Nanogranites in migmatites: deciphering the chemical differentiation in the continental crust

• Applicant: Professor Dr. Patrick J. O' Brien, since 11/2021
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2015 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 279014837

Chemical differentiation of the crust influences the evolution of our planet, it fuels the technological evolution of humankind by re-concentrating elements in ore deposits and it shapes the atmosphere by releasing volatiles. Such re-distribution process begins at depth when the continental crust melts, and the investigation of such deep melts is thus key to understand differentiation. Natural rocks which experienced melting, i.e. migmatites, still contain melt as micrometric inclusions of primary nature. Such melt inclusions (MI) -also called nanogranitoids- were proven in the last decade to represent the most reliable and robust tool to gauge the composition and evolution of melt at depth. In the last three year, the applicant jump-started and extensively developed the investigation of melting and melt-related processes in mafic/ultramafic systems by characterizing in detail two case studies of MI from melting of amphibolites and metagabbros. Moreover, within pyroxenites and eclogites the applicant and his collaborators also discovered the first "metasomatic" nanogranitoids, i.e. containing metasomatic melt originated outside the rock in which they can be currently found. Moreover, the enlargement of the MI database has clearly shown that the metastable polymorphs of quartz and plagioclase are far more common than originally expected, and often they coexist with new phases never reported before -all discoveries warranting more investigations.The renewal of the present project will allow the applicant to further our understanding of chemical differentiation by combining for the first time two powerful petrogenetic tools: nanogranitoids and zircon studies. Zircon is a robust geochronometer, a versatile petrogenetic tool to gauge the physicochemical conditions characteristic of deep processes, and unsurprisingly a very reliable inclusion container. The applicant will combine the information obtained via petrology of metamorphic zircon with a detailed study of the melt preserved in it as MI by means of nanogranitoids re-homogenization and in situ analyses studies – in terms of major and trace elements, volatiles and isotopes. A pilot experiment was successful in re-melting former nanogranitoids in zircon, paving the way for the present project. Such combined approach has the potential to produce significant advancements in our knowledge on deep melt-related processes, and it is at present an entirely unexplored asset in metamorphic petrology. Moreover, part of the project will be dedicated to investigate the crystallization prodcuts of the melt confined in inclusions, in particular synthesizing the polymorphs and characterizing the new phases recently identified.Overall, the present project will lead to take a significant leap forward toward a better understanding of how crustal differentiation at depth starts and evolve through time and how deep melts behave on crystallization.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Professor Dr. Silvio Ferrero, until 10/2021