Metastabile paramagnetische Zentren in Quarz aus Gesteinen des Gotthardmassivs (Schweiz) als mögliche Indikatoren geologischer Prozesse

Metastable paramagnetic defect centers in quartz, induced by rock radioactivity, were investigated using EPR. Three tunnel profiles in the Gotthard massif (Swiss Central Alps), providing well documented rock samples, were selected to evaluate the applicability and reliability of analyzing paramagnetic defect centers in order to understand geological processes. The probed profiles of the Bedretto tunnel, Furka base tunnel and Gotthard highway tunnel (safety tunnel) intersect series of polymetamorphic para- and orthogneisses as well as hercynic (permo-carboniferous) granite intrusions (Rotondo granite, Fibbia- and Gamsboden granitic gneiss), the latter showing a different metamorphic overprint. In contrast to the EPR investigations of rock profiles carried out previously, which were mostly vertical drilling profiles, for the first time horizontal rock profiles were investigated, where all quartz samples come from the same temperature range (20 – 32 °C), meaning that no significant temperature gradient exists in the profiles. Thus the progressive effect of increasing temperature with depth on the defect centers could be eliminated in this work. Several paramagnetic defect centers could be detected, among which [AlO4]0- and [TiO4/Li+]0-centers were of particular interest for the subsequent investigations. For a few selected samples EPR tomography of the [AlO4]0-center was carried out implying a relatively homogeneous distribution of the defects in the quartz grains at least for these samples. Rock radioactivity (dose rate) was determined applying γ-spectrometry. The chemical composition of quartz was analyzed using INAA. Correlations with hereby measured Al- and Ti-concentrations with Al and Ti substituted at lattice sites in quartz (measured by EPR) could not be found. This is mainly because a fraction of Al and Ti in quartz is not bonded in the crystal lattice but exists for instance as mineral inclusions. For a couple of selected samples, measurements with LA-ICP-MS on distinct quartz grains of a sample and measurements at different positions within a crystal were performed. Significant inhomogeneities within the grains as well as from grain to grain could be observed. The dose rate contributions from rock and quartz were derived from γ-spectrometry of the rock and element concentrations of U, Th, and K in quartz. They were correlated with the concentrations of [AlO4]0- and [TiO4/Li+]0-centers, showing complex dependencies. Trends for the different rock types could be distinguished, while scattering of the values was relatively high. For quartz from granites and granitic gneisses they were in similar ranges and could be separated from samples of the polymetamorphic series. Here, the values for both type of centres in each case were contiguous. Quartz samples from polymetamorphic rock series, embedding the granites and granitic gneisses, exhibited much stronger scattering, especially for [TiO4/Li+]0-centers and formed a different trend area. All tunnel profiles displayed similar trend areas. [AlO4]0-centers on average showed higher concentrations at higher dose rates, as expected. [TiO4/Li+]0-centers behaved opposite and, on average, lower concentrations were observed at higher dose rates, indicating center saturation. The dependencies clearly demonstrate a complex situation, which partially has a rock specific origin. For samples from the Bedretto tunnel, defect center concentrations normalized by the dose contribution of rock and quartz were correlated with γ-sensitivity, determined by artificial 60Co-γ-irradiation (1000 Gy). No simple principles could be derived, which indicates hitherto unknown factors for the center accumulation. Calculation of age data from accumulated defect center concentrations in quartz from the Bedretto tunnel again revealed a strong scattering, indicating a distinct dose accumulation pattern for the different quartz samples. The application of CL to selected quartz samples from the Bedretto tunnel and Gotthard highway tunnel revealed the effect of a relatively weak metamorphosis and recrystallization. An unambiguous assignment of characteristic CL-peaks to [AlO4]0- and [TiO4/Li+]0-centers was not possible. For the determination of the accumulated center concentration and their radioactive as well as thermal dependencies, defined irradiation experiments with 60Co-γ-radiation and annealing experiments at different temperatures were carried out. Irradiation experiments generally showed different center accumulation curves for various samples. Particularly [TiO4/Li+]0-centers started to display saturation at absorbed doses greater than 1000 Gy, for [AlO4]0-centers saturation started only from 10000 Gy. Here, for the first time γ-irradiation experiments at elevated temperatures were carried out and indicated an increase in center accumulation between 30 °C and 100 °C, possibly affected by an enhanced mobility of charge carriers in the lattice. Isothermal heating experiments for [AlO4]0-center studies revealed a complex decay kinetics, different for the respective samples. The results indicate a mixed order of the decay reactions. The particular decay behavior demonstrated different thermal stability of identical defect centers in distinct samples, which in turn might have contributed to the variance of center concentrations in relation to the dose rate. [TiO4/Li+]0-centers in these samples generally exhibited a lower thermal stability compared to the [AlO4]0-centers. In the present thesis for the first time long-term annealing experiments at 75 °C over more than 2 years were performed. They revealed very complex reaction kinetics. The results implied a composite mechanism, which suggests, that decay kinetics for high temperature regimes cannot be implicitly transferred to the behavior at low T or room temperature. The complexity of the measured data implies, that the concentration of defect centers depends on different factors: on the one hand rock type and radiation intensity and on the other hand the concentration of substituted Al-, Ti-, and Li-atoms are relevant. Quartz in various rock types represents unsurprisingly different geological processes, which can also be reflected in the defect centers and their concentration. However, here even in the same rock type with very similar geological surrounding and history a complex situation appears. This implies that not only the general origin of quartz but also its recrystallization history, for instance during the alpidic metamorphosis could matter. The rocks of the investigated profiles display recrystallization phenomena under non-equilibrium conditions, often characteristic for retrograde metamorphism. The complex findings indicate diverse dependencies. It is presumed that small-scale geological processes, predominantly selective recrystallization, thermal anomalies, solution related chemical-radioactive transport and tectonic movements could be responsible for the complex results, hence radiation induced defect centers could be suitable for detection of aforementioned processes.