Diffusion-driven Fe-Mg and Li isotope fractionation in olivine: An experimental investigation and new modeling approach
A major challenge of diffusion chronometry is to distinguish between compositional zoning produced by diffusion from that produced by other igneous processes such as crystal growth at changing melt compositions, e.g. during fractional crystallisation or due to magma mixing. In any case, initial and boundary conditions of diffusion models are assumed based on petrographic inferences, and these introduce an inherent uncertainty in the modeled timescales (e.g., Costa et al. 2008). As a result of these, a good quality of the model fit does not necessarily guarantee the correctness or the accuracy of the associated time scale and it is desirable to develop quantitative tools that would allow diffusion profiles to be identified and appropriate initial and boundary conditions to be defined.
While various tools such as the use of diffusion anisotropy in minerals or the shapes of profiles of elements that diffuse much slower than the ones being modelled have been used, a recent novel approach to identify diffusion profiles as well as to place constraints on the initial and boundary conditions under which diffusion occurred in natural samples has been through the use of stable isotope fractionation induced by chemical diffusion processes. This fractionation is much larger, and often in a different direction, than those induced by equilibrium fractionation. Presence of distinctly coupled chemical and isotopic zoning provide strong criteria for clearly identifying diffusion-related processes.
Recent advances in femtosecond laser ablation systems and multicollector-ICP-mass spectrometers enable high-precision spatially resolved analyses of the isotope composition of metals, such as Mg and Fe (Oeser et al., 2014). The combined information recorded in the chemical and isotope zoning allows to better constrain the diffusion model and improves significantly the accuracy of the derived time scales.
This project aims to generate more experimental data on isotopic fractionation caused by diffusion at different conditions in order to better constrain theoretical models and develop more robust diffusion chronometers.
Investigators
Martin Oeser, Leibniz Universitaet Hannover
Stefan Weyer, Leibniz Universitaet Hannover
Ralf Dohmen, Ruhr-Universitaet Bochum
References
Costa F, Dohmen R, Chakraborty S (2008) Time Scales of Magmatic Processes from Modeling the Zoning Patterns of Crystals. Reviews in Mineralogy and Geochemistry 69:545–594. https://doi.org/10.2138/rmg.2008.69.14
Oeser M, Weyer S, Horn I, Schuth S (2014) High-Precision Fe and Mg Isotope Ratios of Silicate Reference Glasses Determined In Situ by Femtosecond LA-MC-ICP-MS and by Solution Nebulisation MC-ICP-MS. Geostand Geoanal Res 38:311–328. https://doi.org/10.1111/j.1751-908X.2014.00288.x