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138La-138Ce and 147Sm-143Nd isotope composition of the main silicate reservoirs and implications forthe rare earth elements pattern of the Bulk Silicate Earth

Abstract : Compositional and evolutional models of the Earth need to constrain the chemical and isotopic compositions of its main reservoirs. For this purpose, we propose to combine the measurementsof two isotope systems:138La-138Ce and147Sm-143Nd. Parent and daughter elementsare from the rare earth element group (REE) and their behaviour are well-known during magmatic processes.Furthermore, chemical and analytical procedures recently developed allow us tofurther explore the domain of Ce isotope systematics in the context of planetary evolution. We report high precision Ce and Nd isotope measurements of chondrites, mid-oceanic ridge basalts (MORB), ocean island basalts (OIB) and aeolian sediments. The mean value of 9 chondrites gives the reference for the CHondritic Uniform Reservoir (CHUR): 138Ce/142CeCHUR= 0.02256577 ± 66 (2 s.d.). MORB and OIB define a negative, linear correlation called the mantle array in the ε143Nd vs. ε138Ce diagram. The corresponding best fit is: ε143Nd = -7.93 x ε138Ce + 0.37. The CHURplots on the mantle array. This is also the case for twoearly-depleted reservoirs we modelled assuming Sm/Ndincreases of 3.6% and 5.6%(relative to chondrites)in order toexplain 142Nd excesses of 10 and 16 ppm in modern terrestrial samples relative to chondrites.The precise determination of the mantle array does not provide any constraint on the La/Ce and Sm/Nd ratios of the Bulk Silicate Earth (BSE).We went further by modelling a consistent Ce isotope composition for the continental crust (upper-middle-lower) assuminga BSE with chondritic REE ratios. Our isotopic valuesforthe continental crust highlight a stronger fractionation of the La/Ce ratio between the crustal reservoirs than that observed for the Sm/Nd ratio.
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https://hal.uca.fr/hal-02526214
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Submitted on : Tuesday, March 31, 2020 - 2:21:03 PM
Last modification on : Wednesday, November 3, 2021 - 6:43:39 AM

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  • HAL Id : hal-02526214, version 1

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Claudine Israel, Maud Boyet, Régis Doucelance, Pierre Bonnand, P. Frossard, et al.. 138La-138Ce and 147Sm-143Nd isotope composition of the main silicate reservoirs and implications forthe rare earth elements pattern of the Bulk Silicate Earth. Goldschmidt 2019, Aug 2019, Barcelone, Spain. ⟨hal-02526214⟩

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