Evidence for Preferential Melting of the Enriched Components in Polynesian Plumes

Although much work has been and is being done on the subaerial parts of ocean island volcanoes, we still know relatively little about the submarine stages of their growth. Drilling on islands (e.g. Stolper et al. J. Geophys. Res. 101, 11593-11598, 1996) will help to answer some of the outstanding questions, it must, however, be complemented by sampling of present-day submarine activity. The subaerial parts of the Pitcairn and Society hotspot traces in Polynesia consist of isolated islands constructed from one or at most two volcanic edifices. This is in stark contrast to the seafloor expressions of the active hotspots, which consist of many small volcanoes associated with several larger ones (e.g. Binard et al. Tectonophys. 186, 293-312, 1991; Tectonophys. 206, 245-264, 1992). Samples from these small volcanoes show them all to be erupting highly fractionated magmas derived exclusively from the enriched, isotopically-extreme plume components (EM-I at Pitcairn, EM-II at Society), evidence that these components have the lowest melting temperatures and are the first product of melting of the plume source. We conclude that the first melt batches from the plume source establish a new conduit system to the surface and are highly cooled and fractionated in the lithosphere in the process. Only if subsequent magma batches from the same source region re-use the same plumbing system do the conduit walls become sufficiently heated for basic magmas to traverse the lithosphere and erupt, leading to the growth of a larger volcano and possibly eventually an island.