The speed of intracontinental subduction

Funded by the Austrian Science Fund (FWF) (2021-2023; FWF Project Nr. M2899-N)

PI (Lise-Meitner Fellow): Dr. Kathrin Fassmer

Host: Univ-Prof. Dr. Bernhard Fügenschuh

Garnet is a characteristic mineral which is formed during rock metamorphism. During this process, rocks are transformed under high pressures and temperatures and new minerals are formed. Garnet is formed in the course of increasing rock metamorphism by burying rocks into the earth's crust (subduction zones). Through tectonic processes, this now garnet-bearing rock then returns to the earth's surface, where it can be found in mountains today. For this reason, the chemical composition of garnet is a kind of "memory" for the pressure-temperature conditions and the duration of garnet growth. By means of analytical methods, the geological history of garnet growth as a function of time can be precisely reconstructed.

In this project, the duration of garnet growth will be determined by dating different parts (core to rim) of individual garnet crystals. The age of garnet growth can be determined very precisely by isotope dating. This is based on the fact that below a certain temperature there is no element exchange between the minerals of a rock (closure temperature). The chemical composition of the garnet at the time of falling below this temperature is thus "frozen" and a chemically closed system is created in which radioactive isotopes of lutetium and samarium decay to hafnium and neodymium over time. The date of the closure of this system is therefore dated. The closure temperature of garnet is close to the maximum temperature of metamorphism of the studied rocks, which allows to date the garnet growth from the beginning of increasing metamorphism to the peak of metamorphism.

The investigated rocksamples were taken from Tyrol (Ötztal, Schneeberg complex) and Carinthia (Radenthein, Radenthein complex). These rocks contain large garnet crystals of up to 10 centimetres in diameter, which makes them particularly suitable for dating, as the individual crystal parts can be separated more easily and precisely. This also affects the accuracy of the dating. In addition to garnet dating, rutiles are also dated. Information about the metamorphic history of the rock can also be drawn from the chemical analysis of rutile, since (1) the formation temperature of rutile can be deduced by means of trace element thermometry, and (2) the closure temperature of the rutile is lower than that of the garnet, and therefore the beginning of the rock's uplift can be dated. This makes it possible to date both sinking and re-rising in a single sample. In addition to the dating, the pressure and temperature conditions of metamorphism are modelled and thus different parts of the pressure-temperature path can be assigned to absolute dates. We hope that this will contribute to the understanding of the duration of mountain building in the Eastern Alps.

Hannah Pomella (Department of Geology, University Innsbruck)
Peter Tropper (Department of Mineralogy and Petrography, University of Innsbruck)
Gerald Degenhart (Radiology, University of Innsbruck)
Christoph Hauzenberger (Institute for Earth Sciences, University Graz)
Carsten Münker (Institute for Geology and Mineralogy, University Cologne)
Axel K. Schmitt (Institute for Geosciences, University Heidelberg)

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