Garnet compositions have two distinct trends in CaO-Cr2O3 space: 1) increasing CaO at constant Cr2O3 within the harzburgite field, and 2) decreasing CaO and Cr2O3 within the lherzolite field, moving from ultramafic compositions of Udachanaya toward more mafic compositions of Obnazhennaya. Zoned garnet grains have sinusoidal-REE patterns within their cores and display a gradational change to flat MREE-HREE profiles at the rims. Clinopyroxenes typically are LREE-enriched and have high Ti/Eu, indicating kimberlite rather than carbonatite metasomatism. Re-constructed melts in equilibrium with garnet REE chemistry indicate that Obnazhennaya garnets were overprinted by plume-derived basaltic fluids, whereas Udachnaya garnets were overprinted by kimberlite fluids. The ubiquitous plume signatures from the younger Obnazhennaya garnets is clear evidence for extensive metasomatism by mafic fluids within the SCLM during Siberian flood basalt (SFB) emplacement.
We present a four-stage model for the temporal evolution of the SCLM over the life cycle of the Siberian superplume: 1) Early-stage metasomatism from fluids circulating within the SCLM, resulting in refertilization from harzburgite to lherzolite in the SCLM; 2) kimberlitic metasomatism, caused by small-degree partial melting of SCLM, induced by superplume upwelling; 3) Syn-SFB basaltic metasomatism, as the result of extensive percolation of basaltic fluids derived from the main stages of superplume activity; and 4) Post-SFB local kimberlitic metasomatism, resulting in LREE enrichment of Obnazhennaya clinopyroxenes related to the eruption of the host kimberlite.