Atmospheric argon contamination of ocean island basalt olivine phenocrysts

^(40)Ar/^(36)Ar and helium and argon concentrations have been repeatedly measured on olivine phenocrysts in a single tholeiitic basalt (PIN-12) from the Juan Fernandez hotspot. Forty olivine splits were analyzed by crushing of bulk samples or laser fusion of single crystals. The measured ^(40)Ar/^(36)Ar ratios span a very large range (400–7700) and are consistent with binary mixing of two argon components. Unlike argon, helium in repeated measurements of this single basalt flow has a reproducible isotopic ratio, 17 times the air ^3He/^4He value. It is unlikely that such large variations in ^(40)Ar/^(36)Ar represent microscale mantle heterogeneity. Rather the results indicate highly variable mixing proportions of a mantle-derived radiogenic argon component (^(40)Ar/^(36)Ar > 7700) and an isotopically air-like endmember that is almost certainly an atmospheric contaminant. This air-like constituent cannot be removed by physical and chemical treatments of the olivines. Analysis of individual crystals by laser fusion shows that both the radiogenic and the contaminant components are in fluid inclusions. Siting of the contaminant in inclusions requires the addition of airderived noble gases to hotspot magmas prior to or during emplacement, a process that may occur by assimilation of altered crust during crustal storage or, alternatively, by direct addition of air or seawater to the magma. In either case the olivines must continue to trap argon, presumably by fracture annealing and/or bubble enclosure, after the contaminating event. If atmospheric contamination is a general phenomenon, the ^(40)Ar/^(36)Ar composition of olivines (and possibly of basalt glasses as well) must be only a lower limit for the mantle source ratio. With the exception of helium, the other noble gases may be similarly compromised. Our results support contentions that lavas with near-atmospheric noble gas compositions reflect severe atmospheric contamination, rather than the air-like signature of an undegassed primitive mantle reservoir