Tetrad-Effects in REE Pattern of Granitoid Rocks: The Result of Liquid Immiscibility in Fluorine-Rich Silicate Melts

摘要

The smooth shape of chondrite-normalized spectra REE distribution in granitoid rocks is often violated by regular minima and maxima or tetrad-effects. Such peculiarities result from disruption of coherency of periodic filling of the f-sublevel in electron configuration of lanthanides in four groups or tetrads: La-Nd, Pm-Gd, Gd-Ho, and Er-Lu. Tetrad-effects of the W-type with concave curves between the boundary elements of tetrads are observed in REE patterns of seawater, algae, sponges, shell limestone, and some underground waters. Regular maxima of the M-type with convex curves in REE patterns were originally described in granite of Southern China [1]. Later, it was established that such REE distributions were typical for many granitoid rocks (rare-metal granite, ongonite, elvan, fluorine-rich rhyolite, granitic pegmatite), as well as for minerals composing them. The first (La-Nd), the third (Gd-Ho), and the fourth (Er-Lu) tetrads are applied for numerical description of tetrad-effects in chondrite-normalized REE distribution. The second tetrad (Pm-Gd) is not accounted for because of the Eu minimum and the absence of Pm under terrestrial environments.