Seismic evidence for a dunite-rich layer beneath mature South Atlantic Ocean crust

Oceanic crust is formed from primitive basaltic melt derived from upwelling mantle at mid-ocean ridges. As the melt migrates toward the surface to form oceanic crust, it reacts with mantle rock at the crust-mantle boundary, the Mohorovičić transition zone (MTZ), but how this reaction affects the MTZ and melt extraction remains unclear. By applying full waveform inversion to wide-angle seismic data acquired along a ridge-parallel profile sampling 30.6 Ma crust in the Atlantic Ocean at 31°S, we discovered an ∼1-km-thick high-velocity layer at the top of the upper mantle, indicating the presence of a dunite-rich layer. This layer is overlain by an ∼1.1-km-thick MTZ exhibiting rapid increases in velocity with depth, which could be due to the downward increase in olivine content and decrease in frozen melt sills. The dunite-rich layer is underlain by a low-velocity region in the upper mantle, possibly representing mantle harzburgite. The presence of a dunite-rich layer at the top of the upper mantle suggests that the melt percolated through, resided in, and reacted with the mantle just below the MTZ.