New Constraints on the Origin of the EM‐1 Component Revealed by the Measurement of the La‐Ce Isotope Systematics in Gough Island Lavas

Abstract : Hot spot lavas show a large diversity of isotope compositions resulting from the recycling of surface material into the convective mantle. Amongst the mantle end‐members, EM‐1 (enriched mantle) is widely debated and scenarios involving either old pelagic sediments subducted into the deep mantle or subcontinental lithospheric material incorporated at shallow depths are commonly evoked. We selected 12 lavas from Gough Island (south Atlantic Ocean) for the measurement of 138La‐138Ce, 147Sm‐143Nd, and 176Lu‐177Hf isotope systems. Results show limited ranges for ε143Nd, ε138Ce, and ε176Hf values, and Ce/Ce* do not correlate with measured isotope ratios. Cerium isotope compositions allow us to exclude the contribution of old sedimentary material carrying a negative, elemental cerium anomaly in the mantle source. Pelagic sediments are indeed characterized by strongly negative, elemental cerium anomalies and also high La/Ce ratios. Modeling a primitive mantle source contaminated by 0.4% to 2.2% of different 2.5 Ga‐old pelagic components is able to reproduce the lowest Ce/Ce* values. However, the cerium isotope measurements show ε138Ce values between −0.39 and 0.15, too low to give support to the incorporation of recycled pelagic sediments in the mantle source of the lavas. Our results suggest that the incorporation of subcontinental lithospheric material at shallow depths during the plume ascent is a more suitable model to explain the formation of the EM‐1 component. Hafnium and Nd isotopes also support this scenario. Subcontinental lithosphere sampled via kimberlites and lamproites has isotopic compositions that plot generally below the mantle array, a signature that is also seen in Gough lavas.
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M. Boyet, R. Doucelance, C. Israel, P. Bonnand, Delphine Auclair, et al.. New Constraints on the Origin of the EM‐1 Component Revealed by the Measurement of the La‐Ce Isotope Systematics in Gough Island Lavas. Geochemistry, Geophysics, Geosystems, AGU and the Geochemical Society, 2019, ⟨10.1029/2019GC008228⟩. ⟨hal-02156044⟩

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