First geochemical data from fumarolic gases at Lascar volcano, Chile Tassi, F.(1), Viramonte, J.(2), Vaselli, O.(1), Poodts, M.(2), Aguilera, F.(3), Martinez, C.(3), Rodriguez, L.A.(4), Watson, I.M.(4) (1)

Dip. di Scienze della Terra, Universita degli Studi di Firenze, Via La Pira 4, 50121, Italia. Instituto GEONORTE, Fac. de Ciencias Nat., Univ. Nac. de Salta, Buenos Aires, 177, Salta, Argentina. (3) Dep..de Ciencias Geol., Univ. Catolica del Norte, Av. Angamos, 0610, Casilla, 1280, Antofagasta, Chile. (4) Geol. and Mining Engineering & Sciences, Michigan Technological University, Houghton, MI 49931. (2)

Lascar is the most active volcano of the northern Chilean Andes. This andesitic-to-dacitic stratovolcano consists of six nested-cones craters. Its activity began at the easternmost volcano and then shifted to the westernmost cone. Lascar volcano has been characterized by the occurrence of frequent small-to-moderate explosive events, along with periodic larger eruptions, that produced ash-fall hundreds of kilometers away from the volcano. The largest historical eruption took place in 1993 and produced pyroclastic flows that extended up to 8.5 km NW of the summit (Viramonte et al., 1994). Comparison with previous descriptions (Matthews et al., 1997) indicated that, after the last eruptive event (July 2000; GVN Bulletin v.25, no. 6), several changes in crater morphology and locations of the gas emissions have been observed. Presently, the active crater, 800 m in diameter and over 400 m deep, hosts several high-flux fumaroles, producing a plume visible from the city of San Pedro de Atacama, at about 60 kilometers WNW. On the 1st of November 2002, two fumaroles, located along the upper collapse ring border in the NW internal flank of the active crater, have been sampled. They represent the first gas samples ever collected in the crater. A maximum temperature of 385°C was measured. Gas composition is mainly characterized by CO2 (up to 690,000 µmol/mol), H2O (up to 170,000 µmol/mol), high SO2 (up to 69,000 µmol/mol), H2 (up to 68,000 µmol/mol), HCl (up to 9,700 µmol/mol), HF (up to 1,600 µmol/mol), and CO (up to 945 µmol/mol) contents have been measured. N2 contents are also relatively high (N2/Ar=1000), typical of gases of andesitic volcanoes, while H2S resulted below detection limits (<0.01 µmol/mol). Gas samples presents minor air contamination, has testified by the low Ar, O2 and Ne contents, while, among the organic species only CH4, C2-C3 alkane-alkene pairs, iso-butene and benzene have been detected. These features suggest that, at Lascar volcano, gases are mainly produced by direct degassing of a magmatic body. Moreover, they seem to be almost not affected by both water-rock interactions and the presence of aquifers, possibly due to the rapid flux of the ascending fluids and/or the proximity the magmatic source to the surface. References: Matthews, S.J., Gardeweg, M.C., Sparks, R.S.J., 1997. Bull. Volcanol., 59, 72-82. Viramonte, J.G., Seggiaro, R.E., Becchio, R.A., Petrinovic, I.A., 1994. 4th Int. Meeting of Colima Volcano, Mexico, 149-151.