Origin of Argon-Lead Isotopic Correlation in Basalts

Sarda et al. (1) show that there is a correlation between Pb/Pb and maximum Ar/ Ar values in Atlantic basalts. The authors argue that this correlation is due to mixing between a degassed-depleted upper mantle (Ar/Ar . 40,000 and Pb/Pb ' 17.8) and a recycled component in the mantle (HIMU) that has unradiogenic Ar resulting from recycling of atmospheric Ar (Ar/ Ar 5 300). 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. From the curvature of the hyperbola in Ar/Ar–Pb/Pb space, Sarda et al. (1) suggest a K/Ar ratio of 3000 to 6000 in the recycled endmember. The K content of the HIMU endmember is difficult to constrain, but is unlikely to be less than 0.1 wt % K2O (2), implying a minimum Ar content of 20 3 10 cm STP g in the HIMU endmember. This is similar to the highest values measured in basalts (' 2 2 20 3 10 cm STP g) (3), and implies that oceanic crust is subducted without Ar loss. If average basalt was subducted (' 6 3 10 cm STP g) without volatile loss, the r coefficient in Pb/Pb–Ar/Ar space would be ' 1 (a straight line). It is reasonable to expect that gas loss during subduction would result in r values .. 1 (the opposite curvature to that observed). It is difficult to envisage a scenario where subducted Ar can result in an r correlation of 0.15 as reported by Sarda et al. (1); it is therefore necessary to examine an alternative mechanism for the observed correlations. There is a broad anti-correlation between Ar/Ar and eruption depth (Fig. 1) in the data presented by Sarda et al. (1). This can reasonably be attributed to more extensive magmatic degassing at shallow levels, producing basalts more susceptible to atmospheric contamination. Basalts with elevated Pb/Pb ratios in the Atlantic are commonly associated with topographic highs. Yet the correlation between Ar/Ar and Pb/ Pb, rather than the consequence of recycled Ar in the HIMU source, may equally be the result of preferential atmospheric contamination of the more shallow erupted HIMU basalts. This argument does not eliminate the possibility of Ar recycling to the mantle; the correlation between Ar/Ar and depth may result from a Ar–rich HIMU component which then produces topographic highs on the seafloor. However, given the problems associated with recycling atmospheric Ar in the quantities required, and the viable alternative explanation to the correlation between Ar and Pb isotopes, subduction of atmospheric Ar is not yet proven.