1. Laboratoire Géochimie et Cosmochimie Institut de Physique du Globe de Paris—Université Denis Diderot, 4 place Jussieu, 75252 Paris Cedex 05, France
2. Institute of Isotope Geochemistry and Mineral Resources, ETH Zurich 8092, Zurich, Switzerland
3. Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, UK
4. Laboratoire des Sciences de la Terre Ecole Normale Supérieure de Lyon, F-69364 Lyon 7, France

Correspondence to: Guillaume Caro¹ Correspondence and requests for materials should be addressed to G.C. (Email: caro@crpg.cnrs-nancy.fr).

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 differentiated^{1, 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 composition^{5, 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  ¹⁴²Nd = -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 ratios^{7, 8}.

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