Evidence for a broadly distributed Samoan-plume signature in the northern Lau and North Fiji Basins

Abstract

Geochemical enrichment of lavas in the northern Lau Basin may reflect the influx of Samoanplume mantle into the region. We report major and trace element abundances and He-Sr-Nd-Hf-Pbisotopic measurements for 23 submarine volcanic glasses covering 10 locations in the northern Lau and North Fiji Basins, and for three samples from Wallis Island, which lies between Samoa and the Lau Basin. These data extend the western limit of geochemical observations in the Basins and improve the resolution of North-South variations in isotopic ratios. The Samoan hot spot track runs along the length of the northern trace of the Lau and North Fiji Basins. We find evidence for a Samoan-plume component in lavas as far West as South Pandora Ridge (SPR), North Fiji Basin. Isotopic signatures in SPR samples are similar to those found in Samoan Upolu shield lavas, but show a slight shift toward MORB-like compositions. We explain the origin of the enriched signatures by a model in which Samoan-plume material and ambient depleted mantle undergo decompression melting during upwelling after transiting from beneath the thick Pacific lithosphere to beneath the thin lithosphere in the northern Lau and North Fiji Basins. Other lavas found in the region with highly depleted isotopic signatures may represent isolated pockets of depleted mantle in the basins that evaded this enrichment process. We further find that mixing between the two components in our model, a variably degassed high-3 He/ 4He Samoan component and depleted MORB, can explain the diversity among geochemical data from the northern Lau Basin.

Geochemical evidence in the northeast Lau Basin for subduction of the Cook‐Austral volcanic chain in the Tonga Trench

“…[]; Price et al . []. (b) The relationship between Pb and Nd isotopic data in the Lau and North Fiji Basins (data from the Tonga Arc are not shown, but young (<3 Ma) ocean island basalt‐type lavas from Fiji have been included).…”

Seismic Evidence for Plume‐Slab Interaction by High‐Resolution Imaging of the 410‐km Discontinuity Under Tonga

“…Within the depressed area, the UUP07 tomography model shows slow seismic velocities (Figure 3c). Isotopic signatures of He-Sr-Nd-Hf-Pb (Pearce et al, 2007;Price et al, 2014) from samples across the North Fiji basin show a gradual change from Samoa island to Fiji island, suggesting a southwestward flow of Samoa plume material. Geochemical evidence supports the plume origin.…”

“…It has been proposed that plume material flows southward from Samoan hot spot toward Fiji into the northern Lau Basin (Chang et al, 2016;Pearce et al, 2007;Smith et al, 2001;Strak & Schellart, 2018). The 3 He/ 4 He (Price et al, 2014) and Hf-Nd isotope ratios (Pearce et al, 2007) change progressively from Samoan hot spot toward the south. The Fijian ocean island basalts (3 Ma to the present) show plume-like geochemical signatures similar to the Samoan magmatism (Gill & Whelan, 1989).…”

Selective ingress of a Samoan plume component into the northern Lau backarc basin

“…Decompressional mantle melting during upwelling has previously been proposed to explain OIB-type volcanism in the northern Lau Basin 42,54 . OIB components are assumed to have lower solidus temperatures at any given pressure than DMM sources, and preferentially melt but mix with and trigger partial melting of the relatively refractory DMM matrix 42,54 .…”

“…OIB components are assumed to have lower solidus temperatures at any given pressure than DMM sources, and preferentially melt but mix with and trigger partial melting of the relatively refractory DMM matrix 42,54 . The twocomponent mixing invoked here to create the trend observed in Fig.…”