THE B ISOTOPIC COMPOSITION OF ARC LAVAS FROM MARTINIQUE, LESSER ANTILLES

Island-are samples from Martinique, Lesser Antilles, are enriched in B relative to oceanic crust or the mantle, with B contents varying betw een 5 and 54 ppm, and have delta(11)B values falling in a range betwee n 1.8 parts per thousand and -5.3 parts per thousand, less than the de lta(11)B of altered oceanic crust. The combination of these results in dicates that both sediments and altered oceanic crust must have been i nvolved in the generation of those magmas, since the abundance of B in the mantle is too low to produce such low delta(11)B values with such high B concentrations. B content and delta(11)B are not correlated, s howing that the amount of B added to the are magma was decoupled from its isotopic composition by the mixing of at least three distinct comp onents. Boron concentrations, considered in conjunction with Sr-87/Sr- 86, Nd-143/Nd-144, delta(18)O and delta(11)B data, 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 magm a is produced in the mantle wedge by the addition of an aqueous fluid derived from dehydrating, subducting lithosphere and then modified dur ing ascent by assimilation of variable amounts of sediment and altered oceanic crust. Most of the B contained in the fluid was probably deri ved from altered oceanic crust plus as little as 3% sediment, while th e material involved in assimilation was apparently largely continental ly derived sediment. A two-stage process like this also explains the d istribution of delta(18)O, within the context of the B, Sr and Nd isot opic compositions of Martinique are lavas.