Re-fertilizing the Earth´s mantle: crust-mantle interaction in (ultra)high pressure settings

• Applicant: Professor Dr. Silvio Ferrero
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 438201206

Understanding the chemical evolution of the lithosphere in collisional settings means to understand and quantify the exchange of material between crust and lithospheric mantle. Such exchange is a complex, two-way process: melts and fluids released from both lithospheric mantle and subducted rocks infiltrate the crust, causing anatexis, element redistribution, and thus crustal differentiation, while at the same time, subducted crustal material re-fertilizes the depleted mantle, renewing its capability for production of basaltic magmas. Crust-mantle interaction processes can be directly investigated in orogenic peridotites, portions of lithospheric mantle embedded in collisional orogens. Orogenic peridotites clearly show evidence of mantle heterogeneity, reflected by the presence of centimetric to metric bodies of pyroxenite and eclogite. Experimental studies suggest that mantle heterogeneity contributes substantially to the variability in composition of basalts, both MORB and OIB. Hence, understanding melt-related processes involving pyroxenite and eclogite is the most direct way to unravel the link between crust-mantle interaction, collisional settings, and mantle-related magmatism.Preserved glassy and crystallized melt inclusions (MI) were recently discovered for the first time in pyroxenites in orogenic peridotites and in eclogites in diamond-bearing metasediments of the Bohemian Massif. The melt in the pyroxenites has granitoid composition, with a clear subduction-related trace element signature, whereas preliminary data on the eclogites show a granitic melt generated at depth >100 km. However, mantle (peridotite) melting is expected to generate basalts, whereas eclogite melting mainly produces melts in the Trondhejmite-Tonalite-Granodiorite (TTG) series. Thus the presence of a granitoid melt during peak metamorphism at mantle depth in these rock types is completely unexpected and, to date, unexplained. Our project aims to solve this enigma with a multidisciplinary approach integrating MI studies in pyroxenites and eclogites, experimental petrology, phase equilibria modeling, and isotopic studies. We will identify melt-producing reactions and source of melts, and describe how the physicochemical parameters at the source region influence melt chemistry. Isotope studies will provide solid constraints to define the timeframe in which these melt-related processes occurred, as well as on the chemical affinity of the melt. We will define which chemical tracers can be used to identify and quantify the different contributions from mantle and crust to magmatism in collisional settings. In conclusion, our ambitious project will merge a wealth of novel geochemical data from natural MI in pyroxenites and eclogites with results from experiments and thermodynamic calculations to clarify lithosphere evolution in collisional settings, providing a complete portrait of how crust-mantle interaction processes shape the orogenic roots of the continental crust.

• DFG Programme: Research Grants