Rare earth abundances and distribution in some spinel peridotite xenoliths from Assab (Ethiopia)

Abstract

REE partition data between solid phases in nine spinel peridotite xenoliths from Assab (Ethiopia) are presented together with bulk-rock REE composition. REE partitioning between clinopyroxene and other coexisting solid phases varies over a relatively wide range. The largest ranges are for LREE in clinopyroxene/orthopyroxene and clinopyroxene/olivine pairs, while clinopyroxene/spinel partitioning of REE is more restricted. The range of REE partition values between coexisting phases is due to compositional dependency effects and is correlated with systematic variations in major element composition of the bulk rocks. The measured REE concentration in the Assab mantle harzburgites do not match with the compositions calculated by mass balance from the modal proportions and REE analyses in individual phases. Inconsistencies for HREE may be due to variable HREE amounts in the clinopyroxene, orthopyroxene phases within a single specimen, while the high LREE contents in the whole rocks are due to contamination during transport to the surface. A geochemical model based on theoretical treatment of the REE partition data suggests that the Assab harzburgites acquired their residual character during a batch melting episode in the upper mantle under the Afar region.