Abstract
The upper mantle is essential for understanding magmatism, crust formation, and element cycling between Earth's interior, hydrosphere, atmosphere, and biosphere. Mantle composition and evolution are inferred through surface sampling and indirect methods. We recovered a 1268 m section of serpentinized mantle peridotite from the Atlantis Massif, with minor gabbroic intrusions, showing depleted compositions and mineralogical variations controlled by melt flow. Located at 30 degrees North on the Central Atlantic Ridge, the Atlantis Massif is an ocean core complex exposing gabbroic rocks and serpentinized peridotite from the lower oceanic crust to the upper mantle. Four previous IODP expeditions were conducted here. IODP Expedition 399 (April 12 - June 12, 2023) by D/V JOIDES Resolution aimed to elucidate the formation of the oceanic core complex, explore reaction processes between the oceanic crust, upper mantle, and seawater, investigate non-biological water-rock reactions representing ancient systems, and evaluate sub-seafloor life activity.
Hole U1601C at the serpentine site in the southern massif was drilled to 1267.8 mbsf, becoming the fifth deepest hard rock hole in IODP history. Continuous temperature, density, porosity, and seismic velocity measurements were taken, along with fluid samples from multiple depths. Dunite zones in these cores exhibit intermediate dips, contrasting with initially steep mantle fabrics, indicating oblique melt transport. Extensive hydrothermal fluid-rock interaction is recorded across the core's full depth, with oxidation in the upper 200 m. Alteration patterns align with vent fluid composition in the nearby Lost City hydrothermal field.
Future analysis of these core samples and data will elucidate the oceanic lithosphere structure, formation of oceanic core complexes, crust alteration, and the generation of hydrogen and methane, essential for the subsurface biosphere.
