V21D-06
Kolumbo active seamount (Greece): A window into the Aegean mantle

Tuesday, 15 December 2015: 09:30
308 (Moscone South)
Andrea Luca Rizzo1, Antonio Caracausi2, Valérie Chavagnac3, Paraskevi Nomikou4, Paraskevi Polymenakou5, Antonios Magoulas5, Manolis Mandalakis6 and Giorgos Kotoulas6, (1)Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Palermo, Italy, (2)National Institute of Geophysics and Volcanology, sez. Palermo, Rome, Italy, (3)GET Géosciences Environnement Toulouse, Toulouse, France, (4)National and Kapodistrian University of Athens, Athens, Greece, (5)Hellenic Center for Marine Research, Anavyssos, Greece, (6)Institute of marine biology, biotechnology and aquaculture, Heraklion, Greece
Abstract:
Submarine volcanism is ubiquitous in active tectonic settings of the earth, but due to depth and hazardousness of these environments the study is a challenge. In May 2014, we performed a cruise in the Aegean Sea aimed to investigate the high-temperature (>200°C) hydrothermal system of Kolumbo active underwater volcano, 7 km northeast off Santorini. Last explosive eruption occurred in 1650 A.D. and killed ~70 people, so plainly the eruptive potential is real.

We sampled gases discharged from seven chimneys located at ~500 m b.s.l. and we investigated their composition. The chemistry indicates that these consist of almost pure CO2 with a small atmospheric contamination. The δ13C-CO2 varies from 0 to 1.5‰ and shows a positive correlation with the concentration of He, H2, CO and CH4 as the result of chemical and isotope fractionation due to variable extents of gas-water interaction. The 3He/4He varies from 7.0 to 7.1 Ra, coherently with the fact that this ratio does not suffer any fractionation due to gas-water interaction. These values are surprisingly higher (more than 3 units Ra) than the measurements performed in gases and rocks from Santorini (Rizzo et al., 2015). They are in the typical range of arc volcanoes worldwide (7-9 Ra; Hilton et al., 2002; Di Piazza et al., 2015), indicating that the 3He/4He ratios measured at Kolumbo are likely the result of direct mantle degassing in a general extensive regime. More importantly, these ratios are the highest in all the South Aegean volcanism, which leads to consider homogeneous (and MORB-like) the He isotope composition of the mantle below the central part of the Hellenic Volcanic Arc and eastward up to Nisyros, which until this study showed the highest ratios (6.2Ra; Shimizu et al., 2005).

Our results strongly emphasize the role of tectonics in the transfer of fluids from the mantle toward the surface. The complicated geodynamics status of the Aegean-Anatolian region, plays a key role in generating crustal stretching and rising up magma-bodies from the mantle into the crust.

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