Structure, Serpentinization and Seismic Reflectivity of the Crust‐Mantle Boundary at Fast‐ and Intermediate‐Spreading Ridges

The mechanisms of complex Moho reflections at fast‐ and intermediate‐spreading ridges remain unclear. A potential key factor is hydrothermal activity, which at mid‐ocean ridges may result in serpentinization of peridotites under the stability field of antigorite (400–600°C) or chrysotile‐lizardite (<400°C). Here, we calibrated the relationship between serpentinization degree, density, and seismic velocities of peridotites at 200 MPa. Based on physical properties of rocks in the oceanic lithosphere, we established seismic velocity and density models of a sharp or layered crust‐mantle boundary with different serpentinization types. Synthetic seismograms demonstrate that the oceanic Moho reflections are controlled by both structure and serpentinization of the crust‐mantle boundary. The impulsive Moho may correspond to a sharp mafic‐ultramafic boundary (petrological Moho) with <10% serpentinization, or the bottom of 60%–80% antigorite peridotites. The Moho reflection becomes invisible when antigorite percentage decreases from 60%–80% (top) to 0% (bottom) in the uppermost mantle for a sharp mafic‐ultramafic boundary. Besides the layered crust‐mantle boundary, the diffusive Moho could also result from the sharp mafic‐ultramafic boundary with 10%–60% or 80%–100% antigorite peridotites, or chrysotile‐lizardite peridotites overlying antigorite peridotites. Using synthetic seismograms, the diffusive Moho at 6–9 km to the west of the East Pacific Rise 9°44′N can be interpreted by 30% antigorite peridotites, suggesting localized hydrothermal circulation close to the fast‐spreading mid‐ocean ridges. Moho reflections and wide‐angle PmP waveforms at mid‐ocean ridges can provide new clues to decipher the interplay between magmatic, tectonic, and hydrothermal processes in the oceanic lithosphere.