There are 2 PhD and one post-doctoral position available at the University of Lausanne, funded
by a Swiss National Science foundation. The goal is to develop quantitative models of crystal
growth in metamorphic and igneous systems. Top of the line analytical methods (FEG-SEM,
FEG-EMPA, μC-Xray Tomography, SIMS, NanoSIMS) will be used to quantify zoning of
minerals and the zoning around minerals in experimental on porphyroblast growth of igneous
minerals and natural examples of metamorphic porphyroblasts. The amount of local
disequilibrium will be assessed, and quantitative 2 and potentially 3-D models for crystal
growth will be developed. It is expected that one PhD student will focus on the metamorphic
study, while the second PhD will focus on igneous and experimental studies. Finally, the
modelling study will be mostly achieved by the post-doctoral fellow, but PhD students are
expected to contribute to the modelling. More details can be found below.
Applications, including contacts for references and a short motivation statement should be sent
to Lukas Baumgartner (Lukas.baumgartner@nullunil.ch). Evaluation of dossiers will start by
October 14th, but applications will be accepted until filled.
Growth models for igneous and and metamorphic minerals: quartz and garnet
This project proposes to use continuum mechanics modelling to understand geochemical and textural aspects of igneous and hydrothermal vein quartz using diffusion-surface reaction models. In these environments, quartz is growing from oversaturated conditions into a structureless continuum. In a first study, I propose to experimentally grow quartz in a rhyolitic melt containing trace elements typical for rhyolite environments. Growth will be induced by changing the saturation of quartz in these melts by changes of water activity, pressure, and temperature in cold seal experiments. Water activity will be changed due to water loss induced by hydrogen gradients between the capsule and the pressure media. Temperature and pressure will be decreased monotonically and cycled to induce polybaric/thermal crystal growth. Rapid quenched capsules will be opened and cut after careful μC-X-ray tomography to locate quartz crystals. Melt halos will be analyzed using low voltage FG-EMPA for major and minor elements and SIMS for trace element composition gradients. SIMS analysis of water and selected cations (Na, Li, Al, H, P) will be performed in the quartz crystals using the newly acquired RF-Hyperion source on the SIMS and NanoSIMS. We will develop a multi element diffusion-reaction kinetics code using initially MATLAB, which will be translated for more complex simulations to graphic cards CPU optimized parallel code using a full Gibbs Free energy minimization.
The second project will explore the conditions under which rhythmically zoned hydrothermal quartz precipitates using the same model, exploring Al poisoning of the surface of quartz as a potential reason for it. Quartz growth in a hydrothermal solution will use the same numerical code, but thermodynamic data for ions and complexes, pH or Na+ as a growth accelerator and Al as a kinetic inhibitor instead. The results will be compared to natural samples of vein quartz. Preliminary work suggests that significant amounts of H+ is included in quartz along with Al3+ to compensate the Si4+ ion in quartz. We will attempt to use the fact that boron isotope fractionation between tourmaline and fluid is strongly pH dependent to see if we can correlate tourmaline isotope composition included in quartz with Al-H zoning of these quartzes.
The final project will explore how far this modelling approach can be used to explain zoning and texture (for example porphyritic) of garnet growth in regional and contact metamorphic environments. To this end we will modify the growth model to include surface kinetic equations for each matrix mineral, modal abundances of the minerals, and a continuum approach to the grain boundary structure. We will attempt to model the zoning of contact metamorphic garnets from the Little Cottonwood stock (Utah, USA) using thermal models. P-T conditions will be monitored using QUIG and Raman in graphite. We will compare our free energy minimization
coupled reaction-diffusion approach with the recently developed effective bulk composition approach (Spear and Wolfe, 2018). In contrast, we expect that the solubility of elements will significantly influence garnet zoning through the development of local depletion and enrichement zones surrounding garnet. Changing surface reaction kinetics for matrix minerals will allow us to approach models which are assuming local equilibrium and Gibbs Duhem relations.
