Mobility of rare earth elements in zones of intense hydrothermal alteration in the Pindos ophiolite, Greece

Abstract

Abstract During the formation of the Jurassic Pindos ophiolite of northwest Greece, a convective seawater-fed hydrothermal system was initiated at or near the spreading centre. As a result, typical ophiolitic background metamorphic assemblages developed during the downward circulation of seawater, and epidosites and stockworks formed in the upwelling part of the system. The epidosites were analysed for major and trace elements including the REE. Substantial chemical changes occur during the evolution of the epidosites from their protoliths. While some elements (Ti, Zr, Y, Nb) appear immobile during this transformation, when a bulk quartz addition is taken into account, the REEs are mobile under these alteration conditions. Analyses show: (a) either enrichment of all the LREEs; or (b) ‘anomalous’ selective enrichment of certain elements (La, Pr, Eu) and depletion of others (Ce, Sm). These patterns can best be explained by: (a) greater mobility in the bulk of the alteration zone of the LREEs compared to the HREEs; (b) variable properties of Ce and Eu due to changes in their valence state; (c) variable water/rock ratio in epidosite zones; (d) heterogeneous permeability of reaction zones; (e) control of the rock alteration mineralogy on the REE distribution. Evidence for the mobility of the REE is also provided by the analyses of hydrothermal solutions from black smokers at 13°N and 21°N on the East Pacific Rise. REE can be very good indicators of the degree of water/rock interaction at high water/rock ratios.