Age constraints on crustal recycling to the mantle beneath the southern Chile Ridge: He-Pb-Sr-Nd isotope systematics

Basalts from the four southernmost segments of the subducting Chile Ridge ( numbered 1-4 stepping away from the trench) display large variations in Sr, Nd, Pb, and He isotope and trace element compositions. Klein and Karsten [ 1995] showed that segments 1 and 3 display clear trace element evidence for recycled material in their source (e.g., low Ce/Pb). The uniformly midocea n ridge basalt (MORB)-like He-3/He-4 and modest variations in Pb, Sr, and N d isotopes of segment 1 (nearest the trench) suggest recent (<20 Ma) introd uction of a contaminant into its source, consistent with recycling of mater ial from the adjacent subduction zone. In contrast, segment 3 lavas display a dramatic southward increase in enrichment, extending to highly radiogeni c Pb and Sr isotopic compositions (e.g., Pb-206/Pb-204 = 19.5) and the lowe st He-3/He-4 yet measured in MORE (3.5R(A)). The segment 3 variations are m ost readily explained by ancient (similar to 2 Ga) recycling of terrigenous sediment and altered crust, but we cannot rule out more recent recycling o f material derived from a distant continental source. The similarity in iso topic signatures of segment 4 lavas to Indian Ocean MORE extends the Dupal anomaly to the Chile Ridge. Like Indian Ocean MORE, the segment 4 isotopic variations are consistent with contamination by anciently recycled pelagic sediment and altered crust and require a complex history involving at least three stages of evolution and possibly a more recent enrichment event. Sou thern Chile Ridge MORE reflect the extensive degree of heterogeneity that i s introduced into the depleted upper mantle by diverse processes associated with recycling. These heterogeneities occur on a scale of similar to 50-10 0 km, corresponding to transform- and propagating-rift-bounded segmentation , and attest to the presence of distinct chemical domains in the mantle oft en bounded by surficial tectonic features that maintain their integrity on the scale sampled by melting.