Noble gas constraints on degassing processes

We compare the rare gas elemental ratios measured in glass samples of mid-oceanic ridges (MORs) and oceanic island basalts to current estimates of the same ratios for upper and lower mantle reservoirs. We selected only samples with high 20Ne/22Ne (>11) in order to minimize atmospheric contamination. The combined use of He, Ne and Ar elemental systematics in these samples allows differentiation between the vesiculation process at mantle plumes, which appears to be in open system, and at normal MORs, where vesiculation occurs in a closed system. Such an open system vesiculation has important consequences for the interpretation of heavy rare gas isotopic ratios in primitive plume-derived materials.