John Lupton (NOAA/PMEL Newport), Marvin Lilley (Univ. of Washington), Dave Butterfield and Joe Resing (both at NOAA/PMEL Seattle)

The northern Lau Basin is marked by a complicated pattern of back-arc spreading centers and distinct volcanic cones and calderas adjacent to the Tofua Arc.  Recent eruptive activity at West Mata volcano and along the NE Lau Spreading Center has focused special attention on this region.  One question that has been difficult to answer is whether these volcanic systems have arc or back-arc affinities.  Helium and carbon isotope ratios and C/³He ratios can help to differentiate between arc vs. back-arc influences.  Another factor is the Samoan hotspot signal, characterized by high ³He/⁴He ratios, which has penetrated southward into the northern Lau Basin through a tear in the downgoing Pacific plate.  Thus far this Samoan influence has only been seen along the NW Lau Spreading Center some distance from the Tofua Arc, where ³He/⁴He ratios in the basalts reach values up to 28 Ra (R = ³He/⁴He and Ra = Rair = 1.4 × 10^-6).   True back-arc systems are similar to mid-ocean ridge (MOR) systems with ³He/⁴He of ~ 8 Ra, δ¹³C - CO₂) of  -4 to -12 ‰, and C/³He of ~1 × 10⁹.  In contrast, arc volcanoes typically have lower ³He/⁴He ratios, heavier δ¹³C of -2 to +2 ‰, and higher C/³He ratios (> 10¹⁰), presumably due to the incorporation of downgoing slab components such as marine carbonates.  West Mata, for example, has ³He/⁴He of 7.3 Ra and C/³He of 10¹⁰, indicating strong arc affinities.  In contrast, the basalts and hydrothermal fluids recovered from the NELSC have ³He/⁴He of 8.0 – 8.6 Ra and C/³He of 1 – 3 × 10⁹, typical MOR or back-arc signatures.  We have estimated ³He/⁴He and C/³He values for a number of other volcanoes in the NE Lau Basin, and as might be expected the presence of an arc component increases with proximity to the Tofua Arc.  In general these carbon and helium isotopic signatures are a useful tool for distinguishing between these arc, back-arc, and hotspot affinities.