Geochemical zoning of volcanic chains associated with Pacific hotspots

摘要

Recent Hawaiian volcanism is manifest as two geographically and geochemically distinct groups of volcanoes~1, the Loa trend in the south and the Kea trend in the north~(2,3). The differences between the Loa and Kea lavas are attributed to spatial variations in the geochemical structure of the underlying Hawaiian mantle plume~(4-9). In turn, the Hawaiian plume structure is thought to reflect heterogeneities in its mantle source~(7,8). Here we compile geochemical data from the Hawaiian and two other volcanic ocean island chains Samoan~(11) and Marquesas ~(12)Marquesas 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, ~(208)Pb */~(206)Pb*, and lower neodymium isotope ratios, I Nd, 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 ~(208)Pb*/ ~(206)Pb* and the lowest ∈ Nd found at Samoa in the south and the lowest ~(208)Pb*/~(206)Pb* and the highest ∈ Nd 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.