Mantle metasomatism constraint: LA-ICPMS U-Pb dating of single garnet xenocrysts from the V. Grib kimberlite, Russia

Mantle metasomatism largely controls the physical and chemical properties of the lithospheric mantle and exerts control over the genesis of kimberlitic magmas and diamond formation. In this context, garnet xenocrysts from kimberlites are fundamental for decoding metasomatic events in the lithospheric mantle and to decipher the variation in rock types with depth. However, dating mantle lithologies and processes is complicated by high ambient temperatures that allow the equilibration of most isotopic systems up to the time of kimberlite eruption.

In this study we investigate garnet xenocrysts from the ~376 Ma V. Grib kimberlite, Russia to evaluate the potential of garnet for dating metasomatic events in the subcontinental lithospheric mantle. Based on major and trace element compositions and REE patterns, harzburgitic, wehrlitic, lherzolitic (4x) and megacrystic garnet xenocrysts are distinguished.

Our preliminary data indicates that garnet can record the ages of different metasomatic events. Garnet from harzburgitic and lherzolithic lithotypes indicate at least 30 myr of mantle metasomatism prior to kimberlite eruption. This age spread reflects distinct metasomatic events, but partial resetting of the U-Pb isotope system also has to be considered.