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Island-arc samples from Martinique, Lesser Antilles, are enriched in B relative to oceanic crust or the mantle, with B contents varying between 5 and 54 ppm, and have δ11Bfalling in a range between 1.8‰ and −5.3‰, less than the δ11Bof altered oceanic crust. The combination of these results indicates that both sediments and altered oceanic crust must have been involved in the generation of those magmas, since the abundance of B in the mantle is too low to produce such low δ11Bvalues with such high B concentrations. B content and δ11Bare not correlated, showing that the amount of B added to the arc magma was decoupled from its isotopic composition by the mixing of at least three distinct components. Boron concentrations, considered in conjunction with 87Sr/86Sr, 143Nd/144Nd, δ18Oand δ11Bdata, show that variable amounts of B were added by a B-enriched fluid to different batches of mantle-derived magma. The B, Sr and Nd isotopic compositions of these samples are consistent with a two-stage process in which enriched magma is produced in the mantle wedge by the addition of an aqueous fluid derived from dehydrating, subducting lithosphere and then modified during ascent by assimilation of variable amounts of sediment and altered oceanic crust. Most of the B contained in the fluid was probably derived from altered oceanic crust plus as little as 3% sediment, while the material involved in assimilation was apparently largely continentally derived sediment. A two-stage process like this also explains the distribution of δ18O, within the context of the B, Sr and Nd isotopic compositions of Martinique arc lavas.