Trace element evidence from seamounts for recycled oceanic crust in the eastern Pacific mantle

We present trace element data for 80 samples from about 50 seamounts in the east equatorial Pacific near the East Pacific Rise. These data indicate that the heterogeneous mantle source that supplies the seamounts consists of two components: (1) an extremely depleted component, much more depleted than estimates of the source of depleted MORB; and (2) an enriched component even more enriched than average OIB. The depleted component shows large variations in Zr/Hf, Nb/Ta, Rb/Cs, Ce/Pb, and Th/U that are correlated with each other and with La/Sm, indicating that these paired elements do fractionate from each other in some oceanic basalts. The order of incompatibility of trace elements we find differs slightly from that found elsewhere. For example, for seamounts, we find that D-Nb approximate to D-Th < D-Ta approximate to D-U. In comparison with Th and U, the enriched component shows anomalous enrichments of Ta and Nb. Since such fractionations are characteristic of subduction zones, we suggest that the most likely ultimate source of the enriched component is recycled ocean crust.