Trace element and isotopic evidence for REE migration and fractionation in salts next to a basalt dyke

Neodymium and Sr isotopic compositions and the rare earth elements (REE) distribution patterns have been determined in salts adjacent to a basaltic dyke along 2 parallel horizontal profiles. The salts, originally consisting of carnallite (KMgCl3 · 6H2O), have been transformed during basalt intrusion mainly into halite (NaCl) and sylvite (KCl) by fluids saturated in NaCl. The Sr isotope data suggests that much more fluid penetrated the upper than the lower horizon. The Nd isotope data shows that in the upper horizon, where fluid flow was stronger, Nd is essentially derived from the basalt. In contrast, in the lower horizon a strong salt Nd component is present. The REE data document in both horizons is a strong depletion of Ce, Pr, Nd, Sm and Eu with increasing distance from the basalt. This depletion of the light rare earths (LREE) is stronger in the upper horizon where fluid flow was stronger. The authors suggest that this REE fractionation is more likely due to precipitation of LREE-enriched accessory minerals such as apatite, than to differential REE solubility caused by selective REE complexation. This finding is of interest for REE behaviour in brines in general, and for the behaviour of radioactive REE and actinides in a salt repository for high-level nuclear waste in particular.