The Fonualei Spreading Centre is a nascent series of en echelon ridges that extend north from near Fonualei volcano, on the Tonga arc front, to the Mangatolu Triple Junction in the northeastern Lau Basin. Fresh basaltic glasses dredged from these ridges afford an excellent opportunity to evaluate geochemical changes with increasing depth to the slab. Here we augment previously published major and trace element data with new Sr, Nd, Pb, Hf and U-Th-Ra isotope data for selected Fonualei Spreading Centre samples as well as present new Hf isotope data from boninites and seamounts to the north of Tonga. The Pb and Hf isotope data are used to appraise interpretations of the extent and distribution of Samoan plume mantle beneath the Lau Basin based on He and Pb isotope data. If elevated ²⁰⁸Pb and lowered Hf isotope ratios in lavas from Niuafo'ou Island and the Mangatolu Triple Junction reflect a Samoan plume influence this is not visible in He isotopes. The boninite and seamount data indicate that the tear in the northern end of the slab may not extend east as far as the boninite locality. Mantle flow is inferred to be oriented to the southwest. In the Fonualei Spreading Centre lavas, Ce/Pb, ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb increase, whereas U/Th, Th/Nb and ¹⁴³Nd/¹⁴⁴Nd decrease, with increasing distance from the arc front. These changes are accompanied by increasing slab surface temperatures (725-940 degrees C) as inferred from decreasing H₂O/Ce ratios. Consistent with experimental data, the geochemical trends are interpreted to reflect changes in the amount and composition of wet pelite melts and aqueous fluids derived from the slab under appropriate conditions. With one exception, all of the lavas preserve both ²³⁸U excesses and ²²⁶Ra excesses. Ba-Yb, Na₈-Fe₈ and U-series isotope systematics suggest that lavas from the Fonualei Spreading Centre and Valu Fa Ridge reflect fluid-fluxed melting. However, there is a change to decompression melting in lavas from the East and Central Lau Spreading Centres where slab surface temperatures reach approximately 900-1000 degrees C. A similar observation is found for the Manus and East Scotia back-arc basins and may reflect the absence of lawsonite in the subducted basaltic crust.