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3D full waveform inversion (FWI) has been applied to the seismic refraction data of the MARINER (Mid-Atlantic Ridge INtegrated Experiment at Rainbow) experiment to create a robust high-resolution model of the seismic velocity structure of the Rainbow massif. The Rainbow massif is an oceanic core complex located on a non-transform discontinuity (NTD) in a magma-starved region of the mid-Atlantic Ridge. Despite the low magmatic input, the core complex hosts a high-temperature hydrothermal vent field (> 340 C) that requires a long-lived magmatic heat source. The FWI results show that deep within the massif,∼ 3-8 km below the seafloor, is a low-velocity body that represents a partially molten sill complex with> 20% gabbro intrusions. The complex extends out north to the AMAR Minor N segment suggesting an increased magmatic input into this segment, forcing the NTD to migrate southwards. Extensive …