We studied the Sm-Nd, Rb-Sr, and Re-Os isotope compositions of mantle xenoliths (eclogites and peridotites) from diamondiferous kimberlites of the Catoca cluster of the Kasai Craton. In the eclogites, the primary strontium isotope composition ⁸⁷Sr/⁸⁶Sr varies from 0.7056 to 0.7071, and the neodymium isotope composition e_Nd, from 1.8 to 2.6. The ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios range from 135 to 80 and from 1.3110 to 1.9709, respectively, which indicates a significant portion of radiogenic Os: y_Os = 129-147. These isotope values exceed the values assumed for model reservoirs (primitive upper mantle (PUM) and bulk silicate Earth (BSE)) and those of chondrites. The isotope composition of the studied systems indicates the formation of eclogites from a rhenium-enriched source, namely, the subducted oceanic crust transformed as a result of metasomatism and/or melting under upper-mantle conditions.