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

Abstract

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 midocean 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 matedhal 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., 2ø6pb/2ø4pb = 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 terdhgenous sediment and altered crust, but we cannot rule out more recent recycling of matedhal 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.

Composition of the Oceanic Crust

“…At the local scale, individual flows do seem to be more homogeneous at fast-spreading ridges Rubin et al, 2001Rubin et al, , 2009 clearly demonstrating that mid-ocean ridge magma chambers act to homogenize melts passing through them, particularly when spreading rates and magma fluxes are high. Nevertheless, as Figure 17 shows, there is no systematic relationship between spreading rate and isotopic variability at spreading rates below about 80 mm year Sr analyses), comes mostly from segments near the Chile Margin Triple junction, and most of the heterogeneity is confined to a single segment (Sturm et al, 1999). It is unclear whether this variability characterizes the entire Chile Ridge.…”

Birth, death, and resurrection: The life cycle of suprasubduction zone ophiolites

“…In this case, a``slab window'' opens inside the subduction zone in response to extinction of the spreading center when it enters the trench. Active spreading in front of the subduction zone continues to form new oceanic crust, leaving a wedge-shaped opening or``window'' in the subducting slab [Karsten et al, 1996;Klein and Karsten, 1995;Lytwyn et al, 1997;Sherman et al, 1997;Sturm et al, 1999]. This window allows subduction-enriched asthenosphere to rise through the slab into the region of active spreading, in the same manner that plume-enriched mantle flows outward along a sublithospheric conduit beneath active spreading centers [e.g., Hanan and Schilling, 1989].…”

“…The best example of this process is where the South Chile Rise is being subducted beneath South America slab [Karsten et al, 1996;Klein and Karsten, 1995;Sherman et al, 1997;Sturm et al, 1999].…”

Separate lead isotope analyses of leachate and residue rock fractions: implications for metal source tracing in ore deposit studies

“…Lead isotope analyses of whole rocks, aimed to trace metal sources in ore deposits and infer source reservoirs in petrogenetic studies, are commonly carried out on the bulk fraction of unaltered rocks (e.g., Tilton and Barreiro 1980;Gunnesch et al 1990;Wareham et al 1998), or on rock fractions previously acid-leached for short time periods (e.g., Sturm et al 1999). Because rocks contain appreciable amounts of uranium and thorium, the measured lead isotope compositions contain in-situ derived radiogenic lead.…”