Constraints on the noble gas composition of the deep mantle by bubble-by-bubble analysis of a volcanic glass sample from Iceland

• Single bubble compositions in a primitive tholeiitic glass, Iceland, are presented.
• The heterogeneity is controlled by an equilibrium degassing of the magma.
• The plume source is not enriched in C over He relative to the normal upper mantle.
• The 20Ne/22Ne ratio, corrected from air-contamination, is similar to neon-B.

Contamination of samples by air-derived noble gases is a well-known problem in noble gas geochemistry, and, as a result, determining the true neon and argon isotopic ratios of the mantle is not straightforward. Here, we directly target individual bubbles in an Icelandic volcanic glass, DICE 11, with a 193 nm excimer laser in order to reduce air-contamination, and analyze He and Ar isotopes, plus 22Ne abundances, on a Helix SFT mass-spectrometer. The CO2 content of the bubbles was measured with a capacitance manometer. In addition, new He, Ne and Ar compositions obtained by crushing on similar samples (DICE 10 and DICE 11) are presented.Our analyses show that 3He/4He ratios are homogeneous in all the vesicles in this glass sample at 17.4 ± 0.4 Ra and are consistent with analyses by crushing. Precise 40Ar/36Ar isotopic ratios were obtained on the largest vesicles only, due to high blank contributions to the smallest vesicles, and are 8600 ± 700, i.e. the highest values measured so far in primitive basalts from Iceland. Considering that the Ar and He isotopic compositions obtained for individual vesicles (by laser ablation) are representative of the true mantle source values, not contaminated by air, we can precisely correct the Ne isotopic analyses obtained by crushing for air contamination; the corrected 20Ne/22Ne ratios are consistent with the presence of neon-B in the sub-Icelandic mantle, which is consistent with the hypothesis that neon in the OIB source region has an irradiated meteorite origin rather than purely solar. In contrast to He and Ar isotopic compositions (which are homogeneous), the relative He–Ne–Ar–CO2 abundances in the different bubbles are heterogeneous and show strong correlations which are compatible with equilibrium degassing of the magma. Because He and CO2 do not fractionate during this degassing, we can precisely constrain the CO2/3He ratio of the mantle source at 6.7 ± 0.5 ∗ 108, at the lower end of the MORB range. Therefore the deep mantle below Iceland is not enriched in carbon compared to the upper mantle (via recycling, for example).