How well do pseudosection calculations reproduce simple experiments using natural rocks: an example from high-P high-T granulites of the Bohemian Massif

Abstract

This study represents a comparison between thermodynamic calculations (pseudosections) and the results of the experimental investigation of high-P/high-T granulites from the Bohemian Massif by Tropper et al. (2005). The experiments were conducted at 750-1000°C at 1.6 GPa, 950°C, 1.4 GPa and 800-900°C at 1.2 GPa in order to model the high-P/high-T evolution of the south Bohemian H O (KNCFMMnTiASH). The calculations revealed that the experimentally obtained phase assemblages could very well be reprodu-2 ced despite the use of different activity models for solid solutions, compositional trends of garnet and feldspars good to satisfactorily.The calculations revealed two major inconsistencies: 1.) the stability of biotite is underestimated in the experiments by ca. 50-100°C and 2.) the amount of melt generated in the experiments is much larger than in the calculations. The reasons for these inconsistencies can be explained by: 1.) electron microprobe analysis of micas from the run products which revealed high F-contents not analyzed previously by Tropper et al. (2005), which is not accounted for in relevant biotite activity models, and by: 2.) the nature of the experimental set-up (lack of eqilibrium in the subsolidus region and fast reaction overstep) which is responsible for generation of abnormously high amounts of melts. Nonetheless the combination of experimental and thermodynamic investigations and its comparison to natural rocks allows gathering deeper insights into the metamorphic evolution of these high-P/high-T granulites. Das Ziel dieser Arbeit war der Vergleich zwischen thermodynamischen Berechnungen (Pseudosektionen) und den experimentellenUntersuchungen bezüglich der Genese der Hoch-T/Hoch-P Granulite aus der Arbeit von Tropper et al. (2005). Die Experimente wurden mit granitischem Ausgangsmaterial bei Temperaturen von 750-1000°C bei 1.6 GPa, bei 950°C und 1.4 GPa und bei 800-900°C bei 1.2 GPa durchgeführt um die P-T Entwicklung dieser südböhmischen Granulite im Experiment zu simulieren. Um die Experimente hinsichtlich ihrer Reproduzierbarkeit zu untersuchen wurden Pseudosektionen mittels der Programme THERIAK-DOMINO

New evidence for the prograde and retrograde PT-path of high-pressure granulites, Moldanubian Zone, Lower Austria, by Zr-in-rutile thermometry and garnet diffusion modelling

“…These biotite flakes have a significant fluorine content, with a maximum of 5.40 wt.% (1.265 apfu) ( Table 2). Rare biotites outside the slightly darker bands also show elevated F contents, typically higher than 0.5 wt.%, as reported from similar rocks by Tropper and Hauzenberger (2015).…”

Garnet Chemical Zoning Based Thermobarometry: Method Evaluation and Applications in the Menderes Massif, Western Turkey

“…These packages use the same input information (garnet composition and effective rock composition) and achieve the same results. Assuming the code of [17] could interface with Perple_X, both programs should produce comparable results provided that the same thermodynamic database and activity models are used [54].…”

“…The GZM method first defines conditions of garnet nucleation, then proceeds to determine the rest of the path. The routine of [17] runs in MATLAB and interfaces with the software package Theriak-Domino [25,54]. The process begins by computing an isochemical phase diagram using a whole-rock bulk composition and at a P-T point outside the garnet stability field.…”