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[1] New geochemical and isotopic data are presented for lavas from three sites in the Havre Trough-Lau Basin back arc and six volcanoes along the Kermadec arc. The back arc basalts range from MORB-like to arc-like in composition and contain a variable contribution from the underlying slab. The least contaminated MORB-like back arc lavas from 24°-29°S are low degree partial melts of a source with Pacific MORB isotopic characteristics. A transition occurs at 30°S between the strongly depleted northern Kermadec (and Tonga) arc lavas and the mildly depleted southern Kermadec arc lavas. This transition does not correlate with changes in the back arc extension rate or width but may reflect inhibited mantle wedge replenishment behind the shallower-dipping northern Kermadec-Tonga slab. Northern Kermadec lavas require mixing between two components: (1) depleted Havre Trough mantle and (2) fluid derived from altered MORB crust with a slight input of sediment lead. Inter-volcano differences in fluid compositions probably reflect local variations on the subducting slab rather than mineralogical variation in the mantle wedge. Southern Kermadec lavas require an additional component: (3) Pacific sediment melt. This sediment melt is only detected where the subduction rate is <7 cm/year, and requires thermal heating of the slab to >650°C before passing through the sub-arc melt generation zone.

[1] Estimates of the relative motion between the Hawaiian and Louisville hot spots have consequences for understanding the role and character of deep Pacific-mantle return flow. The relative motion between these primary hot spots can be inferred by comparing the age records for their seamount trails. We report the seamount chain, including the HEB. This model predicts an age for the oldest Emperor Seamounts that matches published ages, implying that a linear age-distance relationship might extend back to at least 82 Ma. In contrast, Hawaiian age progression was much faster since at least $15 Ma and possibly as early as $27 Ma. Linear age-distance relations for the Hawaii-Emperor and Louisville seamount chains predict $300 km overall hot spot relative motion between 80 and 47.5 Ma, in broad agreement with numerical models of plumes in a convecting mantle, and paleomagnetic data. We show that a change in hot spot relative motion may also have occurred between $55 Ma and $50 Ma. We interpret this change in hot spot motion as evidence that the HEB reflects a combination of hot spot and plate motion changes driven by the same plate/mantle reorganization.

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