Abstract
We present new measurements of 138Ce/142Ce and 143Nd/144Nd isotopic ratios in terrestrial and extra-terrestrial samples. The mean value obtained from nine chondrites defines the 138Ce/142Ce ratio of the chondritic uniform reservoir (CHUR) as 0.02256577±66 (2sd). MORBs and OIBs define the mantle array in the εNd vs. εCe diagram to be εNd=−7.3(±0.5)×εCe+0.4(±0.3). From MORB measurements, we derive the isotopic composition of the depleted MORB mantle (DMM) to be εCe=−1.1±0.6 (2sd). Both CHUR and a modelled early-depleted mantle reservoir plot on the mantle array. Thus, the precise determination of the mantle array does not further constrain the La/Ce and Sm/Nd ratios of the bulk silicate Earth (BSE; i.e., primitive mantle). The composition of 1.8 Ga upper continental crust obtained from aeolian sediments is εCe=1.8±0.3 (2sd; εNd=−11.2), and that of its 2.2 Ga equivalent is εCe=2.3±0.3 (2sd; εNd=−17). Binary mixing models between depleted (DMM) and enriched (upper crust or mafic crust composition) components do not reproduce the linear Ce-Nd mantle array but plots close to the island arc basalt data. When the bulk Ce isotopic composition of the continental crust is calculated from the range of accepted Nd isotope values and a mass-balance budget of the BSE, the mixing curves are closer to the mantle array. However the calculated Ce isotopic composition for the bulk crust is always less radiogenic than measurements. Adjusting the Ce-Nd isotopic composition or the Ce/Nd ratio of the end-members to fully linearise the mixing curve leads to unrealistic values never measured in terrestrial samples. We propose a recycling model to reconstruct the mantle array with the participation of both oceanic crust and sediments in the mantle through time. Cerium is a redox sensitive element, making the La-Ce and Sm-Nd systematics an ideal combination to investigate sediment recycling through time. In this recycling model, the most extreme EM-like signatures require the involvement of oceanic sediments that formed under reduced conditions before the Great Oxygenation Event at 2.4 Ga, and which are devoid of Ce elemental anomalies.
Highlights
Long-lived isotope systematics are remarkable tracers of the long-term evolution of the different terrestrial reservoirs
When the bulk Ce isotopic composition of the continental crust is calculated from the range of accepted Nd isotope values and a mass-balance budget 29 of the bulk silicate Earth (BSE), the mixing curves are closer to the mantle array
We conclude that combining Ce and Nd isotopic measurements in mantle-derived samples does not further constrain the composition of the primitive mantle (PM). 534 Mixing models between the depleted mid-ocean ridge basalts (MORBs) mantle (DMM) and continental crust (CC) end members, both estimated from sample measurements 535, cannot reproduce the Ce-Nd mantle array but better fit Island Arc Basalts (IABs) data
Summary
Long-lived isotope systematics are remarkable tracers of the long-term evolution of the different terrestrial reservoirs. Boyet and Carlson (2005) identified a systematic offset between the 142Nd/144Nd ratios of modern terrestrial rocks and chondrites resulting in an active debate on the refractory lithophile element composition of the PM (see Caro et al, 2008; Jackson and Jellinek, 2013; O’Neill and Palme, 2008). The mean EC 142Nd/144Nd value is lower than that of the BSE without distinguishable variations in their non radiogenic Nd isotopic compositions from Earth (Burkhardt et al, 2016; Boyet et al, 2018). Variations in 142Nd/144Nd values between different EC subgroups show further that the EL3 subgroup matches the terrestrial value (Boyet et al, 2018) These results remove the need of an early fractionation of the Sm/Nd ratio of the Earth if the Earth was purely made of EL3 materials. His regression intercepts the CHUR Nd isotopic composition at a BSE Ce isotopic composition that is 60 ppm less radiogenic than the CHUR value defined by Shimizu et al
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