THE SYSTEMATICS OF BORON ABUNDANCES IN YOUNG VOLCANIC-ROCKS

Boron behaves as a highly incompatible trace element in oceanic settings, while in arcs it shows unique systematics indicative of fluid-rock interactions. Boron analyses conducted on well-characterized mid-ocean ridge basalt (MORB) suites show that B approximates K most closely in its solid/melt distribution behavior, with inferred bulk distribution coefficients of 0.004-0.009 during melting in the mantle and up to 0.07 during low-pressure crystallization. During differentiation processes in volcanic arc lavas B and K also vary similarly, but the B enrichments in basalts from different arc volcanoes are highly heterogeneous relative to those of K, Be, or other incompatibles. Boron shows strong affinities for fluids such as are liberated during the devolatilization of subducting slabs. Boron enrichments correlate directly with extents of melting in arc basalts, and inversely with the enrichments of most other lithophile trace elements. Boron enrichments at arcs are lower in those volcanoes that sample deeper portions of the slab, becoming indistinguishable from MOR.Bs in the rearmost volcanic centers. That such B depletions are evident in lavas entails that magmatic processes and other transport mechanisms efficiently flush B through the mantle wedge and return it to surface reservoirs. The great mobility of boron apparent from the arc data precludes any long-term B enrichment in the sub-arc mantle and requires the existence of strong return fluxes for B in addition to arc volcanism.