Rare gases from the undepleted mantle?

Abstract

If the concept of a depleted/undepleted mantle system is valid, rare gas isotopic composition and abundances should be different in continental and plume areas fed from the undepleted mantle than in oceanic glasses fed from a depleted mantle¹. Ultramafic inclusions in basalt have been studied in attempts to find examples of undepleted mantle trapped gases^2–6. In particular, Kaneoka and Takaoka measured the helium and argon isotopic abundances in olivine nodules and phenocrysts from Hawaiian basalt⁷. The nodules are characterized by midocean ridge basalt (MORB)-type ⁴⁰Ar/³⁶Ar and ³He/⁴He ratios, while the phenocryst data show lower ⁴⁰Ar/³⁶Ar and higher ³He/⁴He; these latter data were interpreted to indicate an undepleted mantle source relative to that for the nodules and oceanic tholeiites. I have measured ⁴He, ⁴⁰Ar and ³⁶Ar abundances in two Hawaiian ultramafic nodules, and find clear evidence of incomplete retention of mantle gases plus significant atmospheric contamination. Comparison with the previously measured phenocrysts indicates the prevalence of these complications in all such ultramafic samples. Conclusions concerning the nature of the rare gases in the undepleted mantle based on such samples are invalid.