Small-scale convection drives oscillatory crustal formation at slow-spreading ridges

Oceanic crust formed at slow- and ultraslow-spreading mid-ocean ridges exhibits quasi-periodic thickness variations over million-year time scales, a pattern not observed at faster ridges. The processes driving this temporal variability remain unresolved. Using numerical models that incorporate hydrous mantle rheology, we show that these oscillations emerge from dynamic interactions between off-axis small-scale convection (SSC) and sub-ridge mantle upwelling. SSC repeatedly initiates beneath young lithosphere and migrates outward, intermittently disrupting sub-ridge upwelling and thus melting flux, leading to oscillatory crustal formation. Higher spreading rates or lower mantle water content shift SSC initiation farther off-axis, weakening these interactions and suppressing crustal variability. Higher water content reduces mantle viscosity, which shortens the developing intervals of SSC and enhances its migration rate, thus generating oscillatory crust with shorter periods. Beyond axial crustal production, SSC creates local undulations in the deepening lithosphere-asthenosphere boundary, explaining short-wavelength deviations from the plate-cooling thermal models. SSC also triggers hydrous melting and accumulates melts beneath plates of various ages, providing a mechanism for the observed low-seismic-velocity and high-electrical-conductivity melt-rich layers. Together, these results offer a dynamic framework for understanding crustal accretion and lithospheric evolution at slow-spreading ridges, extending beyond static plate cooling models.