Geochemical evidence from the Pukapuka volcanic ridge system for a shallow enriched mantle domain beneath the South Pacific Superswell

The South Pacific Superswell is characterized by shallow bathymetry and unusually slow upper mantle seismic velocities, and is also the largest volcanically active intraplate area on the Earth’s surface. Genetic links between the region’s geophysical characteristics and its profuse, isotopically diverse volcanism have been proposed, but not directly shown. We present chemical and isotopic data for volcanic glasses from the 2600 km long Pukapuka ridge system in the eastern Superswell, a feature that appears to have resulted from diffuse lithospheric extension rather than hotspot activity. The samples offer a rare view into the composition of the upper mantle beneath the Superswell away from hotspots and spreading centers. Glasses from the Pukapuka ridges range from transitional basalts with mid-ocean ridge basalt (MORB) characteristics (e.g. La/SmN≈0.8, 87Sr/86Sr≈0.7027, ϵNd≈+9, 206Pb/204Pb≈18.7) to alkalic lavas strongly enriched in incompatible elements and with isotopic signatures similar to the hypothetical lower mantle components ‘FOZO’ and ‘C’ (e.g. La/SmN=7.5, 87Sr/86Sr≈0.7034, ϵNd≈+5, 206Pb/204Pb≈19.8). We propose that the large chemical and isotopic variations in the glasses are controlled by convective mixing of an enriched component upwelling near the center of the Superswell and surrounding normal MORB-source mantle and/or systematically varying degrees of melting of a heterogeneous mixture of these two materials. Weak garnet signatures and trace element evidence for equilibration of the most alkalic lavas with residual phlogopite or amphibole suggest that the Pukapuka lavas were primarily derived from the shallow upper mantle rather than a deep-seated mantle plume. These data are the first direct indication that the upper mantle underlying the Superswell is compositionally as well as thermally anomalous. Additionally, the widespread presence of material of an apparent lower mantle origin in the upper mantle beneath the South Pacific Superswell is consistent with recent mantle convection experiments, in which both Superswell-type features and concentrated hotspot activity are the product of large-scale thermally mediated upwellings in a mildly chemically stratified mantle.