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Abundant volcanic activity occurs in the back‐arc region of the northern Tofua island arc where the Northeast Lau Spreading Center (NELSC) propagates southwards into older crust causing the formation of numerous seamounts at the propagating rift tip. An off‐axis volcanic diagonal ridge (DR) occurs at the eastern flank of the NELSC, linking the large rear‐arc volcano Niuatahi with the NELSC. New geochemical data from the NELSC, the southern propagator seamounts, and DR reveal that the NELSC lavas are tholeiitic basalts whereas the rear‐arc volcanoes typically erupt lavas with boninitic composition. The sharp geochemical boundary probably reflects the viscosity contrast between off‐axis hydrous harzburgitic mantle and dry fertile mantle beneath the NELSC. The new data do not indicate an inflow of Samoa plume mantle into the NELSC, confirming previously published He isotope data. The NELSC magmas form by mixing of an enriched and a depleted Indian Ocean‐type upper mantle end‐member implying a highly heterogeneous upper mantle composition in this area. Most NELSC lavas are little affected by a slab component implying that melting is adiabatic beneath the spreading center. The DR lavas show the influence of a component from the subducted Louisville Seamount Chain, which was previously thought to be restricted to the nearby arc volcanoes Niuatoputapu and Tafahi. This signature is rarely detected along the NELSC implying little mixing of melts from the low‐viscosity hydrous portion of the mantle wedge beneath the rear‐arc volcanoes into the melting region of the dry mantle beneath the NELSC., Plain Language Summary: Volcanic activity is abundant at subduction zones and the chemical analysis of the erupted rocks allows to determine the material transport in the Earth's mantle. The Northeast Lau Spreading Center (NELSC) forms by extension and volcanism behind the northern Tofua island arc. Several large volcanic structures occur east of the NELSC and the lavas of these off‐axis volcanoes are chemically and isotopically distinct implying little mixing with the magmas of the NELSC. The differences suggest decompression melting of relatively dry mantle beneath the NELSC whereas the off‐axis volcanoes reflect melting of water‐rich mantle affected by fluids from the subducting Pacific Plate. The sharp geochemical boundary between the NELSC and off‐axis volcanoes is probably due to a large viscosity contrast between hydrous harzburgitic mantle and dry fertile mantle. Element and isotope ratios indicate that the NELSC magmas form by mixing of enriched and depleted portions of the upper mantle, and we do not find evidence for inflow of the Samoa deep mantle plume from the north. Some of the off‐axis lavas contain a component from a volcanic chain that was subducted some 4 million years ago and that was previously only known in two volcanoes of the Tofua island arc., Key Points: Variably enriched mantle sources melt beneath the Northeast Lau Spreading Center (NELSC) but there is no evidence for Samoa mantle plume inflow. Relatively dry fertile mantle beneath NELSC causes sharp geochemical boundary with hydrous harzburgitic North Tonga mantle wedge. Subducted Louisville Seamount Chain material affects rear‐arc volcanism., Bundesministerium für Bildung und Forschung (BMBF) http://dx.doi.org/10.13039/501100002347, Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659

The D″ region consists of many different structures on many length‐scales and here we test whether an inhomogeneous scattering region could potentially explain two of these seismic observables, namely PKP precursors, which are generated by scattering near the core‐mantle boundary, and reflections of a seismic discontinuity in the lowermost mantle. The focus of this study is on modeling PKP precursors and lower mantle reflections. Testing different heterogeneity models with a range of correlation lengths and velocity perturbations for D″, we find that some of our models can produce both waves. Comparing our synthetic seismograms to real data for precursors in adjacent locations beneath the mid‐Atlantic near South America we find the best fitting model with correlation length of 10 km and velocity perturbation of 5% with a gradual increase of scattering defined by a taper from 200 to 400 km above the core‐mantle boundary., Plain Language Summary: The lowest 300 km of the Earth's mantle (called D″ layer) consists of many different structures that range from small‐scale features such as scatterers to large‐scale structures such as large regions with lower seismic velocity than the surrounding mantle. The structures are visible with a range of seismic waves but many studies are restricted to only one wavetype. In this study, we focus on two seismic waves that are usually associated with different structures. First, PKP precursors that are P waves scattered at small‐scale features in the lowermost mantle and second the PdP wave that reflects off the D″ boundary which is associated with a sharp velocity change. We compute synthetic seismic data assuming a scattering region in D″, testing models of different scatterer sizes and strength and find that some of these models can in fact produce both wavetypes. Additionally, we find a best fitting model by comparing the synthetics with real earthquake data imaging the lower mantle beneath the mid‐Atlantic near South America. We thereby show that perhaps some structures in the lowermost mantle, previously interpreted as different features, are in fact related. This may help to improve our knowledge of the geodynamical processes in the lower mantle., Key Points: We show that PKP precursors and PdP reflections can be produced by the same heterogeneity structure in the lowermost mantle (D″ layer). Our heterogeneity models consist of scatterers of different correlation lengths (10, 50 km) and velocity perturbations (1%, 3%, 5%). Best fitting model for PKP precursors that also generates PdP reflections has a correlation length of 10 km and velocity changes of 5%., https://doi.org/10.7914/SN/3D_2010, https://doi.org/10.7914/SN/KN