Moldanubian Deep crustal Metasomatism (MODEM)

In this project, the processes occurring at the boundary between the Earths crust and the mantle at a depth of about 30 to 50 kilometers will be investigated at fossil crust-mantle contacts. Such contact areas are exposed on the Earths surface in the Moldanubian zone, which is the deepest root zone of the deeply eroded paleozoic Variscan orogen in Central Europe. A Czech (Charles University Prague) Austrian (University of Vienna) research team will undertake sampling campaigns long profiles across several known contacts between crustal- and mantle lithologies. The samples will be subjected to detailed mineralogical, petrographical as well as geochemical analysis to reveal mineral content, microstructural features as well as the major and trace element chemistry using state of the art analytical techniques with a particular focus on electron microbeam analysis. It is known from previous studies that several centimetre to several 100 meter sized mantle fragments that are embedded in crustal rocks show chemical signatures deviating from the original mantle chemistry, which point towards chemical alteration by interaction with fluids or melts of crustal origin. This project aims to determine the conditions at which these fluids or melts produced the chemical alteration, or metasomatism as it is called in technical terms, to identify the underlying processes, and to reveal their temporal sequence. In this context, a relatively early impregnation of the mantle rocks with a crustal fluid, which was very likely derived from subducted continental crust, are of particular interest. Such fluid impregnation was identified from the occurrence of chlorine-, barium-, and strontium-rich as well as Thorium-oxide minerals, which are all exotic in mantle rocks. A second interesting phenomenon is related to conspicuous corrosion of garnet minerals at the lithological contacts between mantle and crustal rocks. The two phenomena allow for determining the absolute age of crust-mantle interaction by radiometric dating of Thorium-oxide and for estimating the duration of the processes underlying crust-mantle interaction by the kinetic modelling of garnet corrosion. An improved understanding of crust-mantle interaction will substantially contribute to a better reconstruction of the geodynamic evolution of the deep root of the Variscan orogen, which has been exhumed to the Earths surface only about 300 million years after its formation. An improved understanding of the deep parts of the Variscan orogen can then be transferred to younger orogens, where the root zone is still hidden in the deep crust and is amenable only indirectly via geophysical methods.

In this project, the processes occurring at the boundary between the Earth's crust and the mantle at a depth of about 30 to 50 kilometers will be investigated at fossil crust-mantle contacts. Such contact areas are exposed on the Earth's surface in the Moldanubian zone, which is the deepest root zone of the deeply eroded paleozoic Variscan orogen in Central Europe. A Czech (Charles University Prague) - Austrian (University of Vienna) research team will undertake sampling campaigns long profiles across several known contacts between crustal- and mantle lithologies. The samples will be subjected to detailed mineralogical, petrographical as well as geochemical analysis to reveal mineral content, microstructural features as well as the major and trace element chemistry using state of the art analytical techniques with a particular focus on electron microbeam analysis. It is known from previous studies that several centimetre to several 100 meter sized mantle fragments that are embedded in crustal rocks show chemical signatures deviating from the original mantle chemistry, which point towards chemical alteration by interaction with fluids or melts of crustal origin. This project aims to determine the conditions at which these fluids or melts produced the chemical alteration, or metasomatism as it is called in technical terms, to identify the underlying processes, and to reveal their temporal sequence. In this context, a relatively early impregnation of the mantle rocks with a crustal fluid, which was very likely derived from subducted continental crust, are of particular interest. Such fluid impregnation was identified from the occurrence of chlorine-, barium-, and strontium-rich as well as Thorium-oxide minerals, which are all exotic in mantle rocks. A second interesting phenomenon is related to conspicuous corrosion of garnet minerals at the lithological contacts between mantle and crustal rocks. The two phenomena allow for determining the absolute age of crust-mantle interaction by radiometric dating of Thorium-oxide and for estimating the duration of the processes underlying crust-mantle interaction by the kinetic modelling of garnet corrosion. An improved understanding of crust-mantle interaction will substantially contribute to a better reconstruction of the geodynamic evolution of the deep root of the Variscan orogen, which has been exhumed to the Earth's surface only about 300 million years after its formation. An improved understanding of the deep parts of the Variscan orogen can then be transferred to younger orogens, where the root zone is still hidden in the deep crust and is amenable only indirectly via geophysical methods.