Multi-stage origin of Roberts Victor eclogites: progressive metasomatism

Xenolithic eclogites are high-P/T garnet-clinopyroxene rocks brought to the surface by kimberlites and other primitive magmas. Their origins are controversial: do they represent subducted oceanic crust or magmatic rocks originally crystallized in the deep earth? The answer has important implications forthe definition of major Earth processes. Roberts Victor (South Africa) eclogites from the base of the lithospheric mantle (180km) can be divided into two Types with five subgroups (IA, IB, IK; IIA, IIB). Type IIA eclogites are the protoliths of Type I, which were progressively metasomatized by carbonatitic-kimberlitic melts/fluids to form Type IAIBIK. Type I rocks show extensive textural disequilibrium, and have abundant fluid inclusions and secondary phases, including diamond. Compared to Type IIA, Type I are richer in LREE and LILE; they have higher 87 Sr/86 Sr and 18 O, lower 143 Nd/144 Nd and 176 Hf/177 Hf, and younger (132+ or -16Ma) isochron ages, synchronous with the kimberlite eruption. Type IIB are broadly similar to IIA, but have lower MgO; their mutual relationships are not clear. Neither Type I nor Type II eclogites are similar to modern or Archean oceanic crust when all the chemical evidence is compared. In addition, the data from Type I eclogites cannot be used to support a "subduction model" of origin because their heavily metasomatised chemical compositions do not retain any primary characteristics. Type II eclogites are the key samples for the study of eclogite origins. Their bulk compositions and microstructures suggest they might have formed by exsolution from high-Al clinopyroxenes, crystallized from mafic-ultramafic magmas in the deep mantle.