Formation of the Ce-Nd mantle array: crustal extraction vs. recycling by subduction

Abstract : 15 We present new measurements of 138 Ce/ 142 Ce and 143 Nd/ 144 Nd isotopic ratios in terrestrial and extra-16 terrestrial samples. The mean value obtained from nine chondrites defines the 138 Ce/ 142 Ce ratio of the 17 chondritic uniform reservoir (CHUR) as 0.02256577 ± 66 (2sd). MORBs and OIBs define the mantle 18 array in the εNd vs. εCe diagram to be εNd =-7.3 (± 0.5) × εCe + 0.4 (± 0.3). From MORB 19 measurements, we derive the isotopic composition of the depleted MORB mantle (DMM) to be εCe =-20 1.1 ± 0.6 (2sd). Both CHUR and a modelled early-depleted mantle reservoir plot on the mantle array. 21 Thus, the precise determination of the mantle array does not further constrain the La/Ce and Sm/Nd 22 ratios of the bulk silicate Earth (BSE; i.e., primitive mantle). The composition of 1.8 Ga upper 23 continental crust obtained from aeolian sediments is εCe = 1.8 ± 0.3 (2sd; εNd =-11.2), and that of its 24 2.2 Ga equivalent is εCe = 2.3 ± 0.3 (2sd; εNd =-17). Binary mixing models between depleted (DMM) 25 and enriched (upper crust or mafic crust composition) components do not reproduce the linear Ce-Nd 26 mantle array but plots close to the island arc basalt data. When the bulk Ce isotopic composition of the 27 2 continental crust is calculated from the range of accepted Nd isotope values and a mass-balance budget 28 of the BSE, the mixing curves are closer to the mantle array. However the calculated Ce isotopic 29 composition for the bulk crust is always less radiogenic than measurements. Adjusting the Ce-Nd 30 isotopic composition or the Ce/Nd ratio of the end-members to fully linearise the mixing curve leads to 31 unrealistic values never measured in terrestrial samples. We propose a recycling model to reconstruct 32 the mantle array with the participation of both oceanic crust and sediments in the mantle through time. 33 Cerium is a redox sensitive element, making the LaCe and Sm-Nd systematics an ideal combination to 34 investigate sediment recycling through time. In this recycling model, the most extreme EM-like 35 signatures require the involvement of oceanic sediments that formed under reduced conditions before 36 the Great Oxygenation Event at 2.4 Ga, and which are devoid of Ce elemental anomalies. 37 38 Keywords: 138 Ce/ 142 Ce; rare earth elements; chondritic bulk silicate Earth; silicate reservoirs; mantle 39 array; ocean island basalts. 40 41 42
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Claudine Israel, M. Boyet, R. Doucelance, P. Bonnand, P. Frossard, et al.. Formation of the Ce-Nd mantle array: crustal extraction vs. recycling by subduction. Earth and Planetary Science Letters, Elsevier, 2020, 530, pp.115941. ⟨10.1016/j.epsl.2019.115941⟩. ⟨hal-02409422⟩

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