Loki's Castle at 73°30′ N along the Arctic Mid-Ocean Ridge (AMOR) provides a natural laboratory to study the evolution of hydrothermal circulation in an ultraslow-spreading environment. In addition, a sedimentary input into the Loki's Castle hydrothermal circulation cell is indicated by vent fluid and gas chemistry. Here we present B and Sr isotope data to investigate interaction between fluid, MORB and sediment and to constrain mass transfer during hydrothermal circulation of a sediment-influenced, black smoker hydrothermal system. The high boron concentrations (1770–2170 μmol/l) and the relatively low boron isotope composition of the vent fluids (δ¹¹B values ranging from + 11.6 to + 15.2‰) reflect not only fluid/rock interaction with mafic crust but indicates a significant contribution from sediments, in agreement with other chemical and isotopic (⁸⁷Sr/⁸⁶Sr) analyses. We present a model of B extraction from sediments at a temperature of 300 °C, which suggests a water/sediment ratio of approximately 2 to 3 at Loki's Castle. At these ratios, relative B extraction from sediment ranges from 35 to 55%.