On rare-earth element behavior in igneous rocks

Abstract

Trends of the past decade in geochemical investigations of trace elements are reflected in studies of rare-earth elements (REE). Substantial improvements have been made in accuracy, sensitivity and convenience of REE analysis, mainly through improved techniques and equipment for neutron-activation analysis and mass spectrometric isotope dilution. The most detailed studies have been on mafic rock systems, with a shift away from broad surveys to detailed examinations of single regions or formations. Ocean ridge tholeiitic basalts have been shown to derive from mantle sources partly depleted by earlier melting. Associated basalts deviating from typical ocean ridge tholeiite have been attributed to invading plumes of more primitive mantle matter. Lunar mare basalts and highland KREEP basalts have been characterized as having REE distributions roughly analogous to terrestrial ocean and continental distributions. Models for genesis of mafic liquids as products of partial melting or fractional crystallization have been developed and applied. These models are dependent on values for distribution coefficients. Such values are not well known or the factors affecting them well characterized. Experimental techniques for estimating distribution coefficients are still being developed and tested. Present models cannot yet account quantitatively for the REE concentrations and distributions observed in even the best understood natural systems. The semiquantitative results on a variety of mafic systems nevertheless constitute important gains in understanding of both trace element behavior and igneous rock petrogenesis.