Geochemical mapping of magmatic gas±water±rock interactions in the aquifer of Mount Etna volcano L. Brusca a , A. Aiuppa a , W. D'Alessandro b, * , F. Parello a , P. Allard c , A. Michel d a Dipartimento di Chimica e Fisica della Terra ed Applicazioni alle Georisorse, Universita Á di Palermo, via Archira® 36, 90123 Palermo, Italy b Istituto Geochimica dei Fluidi, C.N.R., via La Malfa 153, 90146 Palermo, Italy c Laboratoire des Sciences du Climat et de l'Environnement, CEA-CNRS, Orme des Merisiers, 91191 Gif/Yvette, France d Laboratoire Pierre Su Èe, CEA-CNRS, CEN-Saclay, Gif/Yvette, France Abstract Systematic analysis of major and minor elements in groundwaters from springs and wells on the slopes of Mt. Etna in 1995± 1998 provides a detailed geochemical mapping of the aquifer of the volcano and of the interactions between magmatic gas, water bodies and their host rocks. Strong spatial correlations between the largest anomalies in pCO 2 pH and alkalinity) K, Rb, Mg, Ca and Sr suggest a dominating control by magmatic gas CO 2 ) and consequent basalt leaching by acidi®ed waters of the shallow meteoric) Etnean aquifer. Most groundwaters displaying this magmatic-type interaction discharge within active faulted zones on the S±SW and E lower ¯anks of the volcanic pile, but also in a newly recognised area on the northern ¯ank, possibly tracking a main N±S volcano-tectonic structure. In the same time, the spatial distribution of T8C, TDS, Na, Li, Cl and B allows us to identify the existence of a deeper thermal brine with high salinity, high content of B, Cl and gases CO 2 ,H 2 S, CH 4 ) and low K/Na ratio, which is likely hosted in the sedimentary basement. This hot brine reaches the surface only at the periphery of the volcano near the Village of Paterno Á, where it gives rise to mud volcanoes called ªSalinelle di Paterno Áº. However, the contribution of similar brines to shallower groundwaters is also detected in other sectors to the W Bronte, Maletto), SW Adrano) and SE Acireale), suggesting its possible widespread occurrence beneath Etna. This thermal brine is also closely associated with hydrocarbon ®elds all around the volcano and its rise, generally masked by the high out¯ow of the shallow aquifer, may be driven by the ascent of mixed sedimentary±magmatic gases through the main faults cutting the sedimentary basement. q 2001 Elsevier Science B.V. All rights reserved. Keywords: geochemical mapping; magmatic gases; Mount Etna 1. Introduction Mount Etna Eastern Sicily) is a highly active alkali-basaltic strato-volcano built upon tensional faults at the collision boundary of the African and European blocks, Barberi et al., 1974). Its activity and growth over the last 0.2 Ma has been controlled by the intersection of two main fault systems, striking NNW±SSE and NNE±SSW, respectively, and a shallower E±W system, cutting a 18±20 km thick continental crust which upper part is made of carbon- aceous±terrigenous Mesozoic±Pleistocene deposits. Intense seismicity occurs along these regional fault systems. An intense magmatic gas release is known to persist along such weakness zones, together with degassing Journal of Volcanology and Geothermal Research 108 2001) 199±218 0377-0273/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0377-027300)00286-9 www.elsevier.com/locate/jvolgeores * Corresponding author. Present address: Istituto Nazionale di Geo®sica e Vulcanologia, Sezione di Palermo, via Ugo La Malfa, 153, 90146 Palermo, Italy. Tel.: 139-091-6809409; fax: 139-091- 6804949. E-mail address: walter@pa.ingv.it W. D'Alessandro).