Borehole Waters From Hole U1601C (Atlantis Massif) Provide Constraints for a Deep-Sourced Formation Water That (Could) Feed the Lost City Hydrothermal Field

Hole U1601C, drilled during International Ocean Discovery Program (IODP) Expedition 399, reached a depth of 1,268 m below the seafloor (mbsf) in the Atlantis Massif (30°N, 42°W). This borehole represents only one of six boreholes that extends more than 700 m into oceanic rock, penetrating predominantly ultramafic lithologies, mostly composed of variously altered serpentinized peridotite and gabbro. We report geochemical data from borehole water collected after drilling and logging operations, providing a unique opportunity to investigate water–rock and hydrologic interactions and their implications for the origin of hydrothermal fluids at the Lost City Hydrothermal Field (LCHF), ∼800 m to the south. Solute concentrations and isotope compositions reveal trends consistent with four-component mixing. Borehole waters are a mixture of surface seawater used during drilling, freshwater introduced before logging, bottom seawater that entered after drilling, and formation water, which discharges into the base of the borehole. The upper 465 m of the borehole is dominated by seawater-freshwater mixtures, whereas deeper intervals contain increasing proportions of a zero-Mg, high-Ca formation water that constitutes up to 80% of the deepest samples. This formation water reacted with gabbroic and peridotite lithologies at temperatures ≥300°C, enriching the water in Ca, Li, Rb, Cs, and Sr. These findings are similar to models of deep-sourced hydrothermal fluids that discharge from LCHF. Our results provide the first direct evidence for deep, high-temperature fluid circulation through gabbroic and ultramafic lithologies beneath the Atlantis Massif and elucidate subsurface hydrologic processes that source Lost City's unique hydrothermal fluids.