Noble gas constraints on degassing processes

We compare the rare gas elemental ratios measured in glass samples of mid-o ceanic ridges (MORs) and oceanic island basalts to current estimates of the same ratios for upper and lower mantle reservoirs. We selected only sample s with high Ne-20/Ne-22 (>11) in order to minimize atmospheric contaminatio n. 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 c onsequences for the interpretation of heavy rare gas isotopic ratios in pri mitive plume-derived materials. (C) 2000 Elsevier Science B.V. All rights r eserved.