Multistage Melting of Buoyant Mantle beneath the Fiji Triple Junction

Abstract

The geochemical heterogeneity recorded in the submarine basaltic glasses from Fiji Triple Junction (FTJ) at the central region in the North Fiji Basin, one of the most mature back-arc basins, is explained by mantle upwelling of enriched composition within the depleted ambient upper mantle. Anomalous high topography of the FTJ, 500-1500 m above, supports existing of hot buoyant mantle upwelling and melting focusing beneath this region. Oceanic island basalts (OIB)-like 3He/4He ratios, Sr-Nd-Pb isotopes, and volatiles of the FTJ glasses present the same latitudinal patterns along the spreading axis, but the 3He/4He ratio has a low correlation with the other geochemical proxies and exhibits lateral displacement. The most enriched lithophile isotope composition occurs at the summit of FTJ, while the highest 3He/4He value appears in the southern region suggesting multistage melting of the enriched upwelling mantle material. This displaced behavior of the helium isotopes requires constraints in micro-scale mantle dynamics and an explanation of the melting structure beneath the FTJ. Here we present new helium isotopes and volatile data and aim to evaluate an association with high-resolution gravity data.