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 gases from fluid discharges located in the Nea Kameni and Palea Kameni islets (Santorini Island, Greece) was carried out before, during, and after the unrest related to the anomalously high seismic and ground deformation activity that affected this volcanic system since January 2011. Our data show that from May 2011 to February 2012, the Nea Kameni fumaroles showed a significant increase of H2 concentrations. After this period, an abrupt decrease in the H2 contents, accompanied by decreasing seismic events, was recorded. A similar temporal pattern was shown by the F−, Cl−, SO42−, and NH4+ concentrations in the fumarolic condensates. During the sharp Editorial responsibility: G. Giordano F. Tassi : O. Vaselli : L. Giannini Department of Earth Sciences, University of Florence, Via G. La Pira 4, 50121 Florence, Italy F. Tassi (*) : O. Vaselli CNR-IGG Institute of Geosciences and Earth Resources, Via G. La Pira 4, 50121 Florence, Italy e-mail:
[email protected] C. B. Papazachos : E. Karagianni : D. Vamvakaris : D. Panagiotopoulos Geophysical Laboratory, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece G. Chiodini Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Via Diocleziano 328, Naples, Italy G. E. Vougioukalakis IGME—Institute for Geology and Mineral Exploration, 3rd Exit Olympic Village, 13677 Aharne, Athens, Greece
increase of H2 concentrations, when values up to 158 mmol/ mol were measured, the δ13C–CO2 values, which prior to January 2011 were consistent with a dominant CO 2 thermometamorphic source, have shown a significant decrease, suggesting an increase of mantle CO2 contribution. Light hydrocarbons, including CH4, which are controlled by chemical reactions kinetically slower than H2 production from H2O dissociation, displayed a sharp increase in March 2012, under enhanced reducing conditions caused by the high H2 concentrations of May 2011–February 2012. The general increase in light hydrocarbons continued up to July 2012, notwithstanding the contemporaneous H2 decrease. The temporal patterns of CO2 concentrations and N2/Ar ratios increased similarly to that of H2, possibly due to sealing processes in the fumarolic conduits that diminished the contamination related to the entrance of atmospheric gases in the fumarolic conduits. The compositional evolution of the Nea Kameni fumaroles can be explained by a convective heat pulse from depth associated with the seismic activation of the NE–SW-oriented Kameni tectonic lineament, possibly triggered by either injection of new magma below Nea Kameni island, as apparently suggested by the evolution of the seismic and ground deformation activity, or increased permeability of the volcanic plumbing system resulting from the tectonic movements affecting the area. The results of the present study demonstrate that the geophysical and geochemical signals at Santorini are interrelated and may be precursory signals of renewed volcanic activity and encourage the development of interdisciplinary monitoring program to mitigate the volcanic risk in the most tourist-visited island of the Mediterranean Sea. Keywords Santorini Island . Fluid geochemistry . Geochemical monitoring . Seismic crisis
711, Page 2 of 15
Introduction Santorini is the most active volcanic system of the South Aegean Active Volcanic Arc (SAAVA) (southern Aegean Sea), which extends from Methana peninsula to the west to Nysiros Island to the east. Santorini presently consists of a small archipelago of five islands (Fig. 1): Thera, Thirasia, and Aspronisi, which constitute a ring structure bordering the Santorini caldera, and Palea Kameni and Nea Kameni which emerge in the caldera center. Two volcanic systems occur along a NE–SW trending tectonic line produced by a NNW– SSE extensional stress regime (Vougioukalakis and Fytikas 2005; Sakellariou et al. 2010; Dimitriadis et al. 2009), namely, Christiana Islands, 20 km SW of Santorini caldera, and Coloumbo submarine volcano, an oval-shaped crater (1,700 m in diameter and up to 500 m in depth) located 8 km NW of Thera, where more than 20 volcanic cones were recognized (Alexandri et al. 2003; Nomikou et al. 2012a). Since January 2011, several (up to 50 per day) earthquakes (M
