Open AccessAge constraints on crustal recycling to the mantle beneath the southern Chile Ridge: He‐Pb‐Sr‐Nd isotope systematics
Author(s) -
Sturm Marnie E.,
Klein Emily M.,
Graham David W.,
Karsten Jill
Publication year - 1999
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/1998jb900107
Subject(s) - geology , basalt , pelagic sediment , geochemistry , subduction , mantle (geology) , oceanic crust , radiogenic nuclide , mid ocean ridge , terrigenous sediment , trace element , crust , continental crust , ridge , paleontology , tectonics , sedimentary rock
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 3 He/ 4 He 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., 206 Pb/ 204 Pb = 19.5) and the lowest 3 He/ 4 He yet measured in MORB (3.5 R A ). 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.