Possible contributions of HIMU and EM1 components in magma generation beneath the northern Lau back-arc Basin

Abstract

The northeast Lau Basin is seemingly uninfluenced by OIB or hotspot, based on their MORB-like 3He/4He values (~8 R/Ra), while the northwest region with high helium isotope ratios upto 22.6 R/Ra is considered to be influenced by toroidal flow of Samoan plume around the slab edge. However, the northeast Lau Basin spreading ceter (NELSC) lavas show radiogenic Sr-Nd-Pb isotopic compositions and incompatible element-enrichment, which are distinguished from the lavas from the southern back-arc spreading centers (i.e. East Lau Spreading Center and Valu Fa Ridge) showing typical characteristics of the deplepted back-arc basin basalt. This discrepancy between geochemical parameters of the NELSC allowed us to consider some other enriched mantle source, yet helium-degassed, such as subducted hotspot trails. We here constrain possible mantle sources contributed on the distinctive geochemical chracteristics of the northeast Lau Basin using isotopic mixing models. First, longitudinal variations of Sr-Nd-Pb isotopes within the region, from Niuafo’ou island to northern Tonga arc, intimate significant contribution of HIMU (high 238U/204Pb)-type component at the NELSC relative to the western region. The Pb isotopic compositions of NELSC are more radiogenic than even the northern Tonga arc (Tafahi and Niuatoputapu), which are thought to be influenced by subducted Louisville volcanic chain, indicating requirment of the stronger HIMU