Boron isotopes in Central American volcanics indicate a key role for the subducting oceanic crust

The geochemistry of arc magmas can shed light on chemical outfluxes from subducting slabs to the overlying mantle. Boron (B) abundances and isotope ratios are valuable tracers of slab-derived components due to the distinct compositions of the mantle and subducting materials and distinctive isotopic fractionation during dehydration. New Be/B and δ¹¹B measurements in olivine-hosted melt inclusions (MIs) from three Nicaraguan volcanic centers (Telica, Cerro Negro, and Masaya) are consistent with a B-rich slab component that has δ¹¹B ranging from +2.9‰ to +5.9‰, slightly higher than new measurements of hemipelagic (δ¹¹B = +0.7‰±0.03 and +2.1‰±0.08; 1σ n = 3) and carbonate (δ¹¹B = +2.9‰±0.06 and 3.7±0.09; 1σ n = 3) sediments sampled by DSDP Hole 495 on the Cocos plate. A thermochemical model of the Nicaraguan subduction zone is used to quantitatively model B loss and isotopic fractionation during slab dehydration and melting. In contrast to previous studies regarding B systematics in Central America and elsewhere, this model reproduces the range of δ¹¹B preserved in Nicaraguan olivine-hosted MIs without the involvement of serpentinite-derived fluids. The model indicates that Nicaraguan MI δ¹¹B signatures are primarily controlled by input from subducted altered oceanic crust (AOC), with a minor contribution from subducted sediments. This finding implies that the volatile element budget delivered from the slab to the volcanic arc is also mostly derived from the ocean crust, and that volatiles carried in deeper layers of the slab may be recycled beyond the arc into the deeper mantle beneath Central America.