VARIATION WITH SPREADING RATE OF OCEANIC CRUSTAL THICKNESS AND GEOCHEMISTRY

Away from the influence of fracture zones, hotspots and marginal basins, oceanic crust exhibits remarkably uniform crustal thickness, rare earth element concentrations and bulk composition at all but the most slowly spreading ridges. At full spreading rates below 15 mm/a, however, all these observed parameters show marked and abrupt changes. The seismically determined crustal thicknesses decrease sharply from their average of 7 +/- 1 km, as do melt thicknesses inferred from rare earth element inversions of basalts. Bulk element compositions of basalts show an increase in the percentage of Na2O, and decreases in the FeO content and CaO/Al2O3 ratio as the full spreading rate drops below 15 mm/a. We present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts. Changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts. We model the observed behaviour with normal mantle potential temperatures of 1300 +/- 20-degrees-C. Greater crustal thicknesses are generated at the North Atlantic spreading centre adjacent to the Iceland mantle plume, suggesting mantle temperatures about 80-degrees-C hotter than normal in this area.