Evidence for distinct proportions of subducted oceanic crust and lithosphere in HIMU-type mantle beneath El Hierro and La Palma, Canary Islands

摘要

Shield-stage high-MgO alkalic lavas from La Palma and El Hierro (Canary Islands) have been characterized for their O-Sr-Nd-Os-Pb isotope compositions and major-, trace-, and highly siderophile-element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances. New data are also reported for associated evolved rocks, and entrained xenoliths. Clear differences in Pd/Ir and isotopic ratios for high Os (>50ppt) lavas from El Hierro (δ~(18)O_(olivine)=5.17±0.08‰; ~(87)Sr/~(86)Sr=0.7029 to 0.7031; ε{lunate}_(Nd)=+5.7 to +7.1; ~(187)Os/~(188)Os=0.1481 to 0.1750; ~(206)Pb/~(204)Pb=19.1 to 19.7; Pd/Ir=6±3) versus those from La Palma (δ~(18)O_(olivine)=4.87±0.18‰; ~(87)Sr/~(86)Sr=0.7031 to 0.7032; ε{lunate}_(Nd)=+5.0 to +6.4; ~(187)Os/~(188)Os=0.1421 to 0.1460; ~(206)Pb/~(204)Pb=19.5 to 20.2; Pd/Ir=11±4) are revealed from the dataset. Crustal or lithospheric assimilation during magma transport cannot explain variations in isotopic ratios or element abundances of the lavas. Shallow-level crystal-liquid fractionation of olivine, clinopyroxene and associated early-crystallizing minerals (e.g., spinel and HSE-rich phases) controlled compatible element and HSE abundances; there is also evidence for sub-aerial degassing of rhenium. High-MgO lavas are enriched in light rare earth elements, Nb, Ta, U, Th, and depleted in K and Pb, relative to primitive mantle abundance estimates, typical of HIMU-type oceanic island basalts. Trace element abundances and ratios are consistent with low degrees (2-6%) of partial melting of an enriched mantle source, commencing in the garnet stability field (≥110. km). Western Canary Island lavas were sulphur undersaturated with estimated parental melt HSE abundances (in ppb) of 0.07 ± 0.05 Os, 0.17 ± 0.16 Ir, 0.34 ± 0.32 Ru, 2.6 ± 2.5 Pt, 1.4 ± 1.2 Pd, 0.39 ± 0.30 Re. These estimates indicate that Canary Island alkali basalts have lower Os, Ir and Ru, but similar Pt, Pd and Re contents to Hawai'ian tholeiites. The HIMU affinities of the lavas, in conjunction with the low δ~(18)O_(olivine) and high _(206)Pb/_(204)Pb for La Palma, and elevated ~(187)Os/~(188)Os for El Hierro implies melting of different proportions of recycled oceanic crust and lithosphere. Our preferred model to explain isotopic differences between the islands is generation from peridotitic mantle metasomatised by <10% pyroxenite/eclogite made from variable portions of similar aged recycled oceanic crust and lithosphere. The correspondence of radiogenic ~(206)Pb/~(204)Pb, ~(187)Os/~(188)Os, elevated Re/Os and Pt/Os, and low-δ~(18)O in western Canary Island lavas provides powerful support for recycled oceanic crust and lithosphere to generate the spectrum of HIMU-type ocean island basalt signatures. Persistence of geochemical heterogeneities throughout the stratigraphies of El Hierro and La Palma demonstrate long-term preservation of these recycled components in their mantle sources over relatively short-length scales (～50km).