Nestmeyer, Mark, and McCoy-West, Alex J. (2026) Neodymium stable isotope fractionation in minerals: Implications for Earth’s differentiation, and planetary formation. Geochimica et Cosmochimica Acta, 421. pp. 342-355.

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Abstract

The application of rare Earth element stable isotope compositions has become of increasing interest in geochemistry. Recently, studies have begun exploring variations in the stable 146Nd/144Nd isotope ratio in geological samples, with limited isotope fractionation observed in igneous rocks but significantly larger fractionations seen in low temperature systems. Experimental and theoretical studies on the equilibrium isotope fractionation of Nd are widely missing which can support understanding the fractionation of Nd isotopes among Earth’s major reservoirs. Here, we have modelled the isotope fractionation factors for 15 common rock forming and accessory minerals to help understand equilibrium stable isotope fractionation during medium to high temperature processes.We estimate that mantle melting will produce minimal isotope fractionation while the residual peridotite ought to retain heavier Nd isotopes which could explain a potentially superchondritic composition of the depleted mantle. This can be predicted because in mantle minerals with Mg sites (e.g. olivine, orthopyroxene) substitution of isotopically heavier Nd is preferred compared to minerals with Ca sites (e.g. clinopyroxene), with the latter being the major contributor to basaltic melts. The Earth’s outer core (i.e. sulfide matte) is a potential host of lighter Nd isotopes which we demonstrate favours sulfides over silicates. However, the low partition coefficients of rare Earth elements into the core leads to a composition of the bulk silicate Earth that is indistinguishable from chondrites. A better understanding of the δ146/144Nd composition of the mantle is warranted to elucidate the isotope fractionation of Nd between Earth’s major geochemical reservoirs.As Nd condenses from the solar nebular gas into primitive material, the mass-independent nuclear field shift effect dominates equilibrium isotope fractionation and produces significant fractionation in δ146/144Nd even at high temperatures (>1000 °C) with the condensed material enriched in lighter Nd isotopes. However, the isotopic compositions reported for refractory inclusions (–1.86 to 2.20 ‰; Hu et al. (2021)) cannot be solely explained by equilibrium isotope fractionation and other mechanisms are required.

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