EXSOLUTION OF GARNET WITHIN CLINOPYROXENE OF MANTLE ECLOGITES - MAJOR-ELEMENT AND TRACE-ELEMENT CHEMISTRY

Eclogite xenoliths from the mantle have experienced a wide variety of processes and P-T conditions, many of which are recorded in the minera l compositions and textures. Exsolution of garnet from clinopyroxene i s one such texture, occurring in a minority of mantle eclogites. New a nalyses of clinopyroxene and garnet of eclogite xenoliths from kimberl ites at Bellsbank (South Africa) and Obnazhennaya (Yakutia, Russia) ar e presented here, and these are combined with data from the literature . Exsolution of garnet from clinopyroxene is generally lamellar, altho ugh lens-shaped garnets are also present. Major- and trace-element cha racteristics show a wide range of compositions and include eclogite Gr oups A, B. and C. Rare-earth element (REE) concentrations of garnet an d pyroxene were determined by SIMS, and the REE patterns are subtly di fferent from those in ''ordinary'' eclogites. Differences include the absence of prominent Eu anomalies in samples of this study and differe nces in the slopes of chondrite-normalized REE patterns. It is possibl e that these ''signatures'' are unique to exsolved eclogites, a result of subsolidus elemental partitioning during exsolution. Some reconstr ucted whole-rock compositions are aluminuous; comparison with ordinary eclogites shows only minor differences, implying a similar origin. If the immediate precursor to the exsolved eclogites was a monomineralic pyroxenite, the excess aluminium was tied up in Tschermak's molecule, although the occasional presence of kyanite exsolution lamellae is in dicative of a Ca-Eskola component. Reconstructed ''pyroxenes'' from ky anite- and corundum-rich samples contain unrealistic amounts of alumin ium for mantle pyroxenes. A protolith (or parental pyroxene) ''thresho ld'' of approximately 24% Al2O3 may exist, above which (as in a plagio clase cumulate) the final assemblage is kyanite- and/or corundum-beari ng.