Diffusion in common mineral phases. Part 2: plagioclase

The major goal of this project is the determination of diffusion coefficients for the coupled exchange of Na+ and Si4+ against Ca2+ and Al3+ (NaSi-CaAl interdiffusion) in plagioclase feldspars and its coupling with trace element diffusion rates. Both, major element diffusion and trace element diffusion have high potential to be used as diffusion chronometer, provided good experimental calibrations are available. Currently available data have high uncertainty due to various experimental and analytical shortcomings. Our new approach is a modification of the classical diffusion couple set up. A thin film of amorphous plagioclase compositions, doped on the 0.1 wt% level with various trace elements ((i.e. K, Li, Mg, Fe, Sr, Ba), is deposited on the polished surface of an oriented plagioclase single crystal using pulsed laser deposition. The layer rapidly crystallizes at the beginning of the diffusion anneal. Pairs of plagioclases with different compositions will be used to investigate the role of anorthite content on cation diffusion, with particular focus on high An contents. Most of the experiments will be performed in internally heated gas pressure vessels which allow pressure variation in the range 10 - 700 MPa. For comparison some experiments will be performed in a gas mixing furnace at ambient pressure. Temperature is limited by the stability of plagioclase, i.e. possible melting reactions. In addition to temperature, pressure and composition of plagioclase, water fugacity (fH2O) and oxygen fugacity (fO2) are parameters which may affect diffusion in plagioclase and, hence, will be systematically varied in the experiments. Oxygen fugacity is controlled in high pressure experiments using argon/hydrogen mixtures as pressure medium. To adjust and vary water fugacity without having a fluid phase in the experimental charge, we will add silicic melts with water contents below the saturation level. After diffusion anneals, concentration profiles will be measured from the thin-film-side using secondary ion mass spectrometry (TOF-SIMS), laser ablation combined with mass spectrometry as well as focused ion beam techniques (FIB-SEM) and electron microprobe. Diffusion coefficients are extracted by fitting the profiles to the appropriate diffusion equations. The quantification of the diffusion rates as a function of composition (major and trace elements) and P, T, fO2, and fH2O is a pre-requisite to develop a quantitative point defect model, which should allow reliable extrapolation of the diffusion laws beyond the experimental parameter space. Application of these data will be tested in case studies in other projects of the researcher group.