Unmasking xenolithic eclogites: Progressive metasomatism of a key Roberts Victor sample

• Progressive metasomatism has been recognized within one Roberts Victor eclogite.
• δ18O of the garnet decreases from 8.2 to 5.7‰ with increasing metasomatism.
• Mantle metasomatism can sweep away all original compositional information.
• Least-metasomatized samples must be studied to reveal the original eclogites.

Most eclogite xenoliths from the Roberts Victor kimberlite (South Africa) have been metasomatized by melts/fluids in the carbonatitic–kimberlitic spectrum, and these can be used to define a progressive metasomatic process. Several stages of this progressive metasomatism have been recognized within one sample (RV07-17), using petrography, major- and trace-element compositions, and Sr- and O-isotope data. Four zones in RV07-17 are distinguished in terms of the compositions of their garnets. From Zone 1 to Zone 4, the microstructure becomes less equilibrated; secondary minerals and fluid inclusions become abundant; the pyrope content of the garnets increases; and clinopyroxene shows progressive enrichment in MgO. The garnets of Zone 1 have flat REE patterns from Lu to Sm, with a strong depletion in the LREE. Toward Zone 4, the relative abundance of the MREE of the garnets drops significantly, giving smoother patterns. A large relict clinopyroxene grain in Zone 1 shows a strong depletion in the LREE, but the LREE/MREE of the recrystallized clinopyroxene increases from Zone 1 to Zone 4. From Zone 1 to 4, 87Sr/86Sr in clinopyroxene increases along with Sr content and δ18O of the garnet decreases from 8.2 to 5.7‰ as the MgO content increases.Trace-element data imply that the fluids that metasomatized RV07-17 were closer to carbonatitic than kimberlitic in composition. Aside from the one relict clinopyroxene, all original compositional information in this xenolith has been swept away, making it difficult to define the protolith. To know the origin of the xenolithic eclogites, and to use them as evidence for different geodynamic/tectonic scenarios, the least metasomatized samples must be studied; unfortunately these make up a tiny proportion of the widely studied xenolith population at Roberts Victor.