THE TWO LONGEST-LIVED PACIFIC HOTSPOTS, DIVERGING SINCE 80 MA

Long-lived age-progressive hotspot tracks, like Hawaii-Emperor and Louisville, provide key information about plate and mantle plume motions, large-scale mantle flow and the geochemical evolution of heterogeneous mantle sources. While the Hawaii and Louisville hotspots provide a clear record back to 80 million years ago (Ma), the record prior to 80 Ma is largely incomplete. Longer-lived hotspot tracks are needed to resolve plume and plate motions at earlier times. Using a combination of rare geochemical signatures and ages, two chains of volcanoes, the Rurutu hotspot (currently active in the Austral islands) and the Samoan hotspot (currently active near the Tonga trench) can both be traced across the Pacific plate to their intersection with, and subduction into, the Mariana trench. At these intersections, the oldest Rurutu and Samoa seamounts are ~120 and 100 Ma, respectively, making them the longest-lived hotspots in the Pacific. The recognition of these tracks through geochemistry and ages, combined with the realization that prior to ~80 Ma plate motion models rely on Pacific large igneous provinces that erupted near-ridge, suggests plate and plume motion should be reassessed for this period. More importantly, the density in reconstructed locations now allows for tracing Rurutu, Samoa, Hawaii, and Louisville back in time, for the first time including both relative longitudinal and latitudinal changes. Prior to 80 Ma, the long-lived Rurutu and Samoa hotspots appear not to have moved with respect to each other. However, starting at 80 Ma, Rurutu and Samoa diverge in reconstructed locations, likely due to relative plume motion. Thus, there are now four hotspot tracks for which the backtracked locations have sufficient detail to constrain relative plume motions in modeling of plate, plume and mantle motion. Two of these tracks extend relative constraints back past ~100 Ma.