Abstract:
At the present, the geochemical influence of the Galapagos hotspot (offshore South America) can be seen only along the Galapagos spreading center, north of the hotspot. It is possible, however, that Galapagos 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 approximately 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 degrees S and 7 degrees N. Lavas formed between ca. 22.5 and ca. 11 Ma show enriched, Galapagos plume-like Pb and Nd isotope ratios (with a peak in enrichment between > or =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 Galapagos 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 Galapagos plume material to the ridge. On the other hand, the arrival of Galapagos-type signatures took place immediately after formation of the Galapagos spreading center, which could have provided a pathway for hot plume material to spread into the main ridge network.