The ice-covered Aurora hydrothermal vent field, Gakkel Ridge, Arctic Ocean: ultramafic-influenced venting at a mafic axial volcano on Earth’s slowest spreading center

• First detailed geological exploration and sampling of an Arctic Ocean vent field. • Aurora sulfide chimneys are thin-walled with primarily high-temperature mineralogy. • Chimney compositions indicate subsurface fluid interaction with ultramafic substrate. The Aurora vent field, located on Gakkel Ridge, Earth’s slowest spreading mid-ocean ridge, is covered by perennial Arctic sea ice. Previous expeditions briefly glimpsed active venting near an axial volcano summit, but challenges of operating in drifting sea ice prevented detailed exploration and sampling. During the 2021 HACON expedition, Aurora was extensively imaged and sampled with a remotely operated vehicle adapted to these conditions, marking the first successful geological exploration of hydrothermal vents under ice. We present results of visual surveys and mineralogical and geochemical analyses of rocks and fluids collected at Aurora in the context of hydrothermal deposits in ultraslow settings. Active black smokers (350–356 °C) and extinct partially buried chimneys were documented over mafic lava flows covered in up to ∼5 m of sediment. Venting of extremely H 2 -rich (near 0.05 M) and Si-poor acidic fluids forms chimneys dominated by high-temperature minerals (anhydrite, isocubanite, chalcopyrite, pyrrhotite, magnetite, bornite). The chimneys are unusually thin-walled and friable, and mostly lack moderate-to-lower-temperature minerals typical of seafloor hydrothermal deposits. Minor pentlandite, elevated Ni and Co, and low Si collectively support putative subsurface interactions of hydrothermal fluids with ultramafic substrate, despite no such lithologies observed at the seafloor. Inactive chimneys are unusually coated with calcite, which may impact long-term deposit preservation. This study highlights the increasing accessibility of ice-covered Arctic Ocean seafloor for geological investigations. The results will inform future exploration on Gakkel Ridge, serving as an ultraslow-spreading endmember against which to compare yet undiscovered vent fields along this largely unexplored ridge.