ATMOSPHERIC ARGON CONTAMINATION OF OCEAN ISLAND BASALT OLIVINE PHENOCRYSTS

Ar-40/Ar-36 and helium and argon concentrations have been repeatedly m easured on olivine phenocrysts in a single tholeiitic basalt (PIN-12) from the Juan Femandez hotspot. Forty olivine splits were analyzed by crushing of bulk samples or laser fusion of single crystals. The measu red Ar-40/Ar-36 ratios span a very large range (400-7700) and are cons istent with binary mixing of two argon components. Unlike argon, heliu m in repeated measurements of this single basalt flow has a reproducib le isotopic ratio, 17 times the air He-3/He-4 value. It is unlikely th at such large variations in Ar-40/Ar-36 represent microscale mantle he terogeneity. Rather the results indicate highly variable mixing propor tions of a mantle-derived radiogenic argon component (Ar-40/Ar-36 > 77 00) 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 indiv idual crystals by laser fusion shows that both the radiogenic and the contaminant components are in fluid inclusions. Siting of the contamin ant in inclusions requires the addition of air-derived noble gases to hotspot magmas prior to or during emplacement, a process that may occu r by assimilation of altered crust during crustal storage or, alternat ively, by direct addition of air or seawater to the magma. In either c ase the olivines must continue to trap argon, presumably by fracture a nnealing and/or bubble enclosure, after the contaminating event. If at mospheric contamination is a general phenomenon, the Ar-40/Ar-36 compo sition of olivines (and possibly of basalt glasses as well) must be on ly a lower limit for the mantle source ratio. With the exception of he lium, the other noble gases may be similarly compromised. Our results support contentions that lavas with near-atmospheric noble gas composi tions reflect severe atmospheric contamination, rather than the air-li ke signature of an undegassed primitive mantle reservoir.