The Tonga‐Kermadec‐New Zealand volcanic arc is an end‐member of arc systems with fast subduction suggesting that the Tonga sector should have the coolest modern slab thermal structure on Earth. New data for boron concentration and isotopic composition are used to evaluate the contrasting roles of postulated subduction components (sediments and oceanic slab lithologies) in magma genesis. Major observations include: (a) Tonga‐Kermadec volcanic front lavas are enriched in B (as recorded by B/Nb and similar ratios) and most have relatively high δ¹¹B (>+4‰), whereas basaltic lavas from New Zealand have relatively low B/Nb and δ¹¹B (<−3.5‰); (b) both δ¹¹B and B/Nb generally increase northward from New Zealand along with convergence rate and overall slab flux; (c) δ¹¹B and B/Nb decrease toward the back‐arc, as observed elsewhere; and (d) low δ¹¹B is observed in volcanic front samples from Ata, an anomalous sector where the back‐arc Valu Fa Spreading Center impinges on the arc and the Louisville Seamount Chain is presently subducting. Otherwise, volcanic front lavas exhibit positive correlations for both B/Nb and δ¹¹B with other plausible indicators of slab‐derived fluid contributions (e.g., Ba/Nb, U/Th, (²³⁰Th^/232Th) and ¹⁰Be/⁹Be), and with estimated degree of melting to produce the mafic lavas. Inferred B‐enrichments in the arc magma sources are likely dominated by serpentinite domains deeper within the subducting slab (±altered oceanic crust), and B systematics are consistent with dominant transport by slab‐derived aqueous fluids. Effects of this process are amplified by mantle wedge source depletion due to prior melt extraction.

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