0.00001 0.071 0.0038 0.0003 0.0001 0.0017 −0.75 AN Apr-2012 31 980 <0.005 18.1 0.359 0.43 1.11 0.00002 0.036 0.0054 0.0006 0.0001 0.0037 −0.87 AN Jul-2012 36 980 <0.005 18.1 0.359 0.43 1.11 0.00002 0.025 0.0061 0.0007 0.0001 0.0044 −0.83 NK Jul-2010 80 332 <0.01 586 0.019 15 65 0.00081 2.20 −0.23 NK1 May-2011 80 488 <0.01 470 0.036 11.8 23.1 0.00057 7.46 −0.81 NK1 Feb-2012 85 511 <0.01 450 0.041 7.98 22.5 0.00042 8.95 −0.57 NK1 Mar-2012 82 521 <0.01 419 0.105 7.12 48.8 0.00035 3.99 0.0008 0.0004 −0.41 NK1 Apr-2012 82 567 <0.01 365 0.126 6.11 59.3 0.00033 2.59 0.0012 0.0009 −0.11 NK1 Jul-2012 85 572 <0.01 377 0.141 5.56 43.5 0.00029 2.11 0.0019 0.0011 −0.06 NK3 Jul-2010 84 462 <0.01 494 0.026 12.5 26.2 0.00061 5.35 −0.26 NK3 May-2011 85 498 <0.01 449 0.041 11.1 24.0 0.00061 17.6 NK3 Feb-2012 78 523 <0.01 417 0.033 7.40 37.0 0.00054 16.5 −0.61 NK3 Mar-2012 81 566 <0.01 379 0.081 6.37 41.9 0.00033 6.67 0.0007 0.0003 −0.36 NK3 Apr-2012 82 701 <0.01 259 0.113 3.61 33.4 0.00023 2.58 0.0013 0.0008 −0.15 NK3 Jul-2012 84 782 <0.01 184 0.133 3.55 27.6 0.00019 2.44 0.0013 0.0009 −0.11 NK4 May-2011 90 658 <0.01 275 0.024 7.15 41.3 0.00037 18.5 NK4 Feb-2012 94 670 <0.01 260 0.065 4.99 31.9 0.00032 33.6 0.0003 0.0002 NK4 Apr-2012 95 935 <0.01 47 0.156 0.75 4.39 0.00004 12.1 0.0014 0.0009 NK4 Jul-2012 95 935 <0.01 38 0.204 0.67 3.11 0.00003 10.3 0.0018 0.0011 NK5 Jul-2010 93 365 <0.01 592 0.029 13.8 24.3 0.00061 5.35 −0.26 NK5 May-2011 92 395 <0.01 512 0.045 13.5 15.5 0.00069 64.6 −0.52 NK5 Feb-2012 95 421 <0.01 395 0.056 8.87 17.0 0.00049 158 NK5 Mar-2012 93 572 <0.01 377 0.114 5.85 22.4 0.00032 22.8 0.0009 0.0005 −0.25 NK5 Apr-2012 92 586 <0.01 353 0.151 5.82 49.6 0.00031 4.81 0.0013 0.0011 −0.13

NK5 Jul-2012 93 784 <0.01 168 0.184 3.33 41.3 0.00017 3.66 0.0017 0.0013 −0.13 711 15, of 7 Page NK10 Feb-2012 97 535 <0.01 340 0.030 6.12 20.7 0.00033 98.7 NK10 Mar-2012 91 597 <0.01 356 0.073 6.12 23.6 0.00033 17.5 0.0008 0.0004 NK10 Apr-2012 89 574 <0.01 354 0.126 6.06 61.3 0.00032 4.85 0.0012 0.0008 NK10 Jul-2012 90 621 <0.01 313 0.148 5.81 55.5 0.00029 4.11 0.0015 0.0009 711, Page 8 of 15 Bull Volcanol (2013) 75:711

Table 2 Chemical composition (F−,Cl−,SO 2−,andNH +) in condensate 4 4 consisting of H2,CH4, and light hydrocarbons, which are samples from the Nea Kameni fumaroles (concentrations are in mg/L) produced at hydrothermal conditions within a reservoir in- − − 2− + sample date F CI SO4 NH4 terposed between the magmatic chamber and the surface. Uprising magmatic–hydrothermal fluids are strongly affect- NK1 Jul-2010 0.03 2.56 4.25 0.04 ed by atmospheric gas contamination (N2,O2, and Ar), a NK1 May-2011 0.13 5.51 11.5 0.24 typical feature of these fumaroles (Vougioukalakis and NK1 Feb-2012 0.19 7.79 13.6 0.26 Fytikas 2005), likely permeating the pyroclastic deposits NK1 Mar-2012 0.16 3.15 8.12 0.06 and highly fractured constituting the emerged portion NK1 Apr-2012 0.06 3.31 5.53 0.08 of Nea Kameni island and interacting with a shallow aquifer. NK1 Jul-2012 0.07 3.12 5.11 0.07 The latter, also controlling the outlet fumarolic tempera- NK3 Jul-2010 0.08 3.11 5.87 0.05 tures, is, at least partially, fed by condensation of the NK3 May-2011 0.22 7.56 10.6 4.12 uprising fluids. This hypothesis is confirmed by the rela- NK3 Feb-2012 0.51 11.1 18.5 0.55 tively low air contamination shown by the AN and AG NK3 Mar-2012 0.43 4.15 7.15 0.12 submerged springs (Table 1). It is worth noting that the

NK3 Apr-2012 0.09 3.56 5.11 0.12 H2/CH4 ratios of these submarine gas discharges (ranging NK3 Jul-2012 0.11 3.66 5.05 0.09 from 0.04 to 0.72) are significantly lower than those NK4 May-2011 0.42 11.1 56.9 0.56 characterizing the Nea Kameni fumaroles (up to 3,292),

NK4 Feb-2012 0.81 15.3 68.5 8.52 a difference that, according to the log(CH4/CO2)vs. NK4 Apr-2012 0.13 4.06 16.1 3.07 log(H2/CO2) binary diagram (Fig. 5), is mainly due to varia- NK4 Jul-2012 0.11 4.13 11.5 2.96 tions of H2 concentrations. Relatively high H2 concentrations NK5 Jul-2010 0.05 2.88 6.66 0.09 in fumarolic gases are generally ascribed to temperature- NK5 May-2011 0.11 3.55 14.7 0.27 dependent gas–water–rock interactions (Martini 1993).

NK5 Feb-2012 0.31 8.45 13.7 0.89 Therefore, the Nea Kameni fumaroles receive H2 produced NK5 Mar-2012 0.35 7.89 9.55 0.26 at the root of the hydrothermal reservoir, where ascending NK5 Apr-2012 0.04 2.48 3.61 0.27 magmatic fluids meet liquid water. Steam condensation oc- NK5 Jul-2012 0.06 2.13 3.55 0.25 curring in the shallower portion of fumarolic conduits likely NK10 Feb-2012 0.27 9.11 9.55 0.32 contributes to higher H2 concentrations since this gas has low NK10 Mar-2012 0.25 8.85 6.87 0.05 solubility in water (CRC 2001). Condensing steam also causes NK10 Apr-2012 0.07 2.01 3.53 0.05 the removal of gases soluble in water such as H2S, explaining NK10 Jul-2012 0.08 2.14 3.66 0.06 decrease of mantle He from east to west along the SAAVAwas explained in terms of difference in (1) crustal contamination at the magma source and/or (2) degassing activity from the hydrothermal–magmatic systems (Shimizu et al. 2005). The 13 δ C–CO2 ratios of the Nea Kameni fumaroles (Table 1)are higher than those of CO2 in mantle-related fluids that usually range between −3and−7 ‰ vs. V-PDB, depending on the different geodynamic settings of the volcanic region (e.g. Hoefs 1973; Rollinson 1993). The relatively high 13Ccontent is likely related to CO2 production from thermometamorphic processes involving the Mesozoic limestone formations that host the Santorini magma chamber. A similar expla- nation was invoked to justify the carbon isotopic signature of CO2 discharged from Nisyros fumaroles (Brombach et al. 2003;Fiebigetal.2004) as well as from other volca- noes in the Mediterranean area, such as Vulcano Island Fig. 5 Log(H2/CO2) vs. log(CH4/CO2) binary diagram for (1) NK1, (Tedesco and Nagao 1996) and Mt. Vesuvius (Chiodini et NK3, NK4, NK5, and NK10 fumaroles (open circles) and (2) Agios al. 2001). Nikolaos and Agios Giorgios submarine springs (open squares). Data – On the whole, fumarolic gases discharging from the of fumarolic gases from other volcanic hydrothermal system of the Mediterreanean area, e.g., Nisyros (Ni), Ischia (Is), Campi Flegrei summit craters of Nea Kameni island originate from mixing (CF), and Vesuvio (Ve) (Chiodini and Marini 1998; Chiodini et al. of deep-originated fluids (CO2 and He) and gases, mainly 2001, 2004; Fiebig et al. 2004), were also reported Bull Volcanol (2013) 75:711 Page 9 of 15, 711

its absence in the fumarolic vents. Therefore, primary H2 In May 2011, the H2 concentrations increased by up production at hydrothermal conditions followed by passive to one order of magnitude with respect to those measured

H2 enrichment due to steam condensation produced the strik- in July 2010. In February 2012, the NK5 gas showed a ingly high H2 concentrations characterizing the Nea Kameni further abrupt H2 increase, whereas those from the NK1 fumaroles, higher than those commonly found in low- and NK3 fumaroles did not show significant H2 temperature volcanic gases from other volcanic–hydrothermal changes. Gas samples collected in March, April, and system of the Mediterreanean area, e.g., Nisyros, Ischia, July 2012 revealed a rapid H2 decrease down to con- Campi Flegrei, and Vesuvio (Chiodini and Marini 1998; centrations measured prior to the seismic crisis (Fig.

Chiodini et al. 2001, 2004; Fiebig et al. 2004). Taking into 7a). The temporal trend of H2 concentrations suggests account the relatively low outlet temperatures of the Nea that a heat pulse from the depth affected the hydrother- − Kameni fumaroles, it seems reasonable to suppose that F , mal reservoir during the seismic crisis since H2,pro- − 2− + Cl ,SO4 ,andNH4 , which were measured in fumarolic duced by H2O thermal dissociation and whose behavior condensates at significant concentrations (Table 2), were car- is regulated by gas–water–rock redox interactions, is ried upwards to the vents as liquid micro-droplets released typically highly sensitive to the variations of the chem- from the boiling hydrothermal aquifer. ical–physical conditions controlling hydrothermal–mag- The AN and AG springs probably represent the lateral matic fluids (Giggenbach 1980, 1987). In July 2010, the outflow of the hydrothermal–magmatic reservoir. Fluids values of H2/H2O log ratios, calculated using theoretical discharged from these sites show a significant enrichment PH2O of saturated steam at outlet fumarolic tempera- of several chemical species, such as Fe, Mn, Al, Si, Ba, Ca, tures, ranged from −2.36 to −2.80, i.e., slightly less K, As, and other trace elements, with respect to standard negative than those typical of hydrothermal reservoirs seawater (Pushkina 1967, 1968; Peeters 1978; Varnavas and (Giggenbach 1987), which was likely due to H2Ocon- Cronan 1988). These chemical features are probably produced densation at shallow depths. In May 2011 and February by the interaction of hydrothermal gases with seawater and 2012, the H2/H2O log ratios increased up to −1.50 volcanic and basement rocks, a process commonly occurring (NK5 fumaroles; Fig. 7b),achangethatwasmainly in other submarine hydrothermal systems in the Mediterranean dependent on the increase of H2 concentrations Sea (Dando et al. 1999). Gas–seawater–rock interactions oc- (Table 1), which is orders of magnitude higher than curring at relatively shallow depth, where oxidizing condi- any expected variation of the PH2O values related to tions are dominant, explain the relatively low H2 the heat pulse affecting the hydrothermal system. Such concentrations of the submerged fluid discharges, which high H2/H2O log ratios, which cannot be produced in were three orders of magnitude lower than those measured hydrothermal–magmatic systems (Chiodini and Marini in the Nea Kameni fumaroles. In contrast, CH4 and light 1998; Lowernstern and Janik 2003) except in those where hydrocarbons, which are present at significant concentra- serpentinization of mafic and ultremafic rocks occurs (Taran et tions in both the AN and AG gases, seem to be less al. 2010), imply a strong chemical disequilibrium, which was affected by these secondary processes because they are likely due to a heat pulse. This event, which was able to regulated by chemical reactions characterized by a relatively destabilize the Nea Kameni fluid reservoir in May 2011– slow kinetics (Giggenbach 1987). February 2012, was likely produced by either a fresh magma According to these considerations, a schematic concep- injection at depth or a permeability increase accompanying the tual model of hydrothermal–magmatic fluid circulation, seismic activity. Whatever the event that triggered such an consistent with that proposed by Chiodini et al. (1998), is abrupt H2 increase, in coincidence with the observed waning shown in Fig. 6. of the seismic activity (Fig. 2), the H2/H2O log ratios showed decreasing trends, achieving values similar to those character- Evolution of fluid chemistry in 2010–2012 izing these gases in July 2010 (Fig. 7b). − − 2− The concentrations of the main anions (F ,Cl ,andSO4 ) + Seismic signals and ground deformation inferred from both and NH4 in the NK1, NK3, and NK5 condensate samples GPS and satellite data indicated activation of the NE–SW- show temporal trends (Fig. 8a–c) similar to that of H2 (Fig. 8a) oriented Kameni tectonic lineament that were interpreted as and consistent with the seismicity pattern (Fig. 2) and the related to a deep magma intrusion which occurred from geodetic data (Newman et al. 2012). The heat input would January 2011 to mid-May 2012 (Newman et al. 2012; have stimulated the hydrothermal aquifer sourcing the Nea Parks et al. 2012). The effects of these events on fumarolic Kameni fumaroles in a such way that liquid droplets rich in fluid chemistry are shown by the compositional evolution of these ions were more efficiently carried up to the surface by three Nea Kameni fumaroles (NK1, NK3 and NK5; Fig. 5) uprising fluids. that were sampled during the six campaigns carried out in A distinct behavior is shown by CH4, whose concentrations 2010–2012: remained almost constant from July 2010 to February 2012 711, Page 10 of 15 Bull Volcanol (2013) 75:711

Fig. 6 Geochemical conceptual model of the hydrothermal–magmatic system of Santorini. Acidic gases (SO2, HCl, and HF) released by the magmatic source completely dissolve in the hydrothermal reservoir, whose boiling produced a fluid rich in CO2, H2S, H2, CO, and CH4. These compounds are partially scrubbed during vapor condensation and cooling, affecting ascending fluids in the upper portion of the fumarolic conduits. The chemical composition of gases from the AN and AG springs, representing the lateral outflow of the hydrothermal–magmatic reservoir, is controlled by gas– seawater–rock interactions

but then strongly increased from March to July 2012 (Fig. 9a) hydrocarbons in the study period since formation of CH4 when significant concentrations of light hydrocarbons (C2H6 through catalytic hydrogenation of CO2, i.e., the so-called and C6H6), not previously detected in these fumaroles, were Sabatier process (Sabatier and Senderens 1897), is strongly also measured (Table 2). The temporal trend of H2 concentra- favored at reducing conditions and decreasing temperatures tions (Fig. 7a) explains the behavior of CH4 and light (Giggenbach 1987). The relatively slow kinetics characterizing this process (Giggenbach 1997) explains the delay between the

peak of H2 concentrations in February 2012 (Fig. 7a)andthe sharp increase of CH4 concentration that started in March 2012 (Fig. 9a). In this period, enhanced reducing conditions may also have favored the production of light hydrocarbons, i.e.,

C2H6 and C6H6. The former would have formed through Fischer–Tropsch reaction (Anderson 1984), whereas the latter was likely produced via cyclicization of short-chain hydrocar- bons such as acetylene, a common process in the chemical industry.

Fig. 7 a, b Temporal evolution from July 2010 to July 2012 of a H2 Fig. 8 a–c Temporal evolution from July 2010 to July 2012 of − − 2− + concentrations and b H2/H2O log ratios of the NK1, NK3, and NK5 F ,Cl ,SO4 ,andNH4 concentrations in the condensate samples from fumaroles the a NK1, b NK3, and c NK5 fumaroles Bull Volcanol (2013) 75:711 Page 11 of 15, 711

related to an enhanced contribution of deep-originated gases, which in subduction zones are typically characterized by

higher N2/Ar ratios with respect to air (Giggenbach 1992). 13 Significant changes have also affected the δ C–CO2 values. Between May 2011 and March 2012, the carbon iso-

topes in CO2 were significantly more negative than those measured in July 2010 and April–July 2012 (Table 1), approaching values typical of gases from magmatic degassing. A possible explanation for the observed variations 13 shown by the δ C–CO2 values during the 2011–2012 seismic crisis is that CO2 contribution from the mantle increased with respect to that occurring in quiescent periods, when CO2 was mostly sourced by thermometamorphic reaction involving limestone of the basement. This would corroborate the desta- bilization caused by the seismic activity at depth, indicating a relatively lower limestone contamination in the Santorini magmatic system. This also implies that the energy release that caused the unrest consisted of a convective heat pulse, i.e., it involved transfer of magmatic fluids up to the surface.

The temporal trends of H2 and CH4 concentrations in the AN spring (Fig. 10) seem to resemble, although at a smaller scale, those of the Nea Kameni fumaroles (Figs. 7a and 9a, respectively), suggesting that the phenomenon that caused the compositional changes of the Nea Kameni fumaroles has

Fig. 9 a–c Temporal evolution from July 2010 to July 2012 of a CH4 also affected the lateral discharge of the hydrothermal sys- and b CO concentrations and c N /Ar ratios of the NK1, NK3, and 2 2 tem. The progressive increase of the C3H8/C3H6 ratios (from NK5 fumaroles 0.8 to 6.4) which occurred in the period of observation was likely caused by a combination of (1) enhanced reducing

The temporal trend of CO2 concentrations (Fig. 9b)is conditions related to the H2 increase until February 2012 consistent with that of CH4 (Fig. 9a). It is worth noting that (Fig. 10) and (2) a decrease of deep fluid temperature the NK4 gas sample collected during our two last campaigns marked by the starting of the H2 decreasing trend in in April and July 2012 shows only minor air contamination, March 2012 (Fig. 10). Propane formation from C3H6, i.e., comparable with that of the submerged gas discharges the process that caused the observed C3H8/C3H6 increase, is (Table 1). Such a decrease of atmospheric gas concentra- indeed favored at reducing conditions and relatively low tions in the fumarolic fluids, which is probably the main cause of the observed CO2 increases (Fig. 9b), may be related to the self-sealing processes of the feeding conduits due to enhanced mineral deposition. However, the enhanced degassing rate of the feeding hydrothemal system, inferred from the temporal trends of ion concentrations in the fuma- rolic condensates (Fig. 8a–c), may also have contributed to increasing CO2 in the fumaroles. The temporal trends of N2/Ar (Fig. 9c) seem to further support this hypothesis. In fact, until May 2011, the N2/Ar ratios of Nea Kameni gases were as low as ~40, indicat- ing that air-saturated water, whose presence is also attested to by the constantly low fumarolic outlet temper- ature, was the dominant source of these two atmospheric gases in the fumaroles. Significant N2/Ar increases were observed in February 2012 (Fig. 9c), a compositional change that cannot be ascribed to increasing air contami- nation since in the same time interval the Ar concentra- Fig. 10 Temporal evolution from July 2010 to July 2012 of the H2 and tions decreased (Table 1). An N2 excess may instead be CH4 concentrations in gases from the AN spring 711, Page 12 of 15 Bull Volcanol (2013) 75:711 temperatures (Seewald 1994). The temporal evolution of the fumaroles can provide important information on the evolution 13 δ C–CO2 values of AN gases seems to not show a specific of the volcanic activity at Santorini. However, on the basis of trend (Table 1), possibly because isotopic fractionation af- available data, it is not possible to assess whether the 2011– fects CO2 at the surface, where gases interact with seawater, 2012 convective heat pulse was an episodic event or, alterna- masking the effects of CO2 source variations. tively, if a further evolution of the hydrothermal–magmatic No significant changes of the outlet temperatures at the system is to be expected. Our results suggest that a periodic fumarolic vents were recorded since they depend on water (bi-monthly?) sampling of both the sub-aerial and submarine vapor condensation occurring at shallow depth, a process gas discharges presented in this study should be performed. that is favored by both fumarolic low flux and the high Acquisition of a new compositional dataset may be helpful to permeability of lavas constituting the emerged portion of improve the geochemical conceptual model of Fig. 6 and test Nea Kameni. Lack of outlet temperature variations in whether fumarolic deep-originated constituents may act as fumarolic discharges from volcanic systems affected by precursory signals of renewed volcanic activity. Collected strong modifications of chemical and isotopic composition data should be broadened to include continuous measure- related to increases of deep fluid degassing had been ments of selected geochemical parameters, such as H2, repeatedly observed (e.g., Fischer et al. 1997; Vaselli et CH4,andCO2 discharged from vents of the Nea Kameni al. 2003, 2010). This underline the importance of geo- crater. Further efforts should also be made to carry out a chemical fluid surveys rather than just temperature mea- periodic inventory of (1) diffuse CO2 fluxes from the soil surements of fumarolic vents in order to detect changes of in selected areas of both Nea Kameni and Thera islands activity of magmatic–hydrothermal systems. and (2) dissolved gas concentrations in water from wells located in correspondence of the Kameni and Coloumbo tectonic lineaments that are considered the most prone sites Conclusions of future eruptive events.

Significant changes of gas chemistry were measured in the Acknowledgments This work was partly supported by ISMOSAV fumarole emissions from the Nea Kameni summit craters, (http://ismosav.santorini.net), the Municipality of Thera, and the Greek Earthquake Planning and Protection Organization and by the Labora- concomitant with an increase of seismic activity and ground tory of Fluid and Rock Geochemistry of the Department of Earth deformation episodes that occurred at Santorini in 2011–2012. Sciences and CNR-IGG of Florence. J. Cabassi is warmly thanked Newman et al. (2012)andParksetal.(2012) interpreted such for his help during the analyses of the fumarolic condensates. We thank physical modifications as due to the injection of a new magma two anonymous reviewers for their useful suggestions that helped us to improve an early version of the manuscript. batch at depth. If this is the triggering event that induced a more efficient heat transfer from depth into the hydrothermal reservoir feeding the fumaroles, it would explain the increas- ing trends of H2 and CO2 concentrations observed from May 2011 to February 2012. However, a similar geochemical be- References havior would probably also be expected if the seismic activity had increased the permeability at depth, facilitating the release Alexandri M, Papanikolaou D, Nomikou P, Ballas D (2003) Santorini of deep fluids and heat into the hydrothermal system, without volcanic field. New insights based on swath bathymetry. Proceedings involving magma movement. Nevertheless, after this period, of the IUGG conference, Sapporo, Japan Anderson RB (1984) The Fischer–Tropsch reaction. Academic, both the seismic activity and the concentrations of deep- London originated gases significantly decreased down to values sim- Arriaga M-CS, Tsompanakis Y, Samaniego F (2008) Geothermal mani- ilar to those measured prior to the crisis. In contrast, light festations and earthquakes in the caldera of Santorini, Greece: an historical perspective. Proceedings of the XXXIII workshop on hydrocarbons, including CH4, whose production is related to Geothermal Reservoir Engineering, Stanford University, Stanford, relatively slow kinetic processes, have started to signifi- California, pp 28–30, SGP-TR-185 cantly increase only since March 2012, when enhanced Arvanitides N, Galanopoulos V, Kalogeropoulos S, Skamnelos G, reducing conditions created during the previous phase Papavassiliou C, Paritsis S, Boström K (1988) Drilling at favored their formation. Self-sealing of fumarolic conduits Santorini volcano, Greece—a joint Greek–Swedish project to explore an ore-forming hydrothermal system. Eos 69:578–579 caused by mineral deposition explains the increasing Arvanitides N, Boström K, Kalogeropoulos S, Paritsis S, Galanopoulos trends of both CO2 concentrations and N2/Ar ratios which V, Papavassiliou C (1990) Geochemistry of lavas, pumice and veins occurred in concomitance with the increases of light in drill core GPK-1, Palaea Kameni, Santorini. In: Hardy DA, Keller hydrocarbons. J, Galanopoulos VP, Flemming NC, Druitt TH (eds) Thera and the Aegean world III, vol 2. The Thera Foundation, London, pp 266– The intimate relation between (1) seismic and deformation 279 – signals and (2) geochemical parameters observed in 2010 Barberi F, Carapezza ML (1994) Helium and CO2 soil gas emission 2012 demonstrates that monitoring of the Nea Kameni from Santorini (Greece). Bull Volcanol 56:335–342 Bull Volcanol (2013) 75:711 Page 13 of 15, 711

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