Abstract
The Easter Seamount Chain and Nazca Ridge are two of the most conspicuous volcanic features on the Nazca plate. Many questions about their nature and origin have remained unresolved because of a lack of geochronological and geochemical data for large portions of both chains. New 40Ar-39Ar incremental heating age determinations for dredged rocks from volcanoes east of Salas y Gomez Island show that, with very few exceptions, ages increase steadily to the east from 1 center dot 4 to 30 Ma, confirming that the two chains are parts of the same hotspot trail and indicating a hotspot location near Salas y Gomez rather than beneath Easter Island some 400 km farther west. Most of the volcanoes appear to have been erupted onto seafloor that was 5-13 Myr old, and no systematic variation in seafloor age at the time of seamount formation is apparent. At about 23 Ma, the formation of the Nazca Ridge ceased and that of the Easter Seamount Chain began, corresponding to a change in the direction of motion of the Nazca plate. Most of the studied rocks are moderately alkalic to transitional basalts. Their geochemical characteristics suggest that they represent relatively small mean amounts of partial melting initiating in garnet-bearing mantle and ending in the spinel facies. Nd-Sr-Pb isotopic compositions are within the range of values previously observed for volcanoes of the Easter Seamount Chain, west of Easter Island; moreover, most of our data cluster in a rather small part of this range [e.g. epsilon(Nd)(t) is between +6 center dot 0 and +4 center dot 0]. The results indicate that the mantle source has consisted of the same two principal components, a C/FOZO-type component and a high-epsilon(Nd), incompatible-element-depleted Pacific mid-ocean ridge basalt-source-type component, since at least 30 Ma. The lack of any geochemical gradient along the chain east of Salas y Gomez implies that no systematic change over time has occurred in the proportions of these end-members.