A UNIQUE METASOMATIZED PERIDOTITE XENOLITH FROM THE MIR KIMBERLITE PIPE (YAKUTIA)

Major- and trace-element chemistry of garnet, clinopyroxene, orthopyro xene, and olivine from a unique metasomatized garnet peridotite, A-143 from the Mir kimberlite, have been determined by electron microprobe and ion microprobe. The pyrope garnets display significant core-to-rim chemical zonations and intergrain compositional variations. Rims are enriched in Cr2O3 compared to the core (4.98 and 8.04 wt.%, respective ly). Clinopyroxenes are homogeneous but exhibit intergrain composition al variations. Contents of Cr2O3 vary from 6.05 to 6.80 wt.%, while Na 2O changes from 6.71 to 7.08 wt.%. Thus, clinopyroxene is unique in co mposition, containing the highest Na2O and Cr2O3 contents yet reported for peridotites. This reflects enrichment in kosmochlor component (Na CrSi2O6) of up to 21%. Chondrite-normalized rare-earth-element distrib ution patterns from garnet and clinopyroxene are convex. Clinopyroxene REE pattern exhibit no intergranular variation. Significant garnet zo nation, unusual garnet-clinopyroxene intergrowths (fingers, and/or sel vedges of clinopyroxene and garnet), and unique clinopyroxene composit ions point to involvement of metasomatic fluids that could have a kimb erlitic'' component, with (La/Nd)(n) = 5-10, and Nd approximate to 100 xCl chondrites, or a Na-enriched eclogitic'' component. Regardless, pe ridotite xenolith A-143 is a mineral assemblage with unique chemical c omposition and unusual textural features that offers insight into mant le metasomatism and enrichment processes.