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International Ocean Discovery Program (IODP) Expedition 399 collected new cores from the Atlantis Massif (30°N; Mid-Atlantic Ridge), an oceanic core complex that hosts the Lost City hydrothermal field (LCHF). Studies of the Atlantis Massif and the LCHF have transformed our understanding of tectonic, magmatic, hydrothermal, and microbial processes at slow-spreading ridges. The Atlantis Massif was the site of four previous expeditions (Integrated Ocean Drilling Program Expeditions 304, 305, and 340T and IODP Expedition 357) and numerous dredging and submersible expeditions. The deepest IODP hole in young (<2 My) oceanic lithosphere, Hole U1309D, was drilled ~5 km north of the LCHF and reached 1415 meters below seafloor (mbsf) through a series of primitive gabbroic rocks. A series of 17 shallow (<16.4 mbsf) holes were also drilled at 9 sites across the south wall of the massif during Expedition 357, recovering heterogeneous rock types including hydrothermally altered peridotites, gabbroic, and basaltic rocks. The hydrologic regime differs between the two locations, with a low permeability conductive regime in Hole U1309D and a high (and possibly deep-reaching) permeability regime along the southern wall. Expedition 399 targeted Hole U1309D and the southern wall area to collect new data on ancient processes during deformation and alteration of detachment fault rocks. The recovered rocks and fluids are providing new insights into past and ongoing water-rock interactions, processes of mantle partial melting and gabbro emplacement, deformation over a range of temperatures, abiotic organic synthesis reactions, and the extent and diversity of life in the subseafloor in an actively serpentinizing system. We sampled fluids and measured temperature in Hole U1309D before deepening it to 1498 mbsf. The thermal structure was very similar to that measured during Expedition 340T, and lithologies were comparable to those found previously in Hole U1309D. A significant zone of cataclasis and alteration was found at 1451–1474 mbsf. A new Hole U1601C (proposed Site AMDH-02A) was drilled on the southern ridge close to Expedition 357 Hole M0069A, where both deformed and undeformed serpentinites had previously been recovered. Rapid drilling rates achieved a total depth of 1267.8 mbsf through predominantly ultramafic (68%) and gabbroic (32%) rocks, far surpassing the previous drilling record in a peridotite-dominated system of 201 m. Recovery was excellent overall (71%) but particularly high in peridotite-dominated sections where recovery regularly exceeded 90%. The recovery of sizable sections of largely intact material will provide robust constraints on the architecture and composition of the oceanic mantle lithosphere. The deepest portions of the newly drilled borehole may be beyond the known limits of life, providing the means to assess the role of biological activity across the transition from a biotic to an abiotic regime. Borehole fluids from both holes were collected using both the Kuster Flow-Through Sampler and the new Multi-Temperature Fluid Sampler. Wireline logging in Hole U1601C provided information on downhole density and resistivity, imaged structural features, and documented fracture orientations. A reentry system was installed in Hole U1601C, and both it and Hole U1309D were left open for future deep drilling, fluid sampling, and potential borehole observatories. 

International Ocean Discovery Program (IODP) Expedition 399 will collect new cores from the Atlantis Massif (30°N; Mid-Atlantic Ridge), an oceanic core complex that has transformed our understanding of tectonic and magmatic processes at slow- and ultraslow-spreading ridges. The exposure of deep mantle rocks leads to serpentinization, with major consequences for the properties of the oceanic lithosphere, heat exchange between the ocean and crust, geochemical cycles, and microbial activity. The Lost City hydrothermal field (LCHF) is situated on its southern wall and vents warm (40°–95°C) alkaline fluids rich in hydrogen, methane, and abiotic organic molecules. The Atlantis Massif was the site of four previous expeditions (Integrated Ocean Drilling Program Expeditions 304, 305, and 340T and IODP Expedition 357) and numerous dredging and submersible expeditions. The deepest IODP hole in young (<2 My) oceanic lithosphere, Hole U1309D, was drilled 5 km north of the LCHF and reaches 1415 meters below seafloor (mbsf) through a primitive series of gabbroic rock. In contrast, during Expedition 357 a series of shallow (<16.4 mbsf) holes were drilled along the south wall of the massif, one within 0.4 km of the LCHF, and serpentinized peridotites were recovered. The hydrologic regime differs between the two locations, with a low permeability conductive regime in Hole U1309D and a high likelihood of deep permeability along the southern wall. Expedition 399 targets both locations to collect new data on ancient processes during deformation and alteration of detachment fault rocks. Recovered rocks and fluids will provide new insights into ongoing water-rock interactions, abiotic organic synthesis reactions, and the extent and diversity of life in the subseafloor in an actively serpentinizing system. We will deepen Hole U1309D to 2060 mbsf, where temperatures are expected to be ~220°C. The lithology is predicted to transition with depth from primarily gabbroic to more ultramafic material. Predicted temperatures are well above the known limits of life, so detectable hydrogen, methane, and organic molecules can be readily attributed to abiotic processes. A new ~200 m hole will be drilled on the southern ridge close to Expedition 357 Site M0069, where both deformed and undeformed serpentinites were recovered. We aim to recover a complete section through the detachment fault zone and to sample material that reflects the subseafloor biological, geochemical, and alteration processes that occur along the LCHF circulation pathway. Borehole fluids from both holes will be collected using both the Kuster Flow Through Sampler tool and the new Multi-Temperature Fluid Sampler tool. Wireline logging will provide information on downhole density and resistivity, image structural features, and document fracture orientations. A reentry system will be installed at proposed Site AMDH-02A, and Hole U1309D will be left open for future deep drilling, fluid sampling, and potentially borehole observatories.