Tracing mantle sources and Samoan influence in the northwestern Lau back-arc basin

Abstract

Helium isotopes are commonly used as a diagnostic fingerprint of Samoan mantle in the northern Lau Basin, but the extent of input from Samoan sources can only be clearly resolved by coupling 3He/ 4He with other geochemical tracers such as trace elements and other isotope systems. We present new major element, dissolved volatile (H 2O, CO 2, S, Cl, F), trace element, and Sr-Nd-Pb-Hf isotope data for new samples from the NW Lau Basin from five distinct regions (Rochambeau Rifts (RR), Northwest Lau Spreading Center (NWLSC), Peggy Ridge (PR), Lau Extensional Transform Zone (LETZ), and Central Lau Spreading Center (CLSC)) that range from distinctively elevated to normal mid-ocean ridge basalt 3He/ 4He. Helium isotopes variations are not correlated with radiogenic isotopes or trace element abundances. Our new data suggest two-component mixing of MORB-like mantle with an enriched mantle source, similar to Samoa, although consideration of a complete regional data set suggests there may be other sources of heterogeneity in the mantle beneath NW Lau. Models of mantle potential temperature (T p) and primary melt equilibration temperatures indicate similar T p of ∼1400°C for NW Lau, suggesting no strong temperature gradient. The pressure of melt equilibration deepens toward the north (∼1.1-1.2 GPa at PR, LETZ, CLSC; ∼1.3-1.4 GPa at RR, NWLSC), consistent with melting mantle of a constant T p but variable H 2O content. Samoan and MORB-like sources are clearly present beneath the NW Lau basin, but geochemical diversity among the existing data suggest that more than two mantle sources may contribute to mantle enrichment beneath NW Lau.