Spatial Variations in the Degree of Upper‐Mantle Depletion in a Mid‐Ocean Ridge–Transform Fault System

Partial melting beneath a mid‐ocean ridge creates a chemically depleted layer in the uppermost mantle. This chemical depletion lowers the density of the lithosphere compared with the unmelted mantle. Furthermore, dehydration associated with depletion leads to an increase in mantle viscosity that may affect the structure and dynamics of the oceanic plate. Previous studies have mainly considered the formation of this depleted upper‐mantle layer in a two‐dimensional mid‐ocean ridge setting, leaving the dynamics of mid‐ocean ridge–transform fault systems largely unexplored. In this study, spatial variations in the degree of depletion in the uppermost mantle are predicted for a mid‐ocean ridge–transform fault system using a three‐dimensional thermomechanical model. The degree of depletion generally increases with increasing half‐spreading rate. Less depletion is predicted beneath the transform fault and fracture zone compared with the surrounding mantle. Lateral differences in the degree of depletion in the ridge‐parallel direction are reduced when plastic yielding is considered. The degree of variation in the predicted depletion is related to the transform fault length especially at a low spreading rate, thereby suggesting that the large scatter in observed abyssal peridotite compositions with slow spreading rates could be partly attributed to the length of the fault. Plain Language Summary Oceanic plates are a key component in the theory of plate tectonics. Partial melting beneath a mid‐ocean ridge, where oceanic plates are formed, results in residual melt‐depleted mantle that is less dense and more viscous than the unmelted mantle. It is therefore important to understand the degree of depletion in this tectonic setting to better constrain the structure and dynamics of oceanic plates. Previous studies have mainly considered the formation of depleted mantle in a two‐dimensional mid‐ocean ridge setting. However, this layer of depleted mantle likely undergoes a more complex formation process if transform faults, which connect neighboring mid‐ocean ridges, are located nearby. In this study, spatial variations in the degree of depletion in the uppermost oceanic mantle are predicted for a mid‐ocean ridge–transform fault system using three‐dimensional computational simulations. The degree of depletion generally increases with the rate of plate motion. The degree of depletion is lower beneath transform faults and their extension compared with the surrounding mantle.