PARTITIONING OF RARE-EARTH ELEMENTS IN THE ECLOGITES OF METAMORPHIC ROCK COMPLEXES

Abstract

Studies of the behavior of the rare-earth elements (REE) in high-pressure metamorphism have established that the partitioning of REE between the pyroxenes and garnets in the eclogites of metamorphic rock complexes differs from that usually cited in the literature for eclogite xenoliths from kimberlite pipes. In particular, there is no marked enrichment of clinopyroxene in the lighter REE in comparison to the garnet. In addition - and this is especially important - the content of light REE in the rock is higher than their concentration in monomineralic fractions. Two alternative hypotheses explaining this result can be advanced. Either a considerable part of the REE is concentrated in the accessory minerals, or else they are contained largely in the intergranular spaces. In connection with this question, we have carried out an experiment in acid leaching of the rocks. In addition, the distribution of the light REE was studied by the electrographic and β-autoradiographic methods. The results of the study of the REE in the monomineralic fractions of the garnets and pyroxenes shows no marked enrichment of the clinopyroxene in the light REE and that the content of light REE in the rock is higher than their concentration in the monomineralic fractions. In view of the fact that a considerable part of the light REE is in the intergranular space and is readily leached out, their inert behavior can be attributed either to the composition of the fluids, in which no complex-forming radical was present, or to a low fluid/rock ratio.