Trace element crystal chemistry of mantle eclogites

Abstract

Rare earth element (REE) concentrations have been measured using instrumental neutron activation analysis on clean separates of primary minerals from 11 eclogite samples from the Bobbejaan and Roberts Victor kimberlites, South Africa. Samples were selected to reflect minimal secondary alteration and represent a broad range of eclogite compositions from coesiteand corundum-grospydites through magnesian bimineralic eclogites. Correlations between REE concentrations and major-element compositions suggest that garnet and clinopyroxene crystal chemistry are the dominant control on REE distribution and that these approach solidstate equilibrium distributions. Reconstructed wholerock REE concentration variation with whole-rock major-element compositions are consistent with an origin by high-pressure igneous fractionation followed by reequilibration to lower temperatures at pressures in excess of three GPa.