Title: Geochemical and Helium Isotopic Variability within the Lau Basin. Abstract approved:

This research incorporates geochemical and helium isotopic analyses of Lau Basin volcanic glasses, along with helium isotopic analyses of water column hydrothermal plumes to better understand the processes that control the formation and evolution of this backarc basin. Lavas newly collected from the four southernmost segments of the Valu Fa Ridge (VFR) reveal gradients in element ratios such as Ba/Nb along axis over the 75 km length of the study area, and show an increase in arc-like geochemical signature to the south. Basalts from the VFR segments and the off-axis volcanoes are derived by 10–20 % melting of a source that was previously melted 0.5–1.5 % in association with arc volcanism. Melt fraction increases southwards along axis from ~10–15 % and ranges up to 20 % for the seamounts. 3He/4He ratios in glasses along the Eastern Lau Spreading Center (ELSC), from 20º–22º45’S, reveal an along-axis gradient. 3He/4He decreases from 9.6 RA in basalts at the Intermediate Lau Spreading Center-Eastern Lau Spreading Center transition, to 5.4 RA in low silica lavas at the southern end of the VFR. The decrease in the maximum 3He/4He towards the south is ascribed to mantle heterogeneity. Th