Dating mantle metasomatism: LA‒MC‒ICPMS U‒Pb dating of garnet xenocrysts from the V. Grib kimberlite, Russia

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Mantle metasomatism largely controls the composition, and thus the physical and chemical properties of the lithospheric mantle. It thereby exerts control over the petrogenesis of alkaline (e.g., kimberlitic and carbonatitic) within-plate magmas, diamond formation, and the generation of, e.g., rare-earth element deposits. Over time, compositionally diverse metasomatic fluids repeatedly overprint variably depleted original mantle wall-rocks. This produces a complex, essentially ubiquitously metasomatised lithospheric mantle, heterogeneous on scales of microns to terranes. Decoding this complex record by identifying significant episodes and processes is essential for reconstructing lithosphere evolution, the nature and origin of Earth’s volatile flux, and the link between deep lithospheric and crustal processes.Garnet is the most widely used phase for characterizing metasomatic events in the subcontinental lithospheric mantle, comprising a large proportion of thick cratonic lithosphere up to ~250 km depth. For age-dating studies, however, garnet has so far only been employed in tedious, costly and time-intensive conventional isotope dating techniques. Very recent advances in analytical and data reduction techniques at the Frankfurt Isotope and Element Research Center (FIERCE) make low-U garnet accessible to in situ age dating for the first time, allowing not only fast through-put but also the detection of mineral-scale heterogeneity. In this project, in situ LA-MC-ICPMS U‒Pb dating will be applied to a set of well-characterized garnet xenocrysts from the V. Grib kimberlite to investigate a temporal and causal link between specific types of mantle metasomatism and kimberlite and diamond formation. Garnet xenocrysts from the V. Grib kimberlite sample the lithospheric column to a depth of ~220 km and provide evidence for silicate and carbonatitic metasomatism. The results will also provide insights into the lithosphere evolution underneath the Baltic shield by linking deep lithospheric to well-dated crustal processes.