Project

Discipline

kinetics; Igneous Petrology; Metamorphism; Numerical modelling; Experimental petrology; Boundary layer; Depleation halo; thermodynamics; SIMS; FEG-EMPA; NanoSIMS

Lead
Das Wachstum großer Kristalle in magmatischem, hydrothermalen oder metamorphem Gesteinen erfordert, dass Nährstoffe für das Wachstum des Kristalls zur Mineral- / Gesteins- oder Mineral- / Schmelzoberfläche transportiert werden. Dies geschieht typischerweise durch Diffusion durch eine Grenzschicht. An der Oberfläche werden sie zu der Stelle transportiert wo diese in den Kristall inkorporiert werden können. Dieser Prozess führt zu Verarmungs- und Anreicherungszonen, die den wachsenden Kristall umgeben. Das Ausmaß der lokalen Konzentrationsänderungen hängt von der Diffusionsgeschwindigkeit im Medium, der Oberflächenkinetik und den geologischen treibenden Kräften (Druck-, Temperatur-, oder Zusammensetzungsänderungen) ab.
Lay summary
Im Rahmen dieser Studie werden effiziente numerische Modelle des Kristallwachstums entwickelt, die die Oberflächenkinetik des wachsenden Kristalls, die Diffusion im Medium (Korngrenze, freie Flüssigkeit, Silikatschmelze) und die globalen und lokalen thermodynamischen Rahmenbedingungen erfüllen. Die Modelle werden anhand von Hochtemperatur- und Druckexperimenten von Quarzwachstum in rhyolitischen Schmelzen getestet, in welchen die Anreicherung und Verarmung an Elementen um die wachsenden Quarzkristalle gemessen werden kann. Diese werden es erlauben, Diffusionsmodelle für siliziumreichen Schmelzen zu erstellen.  Ein weiteres Projekt wird die Zonierungen in natürlich gewachsenem Gangquarz mit Tourmalineinschlüssen analytisch erforschen, und mit Modellen für wässerige Lösungen vergleichen. Das dritte angewandte Projekt wird die Zonierungen in Granat aus Kontakt- und Regionalmetamorphose mit. Modellen des diffusions-limitierten kinetischen Wachstum vergleichen. Allenfalls werden wir lockale Kinetik in der Matrix berücksichtigen. Das Projekt wird die modernsten in-situ-Analysemethoden (SIMS, NanoSIMS, FEG-EMPA) verwenden.

Direct link to Lay Summary

Last update: 31.12.2019

Natural persons

Number	Title	Start	Funding scheme

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. Rapid quenched capsules will be opened and cut after careful µC-X-ray tomography to locate quartz crystals. Melt halos will be analysed using low voltage FG-EMPA for major and minor elements and (Nano)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 optimised parallel code using a full Gibbs Free energy minimisation.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.