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

Abstract

We present new measurements of 138 Ce/ 142 Ce and 143 Nd/ 144 Nd isotopic ratios in terrestrial and extraterrestrial samples. The mean value obtained from nine chondrites defines the 138 Ce/ 142 Ce 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 2 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 LaCe 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. Keywords: 138 Ce/ 142 Ce; rare earth elements; chondritic bulk silicate Earth; silicate reservoirs; mantle array; ocean island basalts. 2008). 142 Nd is a radiogenic isotope partly produced by the decay of 146 Sm (T1/2=103 Ma). Variations in the Nd abundances exist among Solar System materials and they reflect isotopic heterogeneities within the protoplanetary disk (Bouvier and Boyet, 2016; Burkhardt et al., 2016). Determining terrestrial variations relative to chondrites thus requires precise knowledge of the nature of Earth's building blocks. Isotopic similarities between terrestrial samples and enstatite chondrites (ECs) suggest this group of chondrites to be the main terrestrial component (Dauphas, 2017; Javoy, 1995). The mean EC 142 Nd/ 144 Nd value is nevertheless lower than that of the BSE without distinguishable variations in their nonradiogenic Nd isotopic compositio...

Thallium Isotopic Compositions in Hawaiian Lavas: Evidence for Recycled Materials on the Kea Side of the Hawaiian Mantle Plume

“…Overall, additional multi‐isotopic work (including the noble gases, e.g., Xe and Ne; Parai et al., 2019) on the same samples is needed to substantiate the above observations and to better constrain the composition, sources, and age of these recycled materials. However, the comparisons made in this study demonstrate that interesting trends between traditional and non‐traditional stable isotopes in Hawaiian shield lavas can be delineated with high density sampling and corroborates the approach of other recent work that combines novel and traditional isotopic systems (e.g., see also Israel et al., 2020; Mundl‐Petermeier et al., 2020; Starkey et al., 2016; etc. ).…”

“…Higher 138 Ce/ 142 Ce in OIB has been attributed to old (potentially pre-Great Oxygenation Event [GOE]), recycled oceanic sediments (Israel et al, 2020). However, there are not enough Ce isotopic data for Hawaiian shield basalts from all three main geochemical groups (Loa, Kea, and Enriched Loa) to make robust comparisons to Tl (Figure S2).…”

The Role of Sulphur on the Melting of Ca-Poor Sediment and on Trace Element Transfer in Subduction Zones: An Experimental Investigation

“…Ga ago (Cabral et al, 2013;Delavault et al, 2016;Israel et al, 2020). Several authors have proposed (on the basis of isotope data for Hf, O, Si) that Archean juvenile continental crust formed by melting of enriched mafic crust extracted from a still primitive mantle, and that sediment recycling occurred only since the Mesoarchean (e.g.…”

Cerium stable isotope analysis of synthetic and terrestrial rock reference materials by MC-ICPMS

“…In addition to REE elemental abundances, systematics of radioactive decay chains that involves REE ( 138 La-138 Ba/ 138 Ce, 146,147 Sm- 142,143 Nd, and 176 Lu-176 Hf) are primary sources of information about the age, ori gin and formation of terrestrial and extra-terrestrial rocks and their parent bodies [4][5][6][7][8][9].…”