Completing the detrital record of clastic sediments by in situ Rb/Sr dating of mica

• Applicant: Dr. Martin Kutzschbach
• Subject Area: Geology ; Mineralogy, Petrology and Geochemistry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 538765622

Clastic sediments are powerful sentinels of Earth´s history. They contain records of mountain-building processes, plate movements, and phases of profound climatic change. To ‘read’ these environmental archives, one must identify the rocks from which they were formed. An established approach is to obtain a sedimentary age spectrum by dating individual detrital mineral grains. Several geochronometers have been applied for this purpose, e.g., U/Pb in zircon and rutile and 40Ar/39Ar in mica. These, however, have several drawbacks. Zircon only forms at temperatures above 500°C and preferably in intermediate to felsic magmas. Hence, it not appears in many source rocks. Collecting data for a single 40Ar/39Ar age is very time and labor intensive, which severely limits the sample throughput. Consequently, gaps will arise in the sedimentary age spectra. Within the framework of the proposed project, these gaps are to be closed for the first time using Rb/Sr dating of detrital mica by utilizing the recent development of reaction cell tandem plasma mass spectrometry (ICP-MS/MS). To achieve that goal, we will benchmark this novel technique against established provenance tools (40Ar/39Ar in mica and U/Pb in zircon). This is done by sampling modern sediments deposited in the river Rhine and the river Danube. In the second part we will use that knowledge to study complex palaeosedimentary systems (Carboniferous sediments in the vicinity of the Münchberg Massif and drill core samples from the ICDP COSC drilling project in Western Sweden). For dating of detrital mica a small amount of material is ablated from the mineral surface using a laser to carry out Rb/Sr isotope analyses at high spatial resolution, i.e. in situ. Mica is characterized by high Rb/Sr ratios, and thus Rb/Sr (model) ages of individual mica grains can be determined quickly - analogous to U/Pb dating of zircons. This new, revolutionary in situ Rb/Sr methodology will be applied for the first time in the context of provenance analysis. To increase the dating accuracy, novel matrix matched reference materials will be developed and made available to the entire (geo)scientific community. In addition to single spot dating, Rb/Sr age maps will be acquired using shallow ablation with small laser beam sizes. This allows dating of even very thin mica platelets and provides insights into age resetting mechanisms during secondary processes at the single grain scale, which is another valuable source of provenance information. Given mica´s abundant occurrence in both source rocks and sedimentary sinks, the fast acquisition of Rb/Sr data by LA-ICP-MS/MS, and mica´s unique hydrodynamic behavior, Rb/Sr dating of detrital mica is considered a key complementary tool for sedimentary provenance studies. It has an enormous potential to grow into an efficient standard method and will contribute to a better understanding of the processes that shape our planet.

• DFG Programme: Research Grants
• International Connection: Poland, Switzerland
• Cooperation Partners: Professorin Dr. Iwona Klonowska; Professor Dr. Richard Spikings
• Co-Investigators: Privatdozent Dr. Johannes Glodny; Professor Dr. Wolfgang Müller