Geochemical zoning of volcanic chains associated with Pacific hotspots

Abstract

Lavas erupted from volcanoes in the north of the Hawaiian volcanic island chain have a different geochemical signature from those in the south. Analysis of the geochemistry of lavas erupted in the Samoan and Marquesas volcanic chains reveals similar trends, implying that the lowermost mantle beneath the southern Pacific is isotopically enriched. Recent Hawaiian volcanism is manifest as two geographically and geochemically distinct groups of volcanoes1, the Loa trend in the south and the Kea trend in the north2,3. The differences between the Loa and Kea lavas are attributed to spatial variations in the geochemical structure of the underlying Hawaiian mantle plume4,5,6,7,8,9. In turn, the Hawaiian plume structure is thought to reflect heterogeneities in its mantle source7,8. Here we compile geochemical data10 from the Hawaiian and two other volcanic ocean island chains—the Samoan and Marquesas—that formed above mantle plumes upwelling beneath the Pacific plate. We find that the volcanoes at both Samoa11 and the Marquesas12 show geographic and geochemical trends similar to those observed at Hawaii. Specifically, two subparallel arrays of volcanoes exist at both locations. In each case, the southern trend of volcanoes has higher radiogenic lead isotope ratios, 208Pb*/206Pb*, and lower neodymium isotope ratios, ɛNd, than those of the corresponding northern trend. We suggest that geochemical zoning may be a common feature of mantle plumes beneath the Pacific plate. Furthermore, we find that the pattern repeats between island chains, with the highest 208Pb*/206Pb* and the lowest ɛNdfound at Samoa in the south and the lowest 208Pb*/206Pb* and the highest ɛNd observed at Hawaii in the north. We infer that isotopically enriched material is preferentially distributed in the lower mantle of the Southern Hemisphere, within the Pacific low seismic velocity zone.