BUOYANCY OF THE OCEAN FLOOR

Estimates of the age at which ocean floor becomes negatively buoyant as it cools range between 50 Ma and as little as 10 Ma. The buoyancy of the ocean floor - how much lighter the ocean floor is than the underlying mantle - may be calculated from the composition of the crust and its underlying mantle, by calculating the mineralogy of the crust and mantle and determining the density from the oceanic geotherm. I have used a variety of parameterizations of the chemistry of melting of the mantle as it rises at mid-ocean ridges to determine the bulk chemistry of the crust and the chemistry of the residual depleted mantle as a function of depth. I have then used the mineralogy derived from this chemistry to determine the density as a function of age of the ocean floor. Regardless of whether the oceanic crust is considered to be derived from uniform melting of a column of mantle, or from greater melting at shallow depths than at deeper ones due to greater overstepping of the peridotite solidus, calculations indicate that typical ocean floor does not become negatively buoyant, even if it cools at the surface for 200 Ma. The mean density of the ocean floor at 200 Ma is still 10-30 kg/m3 less dense than the underlying mantle. The range in estimates of its buoyancy is more affected by the choice of thickness for the oceanic plate than by the choice of melting parameterization. The positive buoyancy of the ocean floor does not preclude subduction, since older ocean floor is always less buoyant than young ocean floor that would replace it, but it suggests that ocean floor would not subduct spontaneously, even if it did not strengthen as it cooled, and that slabpull forces may typically have been overestimated.