Geochemical and helium isotopic variability within the Lau Basin

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 20°--22°45'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. The increased variability in 3He/4He along the VFR is due to processes occurring during prolonged residence of magma in the crust prior to eruption. Basalt 3He/4He values are similar to those measured from high-temperature vent fluids in the same region (Proskurowski et al., 2006).;Neutrally buoyant plumes with detectable 3He anomalies (delta3He=65--70%) were present in all of the areas sampled, indicating an abundance of hydrothermal activity along the ELSC. Buoyant plumes with higher delta3He (100--225%) were sampled at two of the target areas. The delta3He anomalies revealed by the hydrocast profiles are ∼200 m thick at each site. The depth of the neutrally buoyant hydrothermal plumes shallows from 2300--2600 m in the north to 1600--1800 m in the south. This change occurs in conjunction with the shallowing of the ELSC ridge axis, which is further enhanced by a transition in ridge morphology from axial valley to axial high at 20° 30'S. 3He/heat ratios of 1--2.8x10 -17 mol J-1 are calculated for the neutrally buoyant plumes, while lower values of 0.8x10-17 mol J -1 are calculated for the buoyant plumes. These values are similar to those observed in vent fluids sampled from relatively mature hydrothermal systems along mid-ocean ridges (e.g., EPR at 21°N, Cleft segment of the Juan de Fuca Ridge and TAG at 26°N along the MAR), and about an order of magnitude less than values observed over young lava flows from recently perturbed magmatic systems.