Geochemistry of rare-earth elements in plagioclase

As opposed to many other features, REE distribution in plagioclase (PI) has been studied poorly. First data on this subject were published about 30 ye ars ago, and since then about 300 analyses of REE in PI have been reported by different authors, This study is an attempt to summarize those data by p utting them in a unified database and paying attention to the most importan t features of lanthanide geochemistry in this mineral. The samples of PI an alyzed for REE were taken from a wide variety of petrographic rocks: from m eteorites and lunar mafites to mafic-ultramafic rocks from ophiolitic and o ther complexes as well as basalts, dacites, rhyiolites, and some metamorphi tes. The total REE content in PI varies from initial to several tens of par ts per million; light elements and Eu dominate with a drastically subordina te role of heavy lanthanides. The REE concentration in a mineral demonstrat es a certain dependence on its general composition and the type of componen t rocks. The PI samples from the rocks of ophiolitic complexes and some met eorites are the most depleted in REE. The REE-enriched samples are from aci d and normal effusive rocks, some lunar and earth gabbroids, and metamorphi c rocks. The chondrite-normalized REE patterns in PI 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 expressed as Eu* = 2Eu(n)/(S m+Gd)(n) varies from 6 to 145 on the average. High Eu* values are character istic of the mineral from the rocks formed under reducing conditions, e.g., from meteorites. The REE distribution coefficients in the P1-clinipyroxene system reflect the behavior of lanthanide fractionating during joint cryst allization of minerals. In the Pl-melt system the coefficients normally dec rease from La to Lu (except for Eu) and from acid to magnesial rocks. Using these coefficients, we can probabilistically calculate the REE composition s of model magmatic melts. The small changeability of the coeffiicents of i nterphase REE distribution calculated for P1 samples from the same object c an be used as a criterion for the system equilibrium. Mechanism and forms o f REE inclusion in the Pi structure remain to be studied. Heterovalent isom orphism of Ca2+ and REE3+ with possible participation of Na+, Si4+, Al3+, a nd Sr2+ seems to be likely.