Carbon dioxide released by arc volcanoes is widely considered to originate from the mantle and from subducted sediments. Fluids released from upper arc carbonates, however, have recently been proposed to help modulate arc CO₂ fluxes. Here we use boron as a tracer, which substitutes for carbon in limestone, to further investigate crustal carbonate degassing in volcanic arcs. We performed laboratory experiments replicating limestone assimilation into magma at crustal pressure-temperature conditions and analysed boron isotope ratios in the resulting experimental glasses. Limestone dissolution and assimilation generates CaO-enriched glass near the reaction site and a CO₂-dominated vapour phase. The CaO-rich glasses have extremely low δ¹¹B values down to −41.5‰, reflecting preferential partitioning of ¹⁰B into the assimilating melt. Loss of ¹¹B from the reaction site occurs via the CO₂ vapour phase generated during carbonate dissolution, which transports ¹¹B away from the reaction site as a boron-rich fluid phase. Our results demonstrate the efficacy of boron isotope fractionation during crustal carbonate assimilation and suggest that low δ¹¹B melt values in arc magmas could flag shallow-level additions to the subduction cycle.