Tracing a mantle plume: Isotopic and trace element variations of Galapagos seamounts

Isotopic and trace element analyses of basalts dredged from across the Galapagos Platform confirm the previously established east facing horseshoe pattern of depleted geochemical signatures at the center of the archipelago and more enriched signatures along the periphery. Statistical analysis of the isotopic data indicates that geochemical variations in the Galapagos cannot be explained by mixing between only the plume and the depleted asthenosphere. Instead, four isotopically distinct end-members must be interacting to account for the subtleties of the Sr, Nd, Pb, and He isotopic data. Three of the components are geographically restricted: one in the south, one in the central region, and one in the north. These three plume components then mix with the fourth component, depleted mantle, which is indistinguishable from the MORB source. The central component resembles the high (3)He/(4)He mantle reservoir that may be common to many plumes and has variously been called PHEM, FOZO, and C by others. Whereas this mantle reservoir appears to make a minor contribution to the composition of most hot spot systems, it may constitute the main body of the Galapagos plume. Geographic distribution of the end-members suggests that the plume is centered near Fernandina similar to 92 degreesN but may be significantly diluted by depleted mantle even near the main conduit. The geochemically distinct end-members trace an eastward mantle flow, manifested as decreasing contributions of the plume in the direction of plate motion. The plume may be tilted by shear in the asthenosphere from plate motion and ambient mantle flow. As it is bent, the plume thermally entrains surrounding upper mantle, resulting in the horseshoe-shaped distribution of depleted and enriched material. The end-members may also outline a deep, strong lateral flow of mantle toward the Galapagos Spreading Center, supplying plume material to the ridge system. Overall, our results suggest that the Galapagos hot spot is both compositionally and dynamically complex owing to its tectonic setting adjacent to a mid-ocean ridge.