Numerische Modellierung von metamorphen Reaktionsentwicklungen entlang vorgegebener P-T-Pfade: der Einfluss von Fraktionierung und Devolatisation
Zusammenfassung der Projektergebnisse
The primary aim of the project was to try and quantify disequilibrium phenomena, mineral reactions and phase transitions in naturally-occurring high- and ultrahighpressure rocks in order to better contrain their actual temporal pressure-temperature evolution. This actual evolution is then used to fine-tune models of geodynamic processes by being able to predict the effects of processes occurring today hidden at depths in the earth. Due to the fact that suitable test rocks had already been isolated by us from previous studies we were able to quickly start comprehensive mineral analysis programs to allow a quantitative estimation of degree of non-equilibrium as reflected in compositional zoning in metamorphic garnet. Our results presented, for the first time, an explanation for step-like features on garnet zoning profiles as normal breaks in growth due to fractionation effects. Such an interpretation is critical as the degree of non-equilibrium reflected in such zoning is often ignored by the common simplified equilibrium P-T diagram (i.e. pseudosection) approach. Importantly, we were able to clearly identify the critical reactions leading to steps or change in slope of compositional profiles. Based on this newly-won information we then undertook trace element modelling by the multi-step equilibrium approach incorporating fractionation in garnet rims. The resulting chondritenormalised trace element plots from core to rim of zoned garnet show a characteristic change in slope, contrasting strongly with what many workers at the time were accepting for correlation of garnet-zircon growth episode relationships. Our LA-ICP- MS normalised trace element analysis plots gave a spectacular fit with the modelled results. This was the first study in the world to predict a growth-fractionation mechanism to explain zoning in garnet and at the same time get a near perfect fit between predicted and measured trace element patterns. Several other groups in the world have subsequently taken up where we left off. A side project involved investigation of subtle substructure in garnet as visualised by electron back-scattered diffraction (EBSD). We were able to show that old, relict and new, overgrowth garnet in a polymetamorphic sample had distinct growth microstructures that allowed for a preferential diffusion along domain boundaries and thus more effective compositional resetting than expected for standard models involving homogeneous diffusion in a sphere. Numerous invitations to talk on aspects of this work worldwide, as well as international prizes (the Mineralogical Society of America elected me as Distinguished Lecturer for the year 2005/2006 as the very first professor from a German University to be given this award, and in 2008 I was presented the Emanuel Boricky Medal from the Faculty of Science of Charles University, Prague for excellence in petrology) show the impact of this research.
Projektbezogene Publikationen (Auswahl)
- (2007). Compositional reequilibration of garnet: the importance of sub-grain boundaries. European Journal of Mineralogy, 19, 4, 431-438
Konrad-Schmolke, M., O’Brien, P.J. & Heidelbach, F.
- (2008). Garnet growth at high- and ultra-high pressure conditions and the effect of element fractionation on mineral modes and composition. Lithos, 103, 3-4, 309-332
Konrad-Schmolke, M., O’Brien, P.J., De Capitani, C. and Carswell, D.A.
- (2008). Major- and rare-earth-element modelling in garnet along eclogite P-T paths, examples from the Western Gneiss Region, Norway. Earth and Planetary Science Letters, 272, 488-498
Konrad-Schmolke, M., Zack, T. and O’Brien, P.J. and Jacob, D.
