Boron concentrations and isotope compositions of fluids and lavas from subduction-zone settings show great potential for elucidating mass flux at Earth's modern convergent margins. However, the fluid-mineral-melt behavior of B and its two stable isotopes remains relatively poorly understood. Boron isotope analyses of tourmaline in metasedimentary rocks subducted to 15–90 km depths (1) demonstrate the ability of this mineral to retain information regarding prograde devolatilization history in even highly retrograded rocks and (2) indicate the importance of tourmaline in affecting whole-rock B loss and B isotope evolution during subduction-zone metamorphism. The B lost from micas during metamorphism of subducting sedimentary rocks and altered oceanic crust is isotopically more enriched in 11B than the B retained in the micas. Beneath forearcs and volcanic arcs, the B from micas is either removed from the subduction-zone rocks via metamorphic fluids or sequestered by growing tourmaline, in which the B can be entrained to even greater depths. Here we demonstrate that these metamorphic fluids could contribute to the relatively high δ11B signatures observed in most arc lavas and the across-arc trends of decreasing δ11B observed in several arcs.