Geochemistry of back-arc lava from the Fonualei Rift and Spreading Center and Northeastern Lau Spreading Center: A reconnaissance study on mantle fertility and arc signature in the NE Lau Basin

Abstract

Several volcanically and hydrothermally active spreading centers in the northeastern (NE) Lau Basin were recently reported in several studies; however, only few geochemical studies on volcanic rocks have been reported yet. We present major and trace elements and Sr-Nd-Pb isotopes composition of lavas from two active spreading centers, the Fonualei Rift and Spreading Center (FRSC) and Northeast Lau Spreading Center (NELSC), in the NE Lau Basin. The lavas from the FRSC consist of mostly basalt and basaltic andesite with significant arc affinities. Trace element and isotope composition of FRSC samples show gradual increase of mantle fertility from south to north. Lavas from the NELSC, which lies northeast of the FRSC and parallel to the NE branch of the Mangatulu Triple Junction, also show arc affinities as well as plume-like trace element and isotope composition possibly affected by adjacent the Samoan plume. The observed increase of mantle fertility in the FRSC lavas also can be attributed to plume component from north of the NE Lau Basin. Our result suggest that approximate limit of the plume component locates around 16 45S below which no significant plume signature is observed in FRSC samples. Although latitudinal variation of subduction component with proximity to the volcanic front is unclear along the FRSC, several subduction proxies (e.g. Ba/La and Ba/Th) shows slight correlation with latitude; further work is needed to determine the latitudinal variation of subduction input along the FRSC.