Experimental determination of rare earth fractionation patterns in partial melts from peridotite in the upper mantle

Abstract

Rare earth element patterns of partial melts of a natural peridotite with a chondrite-like rare earth pattern as starting material have been experimentally determined at high pressure. The experimental data are combined with major element data on the same partial melts to test the applicability of models of volatile-free melting of peridotite to rock-forming processes in the upper mantle. Alkali basalts could not have been formed by partial melting of peridotite in the absence of volatiles because the major and trace element abundances produced experimentally are not compatible with those of natural alkali basalts. Abyssal tholeiite without light rare earth element depletion can be derived from volatile-free peridotite by direct partial melting. Suites of abyssal tholeiite and picritic basalt may have been formed by a two-stage melting process from a common peridotite source. The peridotite residue must have lost clinopyroxene and garnet at the time of formation of picritic melt. Abyssal tholeiites with depletion of light rare earth elements could have been derived only from a peridotite source that has been depleted of the same elements prior to the melting event yielding the tholeiite.