Geochemical constraints on the enriched mantle source beneath the North Fiji Basin

Abstract

North Fiji Basin (NFB) is a mature back-arc basin showing geochemical features of Mid-Ocean Ridge Basalt (MORB) with spatial variation in source component. Especially, the origin for Oceanic Island Basalt (OIB)-like lavas found in the the northern region including Fiji Triple junction (FTJ) has been interpreted as a result of influence from Samoan plume as well as the northern Lau Basin. However, our geochemical data including H2O and Sr-Nd-Pb-He isotopes of the basaltic glasses sampled from the Central Spreading Ridge (CSR) in the NFB suggests additional possible origins for the enriched component found from the northern segments beside the Samoa plume. We define two groups of samples, which are the depleted N-MORB lava group distributed in the whole spreading segments and the relatively hydrous (H2O> 0.4 wt.%) OIB-like alkali basalt group with a restriced occurrence in the northern region of the NFB. These distinctive grouping can be described by two-components mantle mixing model between an extreamly depleted DMM (depleted MORB mantle) and an EM1 (enriched mantle 1) components. Similar to the case of Samoan rejuvenated lava, the observed EM1 signature in the northern NFB can be originated from the ~99% helium degassed Samoan plume material metasomatized by Rarotonga hospot (EM1) residue in the lithosphere. Considering the tectonic evolution of the NFB, the EM1 signature of Rarotonga hotspot trail witihin the Paciforthern region including Fiji Triple junction (FTJ) has been interpreted as a result of influence from Samoan plume as well as the northern Lau Basin. However, our geochemical data including H2O and Sr-Nd-Pb-He isotopes of the basaltic glasses sampled from the Central Spreading Ridge (CSR) in the NFB suggests additional possible origins for the enriched component found from the northern segments beside the Samoa plume. We define two groups of samples, which are the depleted N-MORB lava group distributed in the whole spreading segments and the relatively hydrous (H2O> 0.4 wt.%) OIB-like alkali basalt group with a restriced occurrence in the northern region of the NFB. These distinctive grouping can be described by two-components mantle mixing model between an extreamly depleted DMM (depleted MORB mantle) and an EM1 (enriched mantle 1) components. Similar to the case of Samoan rejuvenated lava, the observed EM1 signature in the northern NFB can be originated from the ~99% helium degassed Samoan plume material metasomatized by Rarotonga hospot (EM1) residue in the lithosphere. Considering the tectonic evolution of the NFB, the EM1 signature of Rarotonga hotspot trail witihin the Pacif