Lead sticks in the monazite crystal

How to determine the age and development history of granite and shale?

Recording of monazite crystals (Mnz) in granite-gneiss-rock with the polarizing microscope. The radiation emitted by uranium and thorium has created defects in the crystal lattice in the surrounding mica (Bt, Ms), which are visible as dark hems. © Chair of Mineralogy / University of Erlangen-Nuremberg
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Rocks tell their story - if you can read them. However, tools are needed that are far sharper than the eyes. The age and development history of granite and shale on the mineral monazite contained therein are currently being reconstructed by mineralogists from the University of Erlangen-Nuremberg and the TU Bergakademie Freiberg.

The chemical analysis uses an electron-beam microprobe which excites elements in a rock sample with a particularly strong electron beam to emit X-ray radiation and thereby detects even very small amounts of monazite.

"Affectionate" lead

Monazite has an extraordinary property that is very useful for geoscientists: if lead has formed in the crystal lattice, it will stick there even under extreme conditions, such as temperatures of up to 800 ° C. This "attachment" is all the more remarkable as monazite crystals in their early stages are extremely repellent to the element lead. But the mineral absorbs up to 15 percent of its weight in thorium. In addition, up to 1.5 percent uranium. In geological periods of millions of years, the radioactive elements of thorium and uranium finally break down into lead.

From this unique combination of properties opens the possibility of a geological age determination with the help of the electron microprobe. Introduced in the 1970s, the device enables rapid and micrometer-scale investigations of the structure of solid materials composed of many different elements.

It is mainly used in the investigation of minerals and crystals, the constituents of rocks and numerous geomaterials. However, when analyzing lead in monazite, the microprobe encounters its methodological limitations. The lead content is usually less than one percent of the weight and can therefore only be identified if the monazite is older than 100 million years - enough time to allow relatively high levels of radioactive decay to develop. display

Optimal technical equipment

With a very strong electron beam, however, it is also possible to stimulate monazite of recent time so far to the emission of X-rays, that the very small amounts of lead are to be discovered. The Electron Beam Microprobe at the Department of Mineralogy of the University of Erlangen-N rnberg offers optimum technical equipment for this purpose. In 2006 and 2007, the German Research Foundation (DFG) funded a project to monazit age dating of garnet mica schists from the French Massif Central. It was possible to set up a measuring routine, which is now available for further analysis of monazitic rocks.

Many dozens of rock thorns from other crystalline rock areas - the Alps, the Antarctic, the Armorican Massif in France, the Granite Belt in Cameroon and northwest India - have since been mined by Professor Matthias G bbels of The University of Erlangen-N rnberg and Professor Bernhard Schulz from the TU Bergakademie Freiberg / Saxony to Monazit searched and made many thousands of dating analyzes.

A method with many advantages

The advantages of this method were evident. The age determination of monazite with the electron microprobe is restricted to granitic rocks and mica schist. However, the time and cost required for this method is only about a fifth compared to mass spectrometry.

The measured additional elements in the monazite crystal also provide important additional information about the physicochemical formation conditions of the monazite in the rock transformation. In addition, several thermal and tectonic events with monazite formation can be detected in a crystalline area, and sometimes even in individual samples or grains.

By contrast, other age dating techniques only ever display the most recent thermal event. Because of the multiple metamorphosis in many areas - for example in the Alps - this is a big advantage of monazite dating.

(idw - University of Erlangen-N rnberg, 24.06.2008 - DLO)