Helium and argon from an Atlantic MORB glass: Concentration, distribution and isotopic composition

Data are reported for detailed He and Ar measurements on a single MORB glass from the Atlantic (CH 98-DR 11; 30041'N, 41049'W; depthapprox.=3500 m). Grain-size fractions prepared under ambient conditions are strongly affected by diffusive loss of He from grains and by adsorption of atmospheric argon on grain surfaces. Both effects could be controlled by crushing gram-sized chunks of glass under vaccum and analyzing the powder without any further handling, in particular without its exposure to the atmosphere. Gases from vesicles, released upon crushing, are characterized by a 4He/40Ar ratio of 6+-1 and a 40Ar/36Ar ratio of up to 22,600. In dissolved gases the 4He/40Ar ratio was found to be (7.2+-1.6) times higher and the 40Ar/36Ar ratio to be about ten times lower than in vesicles. The difference of the elemental ratio He/Ar is as anticipated from the ratio of the solubilities (ca. nine) in the investigated basalt of He and Ar. Thus, while elemental abundance ratios are compatible with equilibrium between vesicles and basalt the grossly different isotopic ratios are not. It is proposed that two basalts, chemically very similar but with different 40Ar/36Ar ratios, were mixed shortly before eruption. The overall 4He/40Ar ratio in the vesiculated basalt is 12+-2 so that, for the ratio in the primary magma, we find 6<=4He/40Ar<=12 which is considerably higher than the radiogenic production ratio in all conceivable sources of MORB. Preferential removal from a melt of argon via vesicles is suggested to be the most likely explanation, whereas any metasomatic transfer seems unrealistic. (orig.)