Origin of Argon-Lead Isotopic Correlation in Basalts

Sarda et al. (1) show that there is a correlation between ^(206)Pb/^(204)Pb and maximum ^(40)Ar/^(36)Ar values in Atlantic basalts. The authors argue that this correlation is due to mixing between a degassed-depleted upper mantle (^(40)Ar/^(36)Ar > 40,000 and ^(206)Pb/^(204)Pb ≈ 17.8) and a recycled component in the mantle (HIMU) that has unradiogenic Ar resulting from recycling of atmospheric Ar (^(40)Ar/^(36)Ar = 300). However, devolatilization of subducting material is well documented in arc environments and it seems extraordinary that Ar, a highly volatile element, can be recycled in significant amounts. I aim to show that the Ar content required of the recycled slab is implausibly high and that most of the variation in Ar isotopic composition could be the result of shallow-level atmospheric contamination.