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

The South Pacific Superswell is characterized by shallow bathymetry and unu sually slow upper mantle seismic velocities, and is also the largest volcan ically active intraplate area on the Earth's surface. Genetic links between the region's geoyhysical characteristics and its profuse, isotopically div erse volcanism have been proposed, but not directly shown. We present chemi cal and isotopic data for volcanic glasses from the 2600 km long Pukapuka r idge system in the eastern Superswell, a feature that appears to have resul ted from diffuse lithospheric extension rather than hotspot activity. The s amples offer a rare view into the composition of the upper mantle beneath t he Superswell away from hotspots and spreading centers. Glasses from the Pu kapuka ridges range from transitional basalts with mid-ocean ridge basalt ( MORB) characteristics (e.g La/Sm-N approximate to 0,8, Sr-87/Sr-86 approxim ate to 0.7027, epsilon(Nd) = +9, Pb-206/Pb-204 approximate to 18.7) to alka lic lavas strongly enriched in incompatible elements and with isotopic sign atures similar to the hypothetical lower mantle components 'FOZO' and 'C' ( e.g. La/SmN = 7.5, Sr-87/Sr-86 approximate to 0,7034, epsilon(Nd) approxima te to +5, Pb-206/Pb-204 approximate to 19.8). We propose that the large che mical and isotopic variations in the glasses are controlled by convective m ixing of an enriched component upwelling near the center of the Superswell and surrounding normal MORE-source mantle and/or systematically varying deg rees of melting of a heterogeneous mixture of these two materials, Weak gar net signatures and trace element evidence for equilibration of the most alk alic lavas with residual phlogopite or amphibole suggest that the Pukapuka lavas were primarily derived from the shallow upper mantle rather than a de ep-seated mantle plume. These data are the first direct indication that the upper mantle underlying the Superswell is compositionally as well as therm ally anomalous. Additionally, the widespread presence of material of an app arent lower mantle origin in the upper mantle beneath the South Pacific Sup erswell is consistent with recent mantle convection experiments, in which b oth Superswell-type features and concentrated hotspot activity are the prod uct of large-scale thermally mediated upwellings in a mildly chemically str atified mantle. (C) 2000 Elsevier Science E.V, All rights reserved.