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.

Dynamic Evolution of Back‐Arc Basins Affected by Double Subduction

“…The pink filled area or circles denoting OIB‐like magma are according to Gill and Whelan (1989), Oh et al. (2021), and Mortimer et al. (2007).…”

“…Material circulation beneath back‐arc basins are mostly confined to the upper mantle, featured by the typical lava types, i.e., normal mid‐ocean ridge basalt (N‐MORB) or back‐arc basin basalt (BABB; Pearce & Stern, 2006; Taylor & Martinez, 2003). However, deep‐mantle sourced lavas such as ocean island basalt (OIB) or enriched mid‐ocean ridge basalts (E‐MORB) are found in the North Fiji Basin and South Fiji Basin (Figure 17a; Eissen et al., 1994; Mortimer et al., 2007; Nohara et al., 1994; Oh et al., 2021; Price et al., 2014). The OIB‐like basalts with EM1 signature in the North Fiji Basin are interpreted by the upwelling of hydrous melts triggered by the stagnant Tonga slab in the MTZ rather than the influence of the Soman plume (Oh et al., 2021).…”

“…However, deep‐mantle sourced lavas such as ocean island basalt (OIB) or enriched mid‐ocean ridge basalts (E‐MORB) are found in the North Fiji Basin and South Fiji Basin (Figure 17a; Eissen et al., 1994; Mortimer et al., 2007; Nohara et al., 1994; Oh et al., 2021; Price et al., 2014). The OIB‐like basalts with EM1 signature in the North Fiji Basin are interpreted by the upwelling of hydrous melts triggered by the stagnant Tonga slab in the MTZ rather than the influence of the Soman plume (Oh et al., 2021). Basalts with the signature of common component C without plume influence are also found in the South Fiji Basin (Todd et al., 2011).…”

“…The areas filled with gray, light blue, and dark blue are geometries of the New Hebrides slab, Tonga-Kermadec slab, and South Loyalty slab at depths of 500, 600, and 1,100 km, respectively, imaged from tomography (Schellart et al, 2009;Schellart & Spakman, 2012). The pink filled area or circles denoting OIB-like magma are according to Gill and Whelan (1989), Oh et al (2021), andMortimer et al (2007). The gray filled strips are hot spot chains.…”

“…They were embedded by the New Caledonian and Tonga‐Kermadec face‐to‐face double subduction systems (Schellart et al., 2006; Figures 1c and 2b). Possible influence of double subduction on the evolution of the North and South Fiji back‐arc basins is addressed in the previous studies, such as the anomalous high thermal state due to enhanced mantle upwelling (Faccenna et al., 2010), and the enriched mantle 1 (EM1) signatures of basalts (Mortimer et al., 2007; Oh et al., 2021; Todd et al., 2011). Enhanced mantle upwelling due to double subduction is supported by the observed enriched intraplate magma found in the Southeast Asia (Lyu et al., 2019).…”