Lithium isotopes have been identified as a promising tracer of subducted materials in arc lavas due to the observable variations in related reservoirs such as subducting sediments and altered oceanic crust. The Tonga–Kermadec arc–Lau back-arc provides an end-member of subduction zones with the coldest thermal structure on Earth. Reported here are Li isotope data for 14 lavas from the arc front and 7 back-arc lavas as well as 12 pelagic and volcaniclastic sediments along a profile through the sedimentary sequence at DSDP Site 204. The arc and back-arc lavas range from basalts to dacites in composition with SiO 2 = 48.3–65.3 wt% over which Li concentrations increase from 2 ppm to 16 ppm. Li/Y ratios range from 0.08 to 0.77 and from 0.24 to 0.65 in the arc and back-arc lavas, respectively. The majority of the lavas have δ 7 Li that ranges from 2.5 ‰ to 5.0 ‰ with an average of (3.6 ±0.7) ‰, similar to that reported from other arcs and there is no distinction between the arc front and back-arc lavas. The pelagic sediments have variable Li concentrations (33–133 ppm) and δ 7 Li that ranges from 1.2 ‰ to 10.2 ‰ while the volcaniclastic sediments have an even greater range of Li concentrations (3.6–165 ppm) and generally higher δ 7 Li values (8–14 ‰). However, δ 7 Li in the lavas does not correlate with commonly used trace element ratio or isotope signatures indicative of slab-derived fluids or the sediments. This is probably because the range of δ 7 Li in the lavas and sediments overlap. Calculated sediment mass-balance models require significantly more sediment than previous estimates based on Th–Nd–Be isotopes. This may indicate that a sizeable proportion of the total Li budget in the lavas is provided by Li-enriched fluids from the subducting sediments and/or altered oceanic crust
