Home – Home – Jornals – Russian Geology and Geophysics 2001 number 6

As opposed to many other features, REE distribution in plagioclase (Pl) has been studied poorly. First data on this subject were published about 30 years ago, and since then about 300 analyses of REE in Pl have been reported by different authors. This study is an attempt to summarize those data by putting them in a unified database and paying attention to the most important features of lanthanide geochemistry in this mineral. The samples of Pl analyzed for REE were taken from a wide variety of petrographic rocks: from meteorites and lunar mafites to mafic-ultramafic rocks from ophiolitic and other complexes as well as basalts, dacites, rhyiolites, and some metamorphites. The total REE content in Pl varies from initial to several tens of parts per million; light elements and Eu dominate with a drastically subordinate role of heavy lanthanides. The REE concentration in a mineral demonstrates a certain dependence on its general composition and the type of component rocks. The Pl samples from the rocks of ophiolitic complexes and some meteorites are the most depleted in REE. The REE-enriched samples are from acid and normal effusive rocks, some lunar and earth gabbroids, and metamorphic rocks. The chondrite-normalized REE patterns in Pl always have a negative slope, and relevant (La/Yb)n values vary in a wide range, from 5 to 200. The patterns show a Eu maximum; its intensity is expressed as Eu*=2Eu_n/(Sm+Gd)_n varies from 6 to 145 on the average. High Eu* values are characteristic of the mineral from the rocks formed under reducing conditions, e.g., from meteorites. The REE distribution coefficients in the Pl-clinipyroxene system reflect the behavior of lanthanide fractionating during joint crystallization of minerals. In the Pl-melt system the coefficients normally decrease from La to Lu (except for Eu) and from acid to magnesial rocks. Using these coefficients, we can probabilistically calculate the REE compositions of model magmatic melts. The small changeability of the coeffiicents of interphase REE distribution calculated for Pl samples from the same object can be used as a criterion for the system equilibrium. Mechanism and forms of REE inclusion in the Pl structure remain to be studied. Heterovalent isomorphism of Ca²⁺ and REE³⁺ with possible participation of Na₁₊, Si₄₊, Al₃₊, and Sr₂₊ seems to be likely.