This project is a three-year investigation into the relationship between the geochemical and tectonic evolution of the ocean crust along the circum-Antarctic Plate Boundary. This is a problem that relates directly to the motion of the tectonic plates and to the driving forces in the underlying mantle. In recent years it has become clear that the geochemical variability of basalts can place major constraints on models of upper mantle convection by limiting the rate of melt formation and the mechanism of melt migration through the mantle. The major element composition of ocean ridge basalts reflect the composition of the residual mantle, and so determine the overall degree of melting at the top of the ascending asthenosphere. The trace element composition appears to be a function of the small scale spatial variability in the initial composition of the mantle. The analysis is based on rock samples dredged from the Southwest Indian Ocean Ridge, and the South America-Antarctica Ridge. These ridges are spreading at a very slow and nearly constant rate (between 0.8 and 0.92 cm/year), eliminating spreading rate as a variable, cross two mantle plumes, and include two triple junctions. These characteristics provide a unique opportunity to study the genesis of ridge basalts and the dynamics of mantle melting by allowing the sampling of both the products and the residue of melting of the asthenosphere beneath the ridges. The main objective of the program is to use the trace element composition of sampled basalts and peridotites along the circum- Antarctic Ridge system to determine the variability in overall composition and the scale of heterogeneity of the mantle source in various plate tectonic environments, and use the major element composition of the samples to determine to what extent the variability is accompanied by initial temperature differences, and differences in the depths of mantle upwelling and melt segregation.
