P-T-t-X EVOLUTION OF THE SIBERIAN SUB-CRATONIC LITHOSPHERE

The curiosity about the structure and origin of Earth has always accompanied humankind. However, in contrast to the great achievements obtained during the exploration of Earths surface, the deepest portions of our planet are still largely precluded from observation. As the greatest depth reached by scientific drilling is ~12 km, negligible when compared to the Earth radius (~6371 km), the only way to explore the interior of our planet is by making use of indirect seismic and/or petrological evidence. Most of the insights into the deepest realms of Earth come from fragments of mantle rocks (xenoliths) and diamonds brought to the surface by kimberlitic magmas in cratonic areas. Cratons are the oldest (>2.5 Ga) and most stable regions of continents, where the lithosphere is >250 km thick. The composition and microstructure of diamonds and minerals derived from the mantle transition zone (400-650 km) provide evidence of repeated events of deformation and melt/fluid-rock interactions (metasomatism), suggesting that the Earths mantle is a highly dynamic system. Numerous models were proposed regarding the genesis of cratonic roots, their composition, T-P conditions and onset of deformation and metasomatic reactions in the deep lithosphere as well as the genesis of kimberlitic magmas themselves. Despite substantial achievements, the picture of the lithospheric mantle beneath a craton is still far from fully understood, as its primary signature is usually overprinted by repeated metasomatic events associated with the infiltration of kimberlite-like melts. Furthermore, the limited representativeness of mantle xenoliths, which represent only small portions of the lithosphere, and their widespread alteration make the study of the deep Earth very intricate. Due to their relatively low degree of alteration and lithological variability, the mantle xenoliths brought to the surface by kimberlitic pipes in the Siberian craton are excellent materials for investigating the nature and evolution of the deep lithosphere. In the present project, the study of a unique collection of fresh granular to highly deformed and recrystallized peridotitic xenoliths sampled from different lithospheric depths by two neighbouring kimberlitic pipes (Udachnaya and Sytykanskaya - Yakutia) will enable us to reconstruct the complete pressure-temperature-time-composition (P-T-t-X) path experienced by the mantle beneath Siberia. State-of-the-art high-resolution microstructural, crystal orientation, petrological and geochemical analyses will be combined with thermodynamic, kinetic and deformation/recrystallization modeling to unlock the sequence of metasomatic and deformation events that took place along an entire sub-cratonic lithospheric column. Such a multidisciplinary approach will deepen our understanding of the origin and evolution of Earths mantle and the main processes exerting a key control on global scale tectonics, which ultimately shapes the habitable surface of our planet. 1

The curiosity about the structure and origin of Earth has always accompanied humankind. However, the deepest portions of our planet are largely precluded from observation, and the only way to explore its interior is making use of indirect geophysical or petrological evidence. Most of the insights into the deepest and oldest realms of Earth come from fragments of mantle rocks ("xenoliths") and diamonds brought to the surface by kimberlitic magmas in cratonic areas, i.e. the oldest and most stable regions of continents, where the lithosphere can be >250 km thick. Numerous models were proposed to understand the genesis of kimberlitic magmas, as well as the nature and evolution of the whole lithospheric mantle, its temperature and pressure conditions, and all the events that modified its structure (deformation) or composition (chemical reactions, i.e. metasomatism). However, the mantle xenoliths are repeatedly overprinted by the infiltration of kimberlite melts, but at the same time the melts themselves evolve due to interaction with the mantle they cross during ascent. This causes the lack a primary signature of both melts and the mantle, hindring the identification of a "starting point" for the models. In the Lise Meitner project M-3080-N, we investigated a unique collection of fresh granular to highly deformed and recrystallized mantle peridotite xenoliths, equilibrated at different depths in the Siberian sub-cratonic lithosphere and brought to the surface by kimberlites. We studied the composition and texture of minerals and rocks, combining state-of-the-art high-resolution microstructural, petrological and geochemical analyses with thermodynamic, kinetic and deformation/recrystallization modelling. This enabled us to: - Reconstruct the compositional evolution of the kimberlite melts, from their genesis to their ascent through (and reaction with) the lithospheric mantle and ultimately eruption. The result of this study is the first and complete model applicable to kimberlites worldwide. - Model the timescales of ascent of kimberlite melts, from their interaction with the lithospheric mantle to eruption and cooling. This was done through chronometry studies based on the diffusion of Fe and Mg within single olivine crystals. - Reconstruct the pressure-temperature-time-composition (P-T-t-X) log of the Siberian lithospheric mantle. This was achieved combining the texture of mantle xenoliths to the chemistry of the mineral phase constituents. - Discriminate the metasomatic reactions taking place in the lithospheric mantle prior to the transport of the xenoliths to the surface from the chemical changes induced by the infiltration of kimberlite melts during xenolith transport. All these results contribute to deepen our understanding of the origin and evolution of Earth's mantle and the main processes exerting a key control on global scale tectonics, which ultimately shapes the habitable surface of our planet.