Spatial and temporal distribution of a C‐like asthenospheric component in the Rano Rahi Seamount Field, East Pacific Rise, 15°–19°S

Abstract

Lavas of the Rano Rahi seamount field define a Nd‐Pb‐Sr isotopic array that connects, and substantially overlaps, the arrays of the nearby East Pacific Rise axis and 5.6–28 Ma lavas of the Pukapuka ridge system, situated between the seamount field and French Polynesia. Dating of Rano Rahi samples by 40Ar‐39Ar incremental heating methods yields ages between 0.23 and 4.67 Ma. Most of the dated lavas were erupted within 60 km of the axis, and no systematic isotopic variation with age is observed. The isotopic and incompatible element data indicate a persistent two‐component mantle source comprising variable proportions of normal Pacific ocean‐ridge‐type mantle and a “C”‐like Pukapuka end‐member with lower ɛNd, higher 87Sr/86Sr and 206Pb/204Pb, and relative enrichment in the highly incompatible elements. The Rano Rahi data reveal peaks of low ɛNd, high 87Sr/86Sr, and high 206Pb/204Pb at about 16°–17°S and 18.5°–19°S that appear to correspond to two concentrations of C‐rich mantle, probably streaming eastward from French Polynesia. The northern stream may have reached the axis more recently and/or be less continuous than the southern stream.