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Super-chondritic Sm/Nd ratios in Mars, the Earth and the Moon

Abstract

Small isotopic differences in the atomic abundance of neodymium-142 (142Nd) in silicate rocks represent the time-averaged effect of decay of formerly live samarium-146 (146Sm) and provide constraints on the timescales and mechanisms by which planetary mantles first differentiated1,2,3,4. This chronology, however, assumes that the composition of the total planet is identical to that of primitive undifferentiated meteorites called chondrites. The difference in the 142Nd/144Nd ratio between chondrites and terrestrial samples may therefore indicate very early isolation (<30 Myr from the formation of the Solar System) of the upper mantle or a slightly non-chondritic bulk Earth composition5,6. Here we present high-precision 142Nd data for 16 martian meteorites and show that Mars also has a non-chondritic composition. Meteorites belonging to the shergottite subgroup define a planetary isochron yielding an age of differentiation of 40 ± 18 Myr for the martian mantle. This isochron does not pass through the chondritic reference value (100 × ε142Nd = -21 ± 3; 147Sm/144Nd = 0.1966)6. The Earth, Moon and Mars all seem to have accreted in a portion of the inner Solar System with 5 per cent higher Sm/Nd ratios than material accreted in the asteroid belt. Such chemical heterogeneities may have arisen from sorting of nebular solids or from impact erosion of crustal reservoirs in planetary precursors. The 143Nd composition of the primitive mantle so defined by 142Nd is strikingly similar to the putative endmember component ‘FOZO’ characterized by high 3He/4He ratios7,8.

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Figure 1: The ε 142 Nd signatures of martian meteorites, lunar samples and eucrites compared with the composition of chondrites and terrestrial samples.
Figure 2: Planetary isochrons for Mars, Vesta5 and the Moon4 compared with terrestrial3 12 and chondritic compositions5 6.

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Acknowledgements

We thank T. Kleine for critical discussion and an informal review of the manuscript, and S. Jacobsen for a constructive review. This study was supported by the CNRS research programme PNP.

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Correspondence to Guillaume Caro.

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The file contains Supplementary Tables S1-S2, Supplementary Figure S1 with Legend and additional references. (PDF 433 kb)

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Caro, G., Bourdon, B., Halliday, A. et al. Super-chondritic Sm/Nd ratios in Mars, the Earth and the Moon. Nature 452, 336–339 (2008). https://doi.org/10.1038/nature06760

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