Isotopes of noble gases from fluid inclusions in mafic phenocrysts from extrusives and ultramafic xenoliths of Stromboli (Aeolian Islands, Italy)

We present a study of noble gases abundances (He, Ne, Ar) and isotopic ratios (He, Ar) in olivine- and clinopyroxene-hosted fluid inclusions from basaltic Low Porphyritic (LP) pumices and High Porphyritic (HP) scorias, respectively of paroxysms and mild explosions of the Stromboli present activity [1] and from ultramafic xenoliths of the 2 ka ago San Bartolo lavas [2]. New petrologic studies on such ultramafic xenoliths show that they are mostly wherlites (subordinately lherzolites), with slightly protogranular to porphyroclastic texture. Thermo-barometric estimates on the mineralogical assemblage indicate that ultramafic xenoliths equilibrated at depth well below the crust-mantle transition of Stromboli. While Ar isotopes are dominated by a clear atmospheric signature in all the samples, 3He/4He from LP pumices and S. Bartolo ultramafic xenoliths converge to a value of 4.2-4.7 Ra, with relatively high gas content. Conversely, the low gas content of the HP scorias allowed us to measure chemical concentrations but not their 3He/4He ratio. Inferences found out from gas content are therefore in full agreement with previous knowledge about differences in volatile content between HP and LP magmas. The 3He/4He measured values are much lower than those of most primitive mantle terms of the Sicilian volcanism (7 ± 0.6 Ra [3]), due to contamination at mantle level beneath Stromboli, operated by the Ionian subducting slab. On the basis of the measured 3He/4He in the LP pumices and ultramafic xenoliths, helium in thermal waters from the Stromboli basal aquifer is a mixing of a mantle-derived and an atmospheric term. The 3He/4He values measured in the rocks constitute the upper limit that should be expected in thermal fluids in case of effusions and/or paroxysms. References. [1] Bertagnini, A., Metrich, N., Landi, P., Rosi., M. (2003): J. Geophys. Res., 108, 1-15; [2] Laiolo, M. & Cigolini, C. (2006): Bull. Volc., 68, 653-670; [3] Martelli, M., Nuccio, P.M.., Stuart, F., Di Liberto, V., Ellam, R.M. (2008): Geochem. Geophys. Geosyst., 9, doi:10.1029/2007GC001730.