High 3He/4He ratios in the Manus backarc basin: implications for mantle mixing and the origin of plumes in the western Pacific Ocean

Helium isotope ratios in oceanic glasses provide a high-integrity tracer of contributions from mantle plumes. Despite a diverse array of petrogenetic affinities, glasses from the central part of the Manus Basin - a backarc basin in the western Pacific - have typical plume (or hotspot) 3He/4He ratios that cluster around 12.2[R.sub.A] ([+ or -]1.0[R.sub.A], n = 18, where [R.sub.A] = 3He/4He of air), a value significantly higher than the range found in most mid-ocean-ridge basalts (MORB) ([8 [+ or -] 1][R.sub.A]). Lavas in other parts of the basin have MORB-like or lower 3He/4He values. A wide range of He concentrations characterizes the Manus Basin glasses: This is considered to reflect the high water content of some lavas, which promotes He loss through volatile degassing. For the most part, it is the degassed lavas that do not show the plume He isotope signature. Results of the present study, together with 3He/4He data for lavas and gases from islands to the south and east of the Bismark Sea, indicate that the focus of mantle plume upwelling is either the center of the Manus Basin or possibly the region to the northwest beneath the volcanic islands of the St. Andrew Strait. This region of plume or hotspot 3He/4He ratios coincides with a domain of anomalously low seismic velocities at the underlying core-mantle boundary, and indicates that the provenance of high-3He/4He magmas in the Manus Basin (and possibly elsewhere) is linked to this boundary layer - either by plume entrainment of lower mantle or, more speculatively, through addition of material from the core-mantle boundary.