ALBERT

Description: At the present, the geochemical influence of the Galápagos hotspot (offshore South America) can be seen only along the Galápagos spreading center, north of the hotspot. It is possible, however, that Galápagos plume material also reached the East Pacific Rise in the past. Detecting such influence would be of particular importance for the interpretation of geochemical data from oceanic crust at Ocean Drilling Program (ODP) Site 1256, which formed ~15 m.y. ago at the East Pacific Rise during a Miocene period of superfast spreading, and is considered to be a reference site for oceanic crust produced at fast-spreading ridges. Here we present geochemical data from Miocene basaltic crust (23–7 Ma) drilled at several Deep Sea Drilling Project (DSDP), ODP, and Integrated Ocean Drilling Program (IODP) sites that formed along the East Pacific Rise between 3°S and 7°N. Lavas formed between ca. 22.5 and ca. 11 Ma show enriched, Galápagos plume–like Pb and Nd isotope ratios (with a peak in enrichment between ≥18 and 12 Ma) compared to lavas created shortly before or after this time interval. Despite their enriched isotope composition, these samples generally show depletion in more-incompatible, relative to less-incompatible, trace elements. Derivation from an enriched Galápagos plume source that had experienced recent melt extraction before it melted further beneath the East Pacific Rise can explain the combined incompatible-trace-element depletion and isotopic enrichment of the 22.5–11 Ma lavas. The influence of plume material correlates with the interval of superfast spreading along the equatorial East Pacific Rise, suggesting a causal relationship. Enhanced ridge-plume interaction ("ridge suction") due to superfast spreading could have facilitated the flow of Galápagos plume material to the ridge. On the other hand, the arrival of Galápagos-type signatures took place immediately after formation of the Galápagos spreading center, which could have provided a pathway for hot plume material to spread into the main ridge network.

Description: A detailed bathymetric and magnetic survey of the eastern flank of the East Pacific Rise at 14°14′S covering seafloor ages of 0–10 Ma has been carried out and used, along with a flowline profile on the conjugate western ridge flank, to reveal the spreading history and the temporal ridge crest segmentation. Additional information from basaltic lavas is included to study the relationship between physical and magmatic segment boundaries. The sequence of magnetic reversals indicates a total spreading rate of 150 mm/yr since 10 Ma. Symmetric spreading, however, occurred only since 2.8 Ma. Between 7 and 2.8 Ma spreading was asymmetric, with a higher spreading rate toward the east. Migration events of at least five overlapping spreading centres (OSC) left discordant zones on the Nazca plate consisting of hummocky basins and motley texture of curved lineations striking a few degrees oblique to the strike of the ridge crest. Four of the OSCs were right-stepping and migrated northward and one was left-stepping and migrated southward. By transferring Pacific lithosphere to the Nazca plate, these migration events may account for most of the asymmetric accretion observed. The basaltic samples from the eastern flank have been analysed and back tracked to the position of eruption on the ridge crest. In terms of their geochemical signature (Mg# 0.41–0.68) the samples reveal that the magmatic segment boundary between the Garrett transform and 14°30′S has remained stationary over the last 10 Myr and therefore provide no evidence for a link between magmatic and physical segmentation. We therefore propose that migrating non-transform ridge axis discontinuities are governed by propagating giant cracks; as a crack front advances a melt reservoir is tapped and magma rises passively into the crack and erupts subsequently on the seafloor. Some of the OSCs seem to have originated close to transform faults and therefore argue that far-field stresses, perhaps caused by the evolution of the Bauer microplate, rather than mantle upwelling create non-transform ridge axis discontinuities.