Entgasung und Kristallisation beim Aufstieg dazitischer Magmen zur Erdoberfläche

Abstract

In this study, kinetic processes during experimentally simulated ascent of dacitic magmas, based on eruption products of Taapaca volcano, were investigated. The focus was on degassing of water saturated samples and the resulting increase of liquidus temperatures and accompanying crystallization of plagioclase. Furthermore potential decomposition reactions of amphiboles were examined. Degassing during isothermal decompression at a temperature of 850°C and decompression rates from 6.3 to 450 bar/h was always close to equilibrium water concentrations, which were determined by independent equilibrium experiments. Water concentrations decrease continuously at a decompression rate of 50 bar/h from 4.93 wt.% at 1550 bar towards lower pressures. A systematic correlation between decompression rates and corresponding water concentrations could not be found. The compositional response of the residual melt during decompression is primarily determined by the duration available for the chemical reaction. This is evident from the metrics “Reequilibrationindex” (REI) and “Reequilibrationspeed” (RES) introduced in this study: Experiments with a decompression rate of 21,4 bar/h achieve a more complete reequilibration than the faster decompressed samples, even though the latter ones show RES which indicate ten times faster reaction kinetics. This does not compensate for the shorter durations available for the reequilibration reactions. Crystallization of plagioclase was identified (besides degassing) to be a major process during decompression. Other significant reactions of minerals could not be found. The crystal size distributions (CSDs) of plagioclas at different decompression rates are distinguishable at a final pressure of 500 bar after decompression: Slower decompression results in steeper CSDs with higher population densities at small crystal size classes. At 50 bar such a difference does not occur. Even though different decompression rates do result in very similar CSDs at lower final pressures, the crystallization processes nucleation and growth contribute to different amounts to the crystallization at higher pressures, depending on the decompression rate. The compositions of plagioclase from decompression experiments are richer in An than corresponding crystals of equilibrium experiments. While equilibrium experiments result in increasing K2O concentrations at lower pressures, this is not the case for decompression experiments. Thus there are significant differences between plagiocloase compositions in decompression and equlibrium experiments. Decomposition reactions of amphibole could not be observed in any decompression experiment. The composition of amphiboles is constant and no concentrations profiles within single crystals could be found. No indication of amphibole decomposition was found even though the stability field of amphibole was left early during decompression experiments. Ascent rates of the natural system of Taapaca volcano could not be constrained with the resulting data for degassing, crystallization and the results for amphiboles. The observations from simulated decompression rates between 6.3 bar/h and 450 bar/h are compatible with the natural samples. A systematic error could not be excluded though, as the natural system is not in equilibrium before eruptions happen and the conducted experiments are not able to reflect this. Water solubilities of dazitic melts at temperatures between 1000 and 1250°C and pressures from 250 to 2500 bar have also been determined in this study. At 1200°C solubilities raise from 1.6 wt.% at 250 bar to 6.5 wt.% at 2500 bar. With increasing temperatures water solubilities decreases at constant pressures: At 1 kbar it decreases from 4.3 wt.% at 1000°C to 3.3 wt.% at 1250°C, at 2 kbar from 6 wt.% to 5.5 wt.% within the same temperature interval.