Diffusion-driven isotope fractionation in feldspars, chain silicates and oxides

• Applicants: Dr. Martin Oeser-Rabe; Professor Stefan Weyer, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 405665352

This project aims to quantify the diffusion-driven fractionation of metal stable isotopes (Fe-Mg-Li) in magmatic mineral phases, that have only rarely been investigated for diffusion-generated metal isotope effects by in-situ techniques. As a first objective, Li diffusion and the associated Li isotope fraction in feldspars will be investigated. The fast diffusion of Li in plagioclase has been used in previous studies to determine the duration of short-lived geologic processes through diffusion chronometry. However, the magnitude of the expectable diffusion-driven Li isotope fractionation has not been calibrated experimentally yet, hampering the promising approach of combining chemical and isotopic diffusion profiles to constrain timescales. For K-feldspar, Li diffusion rates and Li isotope fractionation factors are unknown. The same holds true for Li diffusion in amphibole. Thus, we want to close these knowledge gaps by conducting properly designed diffusion experiments and performing high-precision in-situ analyses of Li contents and Li isotope ratios on K-feldspar and plagioclase, using laser ablation plasma mass spectrometry. Chemically well-characterized plagioclase crystals are already available from experimental runs during the 1st funding period of this Research Unit. In the second part of this project, we aim at examining the extent of diffusion-driven Fe-Mg isotope fractionation in pyroxenes, amphiboles and spinel group minerals (SGMs), and of Li isotope fractionation in amphibole. Large Li-, Mg- and Fe isotope variations have been observed for magmatic pyroxenes, amphiboles and SGMs, but due to the lack of an experimental quantification of diffusion-driven isotope fractionation in these minerals, the interpretation of such variations remains challenging. Since Fe-Mg diffusion rates in pyroxenes, amphiboles and SGMs are the investigation targets of Projects #1 and #2 within this Research Unit, the determination of diffusion-generated Fe-Mg isotope fractionation for these mineral phases will result in a strong interaction among the three projects. High-precision in-situ analyses of Fe- and Mg isotopic compositions will be conducted using laser ablation plasma mass spectrometry with a recently developed depth profiling technique which provides a vertical spatial resolution of ~1 µm. Finally, the obtained metal isotope fractionation factors will also help to characterize the diffusion mechanism of the element of interest (Fe-Mg-Li) within the target mineral (feldspar, pyroxene, amphibole, spinel), and eventually will result in a deeper understanding of isotopic fractionation during diffusion in minerals in general.

• DFG Programme: Research Units
• Subproject of: FOR 2881: Diffusion Chronometry of magmatic systems