Systematic variations in the 3He/4He ratio and carbon of fumarolic fluids from active volcanic areas in Italy: Evidence for radiogenic 4He and crustal carbon addition by the subducting African plate?

The isotopic composition of helium from active volcanic areas in Italy differs from those of other are volcanic areas (circum-Pacific region), even though the tectonic settings are similar [1-4]. Helium isotopic values of fumarolic gases were obtained for the southern Italian volcanoes, Mount Etna and the Eolian Islands ([5,6], this work), and from central Italian volcanoes in Neapolitan, Roman and south Tuscany volcanic provinces ([7,8], this work). The He-3/He-4 ratios decrease from south to north, from values slightly lower than those of MORE to near crustal values, including ratios lower than atmospheric (R-a = 1). Contamination by crustal radiogenic helium, derived from the natural decay of uranium and thorium, is the most likely candidate for lowering He-3/He-4 ratios. Increases from south to north in the U and Th contents of volcanic rocks an in agreement with this hypothesis. Local crustal and surficial contaminations may also explain the systematic changes in ratios. Subduction of the African plate beneath the Tyrrhenian Sea seems to be the most logical process to explain the He-3/He-4 ratio decrease. Similarly, carbon isotopic data show higher values from south to north, which suggests increasing contamination of subducted sedimentary carbon into magmatic gases. Correlations with strontium and neodymium isotope data support the above mentioned hypothesis and demonstrate the remarkable link between helium and carbon isotope variations and tectonic processes. Comparison with low helium data from the Banda are (Indonesia), although with a different mechanism, agrees with the presence of a continental end-member below the Tyrrhenian Sea volcanoes. (C) 1997 Elsevier Science B.V.