A Ni-Fe alloy (Ni3.8Fe0.1Co0.1) was found in olivine-poor gabbro recovered from the Integrated Ocean Drilling Program (IODP) Hole U1309D, Atlantis Massif, 30 degrees N on the Mid-Atlantic Ridge. Ni-Fe alloys are one of the unique minerals formed during serpentinization; however, our finding establishes that Ni-Fe alloys are also formed in the gabbroic crust possibly associated with extremely reduced magmatic fluids. The occurrence of the Ni-Fe alloy suggests that oceanic lower-crustal gabbros as well as serpentinites may be potential reservoirs of abiogenic CH4.