Hydrogen concentration in mantle xenoliths from the Veneto VolCanic Province (NE Italy).

Hydrogen contents were quantified in olivine (ol) and clinopyroxene (cpx) grains using the nuclear microprobe. The analysed minerals form spinel peridotite xenoliths with protogranular to porphyroclastic texture, hosted in alkaline basalts from two localities of the Eastern Veneto Volcanic Province (NE Italy). Mineral compositions indicated good reequilibration at T-P upper mantle conditions (T = 850-950°C and P = 1.7±0.04 GPa). A combination of ERDA (Elastic recoil Detection Analysis) and RBS (Rutherford Back Scattering) methods were used for H measurements (Laboratoire Pierre Süe, France). The following analytical conditions were applied: 4He+ energy = 3.0 MeV, beam spot size = 4x4 μm2, 4x16 μm2 for ERDA, scanned on large sample areas, scattering angle = 170°, recoil angle = 30° (see also Bureau and co-workers, this issue). 22 samples were prepared following a specific protocol in order to decrease the effect of H absorbed at the surface of the polished samples. The same grains were analysed with electron microprobe, in order to check possible chemical heterogeneities in the areas investigated by ERDA. Chemically homogeneous ERDA maps were selected using Daudin et al. (2003): results reported below are referred to H depleted zones: Sample Wt ppm H H2O ppm wt oxyde Tot rel uncert Ol-vpol2 24 213 15 Ol-vsol4 17 145 15 Cpx-vppx2 50 444 14 Cpx-vspx3 45 398 13 All cpx grains show H contents in the ranges reported from the literature for mantle analogue phases, obtained by commonly used IR spectroscopy, whereas ol grains have H concentrations close to or higher than maximum values reported for mantle ol (Ingrin & Skogby, 2000). Cpx samples systematically show H contents higher than those found for olivine: this is confirmed by recent IR data (Demouchy et al., 2006) and probably due to the fact that H diffusion at mantle temperatures is faster in olivine than in pyroxene.