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

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doi: 10.1016/0012-821X(94)00257-Y
Authors:Poitrasson, Franck; Pin, Christian; Duthou, Jean-Louis
Author Affiliations:Primary:
Université Blaise Pascal, Département des Sciences de la Terre, Clermont-Ferrand, France
Volume Title:Earth and Planetary Science Letters
Source:Earth and Planetary Science Letters, 130(1-4), p.1-11. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X
Publication Date:1995
Note:In English. 43 refs.; illus., incl. 3 tables
Summary:Although several studies have shown that the Sm-Nd 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 Sm-Nd systematics unperturbed. This contribution describes two cases of such isotopic disturbances of hydrothermalised felsic rocks from two different geological contexts: in rhyolites from the Esterel (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 initial 143Nd/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 Sm-Nd system in these samples was perturbed by hydrothermal alteration. In the two examples studied, the perturbation of the Sm-Nd radiometric system cannot be solely explained by a modification of the Sm/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 contrasting 143Nd/144Nd ratios. This inference, 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.
Subjects:Absolute age; Applications; Chemical reactions; Granites; Hydrothermal alteration; Igneous rocks; Interpretation; Intrusions; Isotope ratios; Isotopes; Leaching; Metals; Metasomatism; Migration of elements; Mobility; Nd-144/Nd-143; Neodymium; Plutonic rocks; Rare earths; Rhyolites; Sm/Nd; Stable isotopes; Trace elements; Volcanic rocks; Water-rock interaction; Corsica; Esterel; Europe; France; Western Europe
Abstract Numbers:95M/3073
Record ID:1995029183
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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