Hydrothermal remobilization of rare earth elements and its effect on Nd isotopes in rhyolite and granite

Although several studies have shown that the SmNd radiometric system is not remobilized during the hydrothermal alteration of felsic rocks, it is doubtful that the rare earth element (REE) mobility noticed under certain hydrothermal conditions always leaves the SmNd systematics unperturbed. This contribution describes two cases of such isotopic disturbances of hydrothermalised felsic rocks from two different geological contexts: in rhyolites from the Estérel (southeastern France) and in the red granite of Porto (northwest Corsica, southeastern France). Although these two igneous bodies were initially very homogeneous as regards their Nd isotopes, some samples display very contrasting Nd isotopic signatures (i.e. comparatively low initial143Nd/144Nd ratios) that are anomalous from a magmatic point of view. Moreover, these isotopically perturbed rocks are invariably associated with peculiar REE patterns that are also difficult to explain in terms of differentiation or contamination at the magmatic stage. It is thus inferred that the SmNd system in these samples was perturbed by hydrothermal alteration. two examples studied, the perturbation of the SmNd radiometric system cannot be solely explained by a modification of theSm/Nd ratio significantly after the emplacement of the igneous bodies. Although the net result of the hydrothermal alteration is leaching of the REEs from the rocks (i.e., mainly the middle and heavy REE for the granite and the light REE for the rhyolites), the isotopic compositions of the samples were modified by a component introduced from a fluid having contrasting143Nd/144Nd ratios. nference, together with structural observations, indicates that REEs were transported over distances exceeding several tens to hundreds of metres, even in the case of the granite studied in this work where mineralised fractures are absent. This is far greater than previous estimates and this conclusion is of great importance for environmental and nuclear waste disposal site studies where light-REE are often taken as natural analogues for artificial radionuclides.