Long Term Geochemical Monitoring of Stromboli Volcano (Italy): Clues On 2002-2003 Eruption From Gas Chemistry, Helium And Carbon Isotopes

Since 1999, a geochemical monitoring has been carried out on both the hydrothermal system and the crater fumaroles of Stromboli volcano. A few (3) thermal waters as well as a low temperature (98° C) fumarole, located on the "Pizzo sopra La Fossa" area, are monthly sampled and analysed for their chemical and isotope (He and C) composition until the onset of 2002-2003 eruption. CO₂ is the main gas species dissolved in thermal waters, with concentrations ranging from 30 to 160 cc/l STP. The sampled fumarole showed a CO₂ dominant composition (about 100% vol.). Measured ³He/⁴He ratios are in the range of 3.6-4.3 Ra (corrected for air contamination), which represents the higher magmatic marker recently recognised at Stromboli (Inguaggiato & Rizzo, 2003). Since June 2002, synchronous variations in the dissolved CO₂ and ³He/⁴He ratios were observed among the sites. Such variations occurred before and during the main periods of volcanic unrest, displaying similar trends with those observed at the crater fumarole. Such findings suggest that episodes of magma degassing and ascent occurred in the plumbing system some months before the onset of the 2002-2003 eruption (28th December), inducing a pressure increase in the shallow levels of the volcano. Similar variations, although with a different range, were recognized before and during the 5th April 2003 explosive event, suggesting again a new input of gas-rich magma batches which induced a pressure increase in the shallow feeding system. Gaseous CO₂ of the fumarole showed a δ ¹³C average value of -2, which represents the typical magmatic marker for this volcano, falls in the range of Mediterranean volcanoes area (0 to -2 ‰ , Inguaggiato et al., 2000). The CO₂ gas isotope data of thermal waters, recalculated from TDIC values, highlighted a wide range of variation, from -9 to 0 ‰ versus PDB linked to a mixing between magmatic and atmospheric end-members(-2 ‰ -7 ‰ respectively). Since June 2002, temporal variations of recalculated δ ¹³C values showed more homogeneous isotope data around -2‰ , suggesting an increased magmatic contribution of CO₂ in the hydrothermal system. After 5th April, the absence of new input of undegassed magma determined a progressive trend towards more negative isotope values in the thermal waters. The long term monitoring of Stromboli volcano allowed us to recognise signals of magma degassing before and during the main episodes of volcanic unrest, thus representing an important tool in forecasting volcanic activity. Inguaggiato S., G. Pecoraino, and F. D'Amore. Chemical and isotopical characterization of fluid manifestations of Ischia Island (Italy). J.Volcanol.Geoth.Res. 99:151-178, 2000. Inguaggiato S. & A.Rizzo. Dissolved helium isotope ratio in ground-waters: A new technique based on gas-water re-equilibration and an application to a volcanic area" Appl. Geochem. Accepted. 2003