Origin of ~3He/~4He ratios in HIMU-type basalts constrained from Canary Island lavas

摘要

New helium isotope and abundance measurements are reported for olivine and clinopyroxene phenocrysts from HIMU-type (high-μ=elevated ~(238)U/~(204)Pb) lavas and xenoliths spanning the stratigraphies of El Hierro and La Palma, Canary Islands. Some pyroxene phenocrysts have suffered post-eruptive modification, either by less than 1% assimilation of crustal-derived He, or by closed-system ageing of He. Olivine phenocrysts record mantle source ~3He/~4He compositions, with the average ~3He/~4He for La Palma olivine (7.6±0.8R_A, where R_A is the atmospheric ~3He/~4He ratio of 1.38×10~(-6)) being within uncertainty of those for El Hierro (7.7±0.3R_A), and the canonical mid-ocean ridge basalt range (MORB: 8±1R_A). The new helium isotope data for El Hierro and La Palma show no distinct correlations with whole-rock ~(87)Sr/~(86)Sr, ~(143)Nd/~(144)Nd, ~(187)Os/~(188)Os, or Pb isotopes, but ~3He/~4He ratios for La Palma lavas correlate with ~(18)O/~(16)O measured for the same phenocryst populations. Despite limited ~3He/~4He variations for El Hierro and La Palma, their He-O isotope systematics are consistent with derivation from mantle sources containing distinct recycled oceanic basaltic crust (El Hierro) and gabbroic lithosphere (La Palma) components that have mixed with depleted mantle, and a high-~3He/~4He component (>9.7R_A) in the case of La Palma. The new data are consistent with models involving generation of compositionally and lithologically (e.g., pyroxenite, eclogite, peridotite) heterogeneous mantle sources containing recycled oceanic crust and lithosphere entrained within upwelling high-~3He/~4He mantle that has been severely diluted by interaction with depleted mantle. We propose that the noble gas systematics of HIMU-type lavas and ocean island basalts (OIB) in general, are most simply interpreted as being controlled by the most gas-rich reservoir involved in mixing to generate their mantle sources. In this scenario, HIMU and enriched mantle (EM) sources are dominated by depleted mantle, or high-~3He/~4He mantle, because recycled crust and lithosphere have low He concentrations. Consequently, high-~3He/~4He OIB would predominantly reflect derivation from a less depleted mantle source with sub-equal to higher He contents than depleted mantle. The available coupled He-O isotope systematics measured for OIB lavas are consistent with this hypothesis.