EM1‐Signature in the North Fiji Basin: Evidence for Stagnant Slab‐Derived Mantle Upwelling Beneath the Trench‐Distal Back‐Arc Basin

Abstract

Basaltic volcanism at the mature back‐arc spreading axis in the North Fiji Basin (NFB) exhibits significant geochemical heterogeneity from depleted mid‐ocean ridge basalt (MORB) to enriched MORB compositions. The volatile and Sr‐Nd‐Pb‐He isotope composition of basaltic glasses from the Central Spreading Ridge (CSR) of the NFB suggests that a second enrichment mantle component is present in addition to the previously observed Samoan mantle plume. The hydrous OIB‐like basalts from the northern CSR have a weak enriched mantle 1 (EM1) isotopic signature and negative correlation between Ba/Th and Zr/Hf ratios, similar to rejuvenated Samoan lavas associated with tectonic driven volcanism. The EM1 signature is distinct from shield‐stage Samoan basalts and back‐arc basin basalts from the Lau Basin by high Ba/La ratio and negative correlation between Ba/Th and Ba/Ta ratios, indicating contributions from dehydrated and altered oceanic crust and sediments. Our new geochemical data suggest that focused melting above the mantle transition zone (MTZ) triggered by dehydration of stagnant slab can be a source of the EM1‐like melts containing both elevated 3He/4He ratios (up to 12.52 RA) and altered oceanic slab components. Given the spatial coincidence of the inferred EM1 basalts with the clustered deep earthquakes, our results support the presence of detached oceanic slabs in the MTZ beneath the northern NFB.