THE SYSTEMATICS OF BORON ABUNDANCES IN YOUNG VOLCANIC-ROCKS

Boron behaves as a highly incompatible trace element in oceanic settin gs, while in arcs it shows unique systematics indicative of fluid-rock interactions. Boron analyses conducted on well-characterized mid-ocea n ridge basalt (MORB) suites show that B approximates K most closely i n its solid/melt distribution behavior, with inferred bulk distributio n coefficients of 0.004-0.009 during melting in the mantle and up to 0 .07 during low-pressure crystallization. During differentiation proces ses in volcanic arc lavas B and K also vary similarly, but the B enric hments in basalts from different arc volcanoes are highly heterogeneou s relative to those of K, Be, or other incompatibles. Boron shows stro ng affinities for fluids such as are liberated during the devolatiliza tion of subducting slabs. Boron enrichments correlate directly with ex tents of melting in arc basalts, and inversely with the enrichments of most other lithophile trace elements. Boron enrichments at arcs are l ower in those volcanoes that sample deeper portions of the slab, becom ing indistinguishable from MOR.Bs in the rearmost volcanic centers. Th at such B depletions are evident in lavas entails that magmatic proces ses and other transport mechanisms efficiently flush B through the man tle wedge and return it to surface reservoirs. The great mobility of b oron apparent from the arc data precludes any long-term B enrichment i n the sub-arc mantle and requires the existence of strong return fluxe s for B in addition to arc volcanism.