Using geochemistry to map mantle flow beneath the Lau Basin

Geochemical tracers can be used to map mantle flow beneath the Lau Basin-Tonga region over the past 6 m.y. and constrain the extent and means by which different components contribute to the magmas erupted there. Helium isotope data track the Samoan plume to show that it infiltrated into the northwestern part of the Lau Basin, at approximately 40 mm.yr (super -1) , through an opening tear in the Pacific plate. In contrast to other recent studies, our analysis indicates that this plume does not contribute to the arc lavas. A Louisville plume component is present in the northern Tonga arc lavas, but this was added from volcaniclastic sediments transported into the arc on the subducting slab and does not require the presence of plume material within the mantle wedge. The Louisville tracer has been used to infer that the mantle wedge beneath the island-arc is downwelling at the rate of 20-40 mm.yr (super -1) , owing to viscous drag against the downgoing Pacific plate. A third geochemical tracer, provided by the distinction between Indian and Pacific mantle, indicates that Indian mantle has migrated eastward at rates of 45-65 mm.yr (super -1) . The inferred rates of migration indicate that the translation of geochemical signatures occurs by mantle flow rather than by the order-of-magnitude-faster movement of partial melts. Except for the flow in the mantle wedge, the directions of mantle flow in this region are unrelated to the overlying plate motions.