Geochemical monitoring of the 2002-2003 eruption at Stromboli volcano (Italy) : precursory changes in the carbon and helium isotopic composition of fumarole gases and thermal waters

• Published in: Bulletin of volcanology 2005-12, Vol.68 (2), p.118-134
• Main Authors: CAPASSO, G, CARAPEZZA, M. L, FEDERICO, C, INGUAGGIATO, S, RIZZO, A
• Format: Article
• Language: English
• Subjects: Air pollution; Crystalline rocks; Degassing; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Fractionation; Gases; Helium; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Isotope fractionation; Isotopes; Lava; Magma; Natural hazards: prediction, damages, etc; Thermal water; Volcanic eruptions; Volcanoes; Volcanology
• ISSN: 0258-8900; 1432-0819
• DOI: 10.1007/s00445-005-0427-5

Significant changes in the helium and carbon isotopic composition of shallow thermal waters vs. gas and a crater fumarolic gas have been recorded at Stromboli prior and during the 2002-2003 eruption. The^sup 3^He/^sup 4^He ratios corrected for air contamination (Rc/Ra), and δ^sup 13^C of fumarolic gases gradually increased from May to November 2002 before the eruption onset. These variations imply early degassing of a gas-rich magma at depth that likely fed both the intense Strombolian activity and small lava overflows recorded during that period. The lava effusion of late December 2002 was shortly preceded by a marked Rc/Ra decrease both in water and fumarolic gases. Comparison of He/CO^sub 2^ and CH^sub 4^/CO^sub 2^ ratios in dissolved gas and with (ProQuest: Formulae and/or non-USASCII text omitted; see image) values rules out the Rc/Ra decrease due to an increasing input of radiogenic^sup 4^He. The Rc/Ra decrease is attributed to the He isotope fractionation during rapid magma ascent and degassing. A new uprising of ^sup 3^He-rich magma probably occurred in January to February 2003, when Rc/Ra ratios displayed the highest values in dissolved gases ever measured before (4.56 Rc/Ra). The increase in He/CO^sub 2^ and CH^sub 4^/CO^sub 2^ ratios and decrease in δ^sup 13^C of dissolved CO^sub 2^ was recorded after the 5 April 2003 explosive paroxysm, likely caused by enhanced gas-water interaction inducing CO^sub 2^ dissolution. No anomalous Rc/Ra values were recorded in the same period, when usual Strombolian activity gradually resumed.[PUBLICATION ABSTRACT]