HIGH HE-3 HE-4 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 trac er of contributions from mantle plumes. Despite a diverse array of pet rogenetic affinities, glasses from the central part of the Manus Basin -a backarc basin in the western Pacific-have typical plume (or hotspot ) He-3/He-4 ratios that cluster around 12.2R(A) (+/-1.0R(A), n = 18, w here R-A = He-3/He-4 of air), a value significantly higher than the ra nge found in most mid-ocean-ridge basalts (MORB) ([8 +/- 1]R-A). Lavas in other parts of the basin have MORE-like or lower He-3/He-4 values. A wide range of He concentrations characterizes the Manus Basin glass es: 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 signa ture. Results of the present study, together with He-3/He-4 data for l avas 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 th e volcanic islands of the St. Andrew Strait, This region of plume or h otspot He-3/He-4 ratios coincides with a domain of anomalously low sei smic velocities at the underlying core-mantle boundary, and indicates that the provenance of high-He-3/He-4 magmas in the Manus Basin land p ossibly elsewhere) is linked to this boundary layer-either by plume en trainment of lower mantle or, more speculatively, through addition of material from the core-mantle boundary.