ALBERT

Description: Along the Galápagos Spreading Center (GSC), 3He/4He varies from 8.5–5.9 RA. High 3He/4He ratios, resembling those in the western and southern Galápagos islands, are absent. This lack of high 3He/4He contrasts markedly with other localities of plume–ridge interaction, such as Iceland, Easter, and Amsterdam/St. Paul. The most striking feature is a 3He/4He gradient, decreasing westward from 8.4–7.0 RA between 89 and 93°W, where the GSC is shallowest and shows “axial high” morphology. The intra-segment 3He/4He variability within this region indicates that magma crosses the mantle/crust boundary at multiple points beneath individual ridge segments, and lateral mixing within the crust and upper mantle is limited. Some of the 3He/4He variability may also reflect transfer of discrete heterogeneity from beneath the northern sector of the Galápagos plateau. One possible explanation for the absence of high 3He/4He along the GSC is that helium is a relatively ineffective downstream tracer of mantle material from the core of the Galápagos plume, due to its preferential extraction beneath the archipelago compared to other incompatible, lithophile tracers. A second explanation is that the heterogeneous Galápagos plume is sheared in the upper mantle by motion of the Nazca Plate relative to the migrating GSC. In this case, plume core material having high 3He/4He (from beneath Fernandina and Isabela) would be dispersed mostly away from the ridge, while plume edge material having low 3He/4He plus enriched Sr and Pb isotope signatures (from beneath the northern periphery of the archipelago) is smeared into the sub-ridge mantle.