ACTIVE SUBMARINE VOLCANISM ON THE SOCIETY HOTSPOT SWELL (WEST PACIFIC) - A GEOCHEMICAL STUDY

The petrography and geochemistry of lavas dredged from five active submarine volcanoes from the southeast end of the Society Islands hotspot trace were studied. Most samples are basic and alkaline, ranging from 16 to 5 wt % MgO, with about 5% normative nepheline. Fractionation modelling based on major and minor compatible element variations suggests that olivine and minor clinopyroxene were the major fractionating phases and implies a maximum range of fractionation of 30-35%. There is indication of a mantle component, enriched by injection of material from a subducted oceanic slab. Correlation of 207Pb/204Pb with 87Sr/86Sr suggests that the subducted material is geochemically old. Mapping the geochemical variations shows that the contribution to the lavas from the subduction component is greater over the north of the hotspot than in the south. It is concluded that the Pacific Superswell is unlikely to be caused by a large convective plume. The Superswell is more probably located above a region in the asthenospheric mantle which, due to its higher content of recycled continental debris, is anomalously hot. -after Authors