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

Abstract

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 mid‐ocean ridge basalt (MORB)‐like 3He/4He and modest variations in Pb, Sr, and Nd isotopes of segment 1 (nearest the trench) suggest recent (<20 Ma) introduction of a contaminant into its source, consistent with recycling of material from the adjacent subduction zone. In contrast, segment 3 lavas display a dramatic southward increase in enrichment, extending to highly radiogenic Pb and Sr isotopic compositions (e.g., 206Pb/204Pb = 19.5) and the lowest 3He/4He yet measured in MORB (3.5RA). The segment 3 variations are most readily explained by ancient (∼2 Ga) recycling of terrigenous sediment and altered crust, but we cannot rule out more recent recycling of material derived from a distant continental source. The similarity in isotopic signatures of segment 4 lavas to Indian Ocean MORB extends the Dupal anomaly to the Chile Ridge. Like Indian Ocean MORB, 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. Southern Chile Ridge MORB reflect the extensive degree of heterogeneity that is introduced into the depleted upper mantle by diverse processes associated with recycling. These heterogeneities occur on a scale of ∼50–100 km, corresponding to transform‐ and propagating‐rift‐bounded segmentation, and attest to the presence of distinct chemical domains in the mantle often bounded by surficial tectonic features that maintain their integrity on the scale sampled by melting.