ABSTRACT
Models for melting at ridges, and the resulting mineralogy, permit precise estimates of the density of oceanic plates. The plates become denser than columns of unsegregated asthenosphere at ridges in 10–15 m.y., and than asthenosphere segregated into crust and depleted mantle almost immediately, but their mean densities never exceed that of the underlying mantle, because low-density spinel peridotite in the plates more than compensates for cooling-related density increases. Cooled oceanic plates are negatively buoyant only if melt material can flood their upper surfaces. Then the buoyancy drive for subduction is comparable to slab pull. If oceanic plates fail at new faults in compression, flooding by melt material is unlikely, and the subduction buoyancy drive is small. These observations strengthen arguments for subduction initiation at former fracture zones, where flooding of oceanic plates by melt is most likely.
