Senckenberg Research

Dating the Brocken (Harz Mountains)

 Brocken granite and antenna on the summit.
Brocken granite and antenna on the summit.

For many geoscientific questions it is important to know how old rocks are. A team of researchers led by Senckenberg scientists was able to show that the rock that forms the peak of the Brocken, the highest Peak of the Harz Mountains in northern Germany is about 293 million years old. Based on this age, some far-reaching geological conclusions can be drawn.

The Senckenberg Natural History Collections in Dresden are equipped with a laboratory for determining rock ages. This laboratory is located at the Geochronology Section of the Museum of Mineralogy and Geology, which is one of the two departments at Senckenberg in Dresden. The determination of the age of rock samples provides basic information that is required for the execution of most geoscientific research projects.

For instance, to provide more precise answers to questions involving the formation of the base mountains of Central Europe (a process known as the ‘Variscan orogeny’), it proved necessary to apply modern methods to the task of estimating the age of the highest part of the Harz Mountains, the Brocken. In addition to its geological importance, the Brocken has great symbolic significance for the division and reunification of Germany, as the Inner-German frontier ran across it.

The project team engaged in determining the age of the rock, from the sampling to the final results, was made up of staff from the Senckenberg Naturhistorische Sammlungen Dresden, the Harz National Park and the Free State of Saxony Agency for the Environment, Agriculture and Geology (LfULG Sachsen).

The team of the Geochronology Section (Senckenberg Dresden) and co-workers after sampling the Brocken granite.
The team of the Geochronology Section (Senckenberg Dresden)
and co-workers after sampling the Brocken granite (from left to right):
Ulf Linnemann (Senckenberg Dresden), Anja Sagawe (Senckenberg
Dresden), Maren Zweig (LfULG Saxony), Friedhart Knolle
(Harz National Park), Andreas Gärtner (Senckenberg Dresden),
Nadine Richardt (Senckenberg Dresden), Sindy Becker
 (Senckenberg Dresden), Mandy Hofmann (Senckenberg Dresden).

The age was determined using the mineral zircon (ZrSiO4) that is commonly found in the Brocken granite of the summit. However, the microcrystals are only 200 to 250 micrometers in size. Following a complex preparation process, the uranium and lead isotopes in the zircon crystals required for the age determination were analysed using a laser connected to a mass spectrometer with inductively coupled plasma (LA-IC P-MS*).

On the basis of the uranium and lead isotopes of the zircon crystals, the age determined for the Brocken summit rock was 293 ± 3 million years. This means that the intrusion of the Brocken granite can be associated with the Lower Permian (Sakmarian) epoch.

Based on the age of the Brocken rock, some far-reaching geological conclusions can be drawn. The Variscan Mountains eventually resulted from the collision of the two supercontinents Euramerica and Gondwana as well as several microcontinents during the Devonian and Mississippian periods. This collision created the new supercontinent of Pangaea. However, the Brocken granite was formed much later, during the Lower Permian epoch. During this period, a large part of the Variscan Mountains was worn away by surface processes. In view of the colour of the sediments, the resulting debris and accompanying volcanics are collectively referred to as the ‘Rotliegend’ lithostratigraphic unit (the ‘underlying red’ sequence of rock strata). The formation of the Brocken granite during the Lower Permian was caused by subterranean processes. These include the thinning of the earth’s crust as well as the deformation of the Variscan mountains following the collision of the continents.

 

View of the geochronological laboratory of the Senckenberg Natural History Collections Dresden (Museum of Mineralogy and Geology).View of the geochronological laboratory of the Senckenberg Natural History Collections Dresden (Museum of Mineralogy and Geology). The uranium-lead age of the Brocken granite was determined here. On the left: a New Wave excimer laser (UP 193). The Fisher Scientific ICP MS* (ELEMENT 2 XR) is on the right hand side. (* Inductively Coupled Plasma Mass Spectronometry).

 

Image of a polished zircon crystalImage of a polished zircon crystal in cathodoluminescence mode obtained using a scanning electron microscope. The zonation (growth pattern) of the crystal is clearly visible. The length of the crystal is about 180 μm.

 

Image of a zircon crystal from the Brocken granite.Image of a zircon crystal from the Brocken granite. Both craters were induced by the laser ablation required for the age dating. The length of the crystal is about 250 μm. The crystallization age of 293 million years was calculated from the proportions of uranium and lead isotopes. This age is considered to be the age at which the Brocken granite was formed. The isotope ratios of 206Pb / 238U and 207Pb / 235U of 30 zircon grains were used.

 

 

Author

Prof. Dr. Ulf Linnemann

Geologist Prof. Dr. Ulf Linnemann is Managing Director of the Senckenberg Natural History Museum in Dresden and Head of the Geochronology Section. He also holds an honorary professorship at the Technische Universität Dresden, where he lectures in geoscience.

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