Isotope geochemistry of Pantelleria volcanic fluids, Sicily Channel rift: a mantle volatile end-member for volcanism in southern Europe

Chemical and isotopic ratio (He, C, H and O) analysis of hydrothermal manif estations on Pantelleria island, the southernmost active volcano in Italy, provides us with the first data upon mantle degassing through the Sicily Ch annel rift zone, south of the African-European collision plate boundary. We find that Pantelleria fluids contain a CO2-He-rich gas component of mantle magmatic derivation which, at shallow depth, variably interacts with a mai n thermal (similar to 100 degrees C) aquifer of mixed marine-meteoric water . The measured He-3/He-4 ratios and delta(13)C Of both the free gases (4.5- 7.3 R-a and -5.8 to -4.2 parts per thousand, respectively) and dissolved he lium and carbon in waters (1.0-6.3 R-a and -7.1 to -0.9 parts per thousand) , together with their covariation with the He/CO2 ratio, constrain a He-3/H e-4 ratio of 7.3 +/- 0.1 R-a and a delta(13)C of ca. -4 parts per thousand for the magmatic end-member. These latter are best preserved in fluids eman ating inside the active caldera of Pantelleria, in agreement with a higher heat flow across this structure and other indications of an underlying crus tal magma reservoir. Outside the caldera, the magmatic component is more af fected by air dilution and, at a few sites, by mixing with either organic c arbon and/or radiogenic He-4 leached from the U-Th-rich trachytic host rock s of the aquifer. Pantelleria magmatic end-member is richer in He-3 and has a lower (closer to MORB) delta(13)C than all fluids yet analyzed in volcan ic regions of Italy and southern Europe, including Mt. Etna in Sicily (6.9 +/- 0.2 R-a, delta(13)C = 3 +/- 1 parts per thousand). This observation is consistent with a south to north increasing imprint of subducted crustal ma terial in the products of Italian volcanoes, whose He and C (but also O and Sr) isotopic ratios gradually evolve towards crustal values northward of t he African-Eurasian plate collision boundary. Our results for Pantelleria e xtend this regional isotopic pattern further south and suggest the presence of a slightly most pristine or 'less contaminated', He-3-richer mantle sou rce beneath the Sicily Channel rift zone. The lower than MORB He-3/He-4 rat io but higher than MORB CO2/He-3 ratio of Pantelleria volatile end-member a re compatible with petro-geochemical evidence that this mantle source inclu des an upwelling HIMU-EM1-type asthenospheric plume component whose origin, according to recent seismic data, may be in the lower mantle. (C) 2000 Els evier Science B.V. All rights reserved.