Abstract

Volcanism in three Pacific Ocean hotspot tracks, Hawaii, Samoa, and the Marquesas Islands, has been shown to be organized into two geographically distinct, subparallel trends. On all three tracks, the pairs of volcanic trends are geochemically distinct, with magmas from the southern trend possessing an enriched isotopic signature (e.g., lower 143Nd/144Nd and higher 87Sr/86Sr and 208Pb*/206Pb*) relative to the northern trend. The systematic difference between northern and southern trends at these three hotspots, with the possible exception of Samoa, has been attributed to their location near the northern boundary of the DUPAL belt (acronym after Dupré and Allègre), a region of isotopic enrichment in the Southern Hemisphere mantle that coincides with a seismic large low shear-wave velocity province in the lowermost mantle beneath the Pacific. This model invokes azimuthal, bilateral zoning within the plume conduits beneath each hotspot; the southern side of the plume conduits entrain isotopically enriched material from the DUPAL region, and the northern sides entrain comparatively depleted mantle. Here we present evidence of volcanism with subparallel trends in a fourth Pacific hotspot track (the Society Islands archipelago) located on the southern side of the DUPAL belt. As with the other three hotspots, magmas from the northern and southern volcanic trends are geochemically distinct. However, in the Society Islands the loci of enrichment are reversed, i.e., the northern trend is isotopically enriched relative to the southern. This reversed pattern of enrichment at the Society Islands archipelago is consistent with the hypothesis that spatial variations in enrichment at each Pacific hotspot reflect its position relative to the DUPAL belt. This indicates that volcanism at hotspots may be used as a window on the geometry of deep mantle reservoirs.

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