Na-K diffusion in alkali feldspar: interdiffusion model

The project Na-K diffusion in alkali feldspar: interdiffusion model deals with the motion of Na and K ions in the crystal structure of alkali feldspar. With 50 volume percent feldspar is the most abundant mineral in the Earths crust. Its crystal structure is comprised of an aluminum-silicon-oxygen framework with cavities, which are occupied by alkali and alkali earth cations, primarily Na, K, and Ca-ions. In alkali feldspar only the monovalent Na and K ions are present, which are mobile at the high temperatures prevailing in the Earths crust. They may thus be effectively re-distributed in the crystal by diffusion, they may also be removed from or introduced into the crystal. The chemical composition a feldspar has obtained at a certain set of pressure-temperature conditions tends to re- equilibrate under changing environmental conditions, and the memory of the feldspar for earlier stages and thus for the conditions of rock formation may be changed or entirely lost. In this context, the interdiffusion of Na and K in alkali feldspar is the most important process. Only when the temperature- and pressure dependence of the interdiffusion rate is known, conditions of rock formation and possibly their evolution in time may be determined from the chemical patterns observed in feldspar. In the literature there is considerable discrepancy between direct experimental determinations of Na-K interdiffusion rates in alkali feldspar and calculations that are based on theoretical interdiffusion models and independently determined rates of Na and K self-diffusion. It is hypothesized that this is due to inadequate quality of Na- and K tracer diffusion coefficients previously determined with out-dated methods, with which it was difficult to unravel underlying diffusion mechanisms. Moreover, due to the composition dependence of the lattice parameters, Na- K interdiffusion induces mechanical stress, which feeds back into diffusion via thermodynamic effects and lattice distortion. Neither the effect of different diffusion vehicles nor the feedback between self-induced mechanical stress and diffusion has been accounted for in interdiffusion models for alkali feldspar so far. With the intended project new data on the self-diffusion will be generated by doing dedicated tracer diffusion experiments and with specific 22Na diffusion experiments on previously applied sharp Na-K composition gradients the feedback between mechanical stress and diffusion will be calibrated. The experimental results will then be used to develop a comprehensive interdiffusion model, which accounts for different diffusion mechanisms and for the feedback between self-induced mechanical stress and diffusion. This broad, combined experimental and theoretical approach has not been applied to alkali feldspar so far and to the knowledge of the proponent is novel in mineralogy. The expected outcomes will yield substantial improvement in the kinetic modelling of mineral reactions involving alkali feldspar. Due to its ubiquitous occurrence in the Earths crust and the considerable level of knowledge about basic thermodynamic and mechanical properties alkali feldspar is best suited for this project.

In the frame of this project the diffusion of Na and K in alkali feldspar was addressed using a combined experimental and theoretical approach. Feldspar is the most abundant mineral group in the Earth's crust contributing about 50% of its volume. The chemical and microstructural signatures engraved in alkali feldspar, a prominent subgroup of the feldspar minerals, are an important source of information for reconstructing the minerals formation conditions and geological history. Estimates of formation pressure-temperature as well as cooling rates provide important constraints on geological processes occurring at depth over geological times. In this context, the diffusion of Na and K, which are the majority species on the alkali sublattice of the alkali felspar, is of crucial importance. It determines at what rate the Na/K proportions can be adjusted to different pressure temperature conditions, whether and over what time scales intracrystalline microstructures such as exsolution lamella can develop, and how robust the "memory" of the feldspar, regarding its geological history is. In the frame of this project, a new type of combined chemical diffusion and tracer diffusion experiments was developed, where isotopically labelled KCl salt with 95% 41K 5% 39K was exchanged with natural gem-quality alkali feldspar with the isotope abundances 6.7% 41K 93.3% 39K to obtain combined Na, 41K, and 39K concentration profiles. The exchange experiments were performed at temperatures between 800C and 1000C, which are relevant for geological processes at depth, and (isotope) chemical analysis was done using secondary ion mass spectrometry, a method that can quantify element and isotope ratios at high precision and at a few nanometer spatial resolution. This is necessary, because the profiles that can be attained with realistic run durations in the laboratory are only a few micrometers long. The approach is to capture the slow intracrystalline diffusion, which occurs over geological times in nature, within the laboratory time scale by miniaturizing the experimental system. A new multicomponent diffusion model for the diffusion of ionic species in ionic crystals was developed to obtain satisfactory fits of the combined chemical and isotopic diffusion profiles. The new model accounts for the classical vacancies mediated mechanism and for direction cation exchange without involvement of vacancies. The model is completely general and applicable to diffusion in any ionic crystals. Apart from this central result, insights into the coupling between Na-K diffusion and lattice strain could be obtained from dedicated fracturing experiments, where the propagation of cracks was controlled by Na-K interdiffusion. Finally, exsolution experiments were performed to quantify the excess energy that is introduced by newly formed concentration gradients by exsolution.