Kohlendioxid in ultrapotassischen Silikatschmelzen: Mechanismen und Raten der Magma-Entgasung und deren Einfluss auf die Intensität und den Stil der Vulkanausbrüche.
Zusammenfassung der Projektergebnisse
Some explosive volcanic eruptions are related to magma with high CO2 contents that originate from a magma plumbing system being rooted in carbonate substrata. This project used high pressure high temperature experiments, to determine melt properties and to simulate magma ascent processes to improve the knowledge on the role of CO2 for volcanic degassing mechanisms and their effect on explosive volcanic eruptions. The first part of this project focussed on the solubility of mixed CO2-H2O volatile phases in K-rich leucititic and phonolitic melts as a function of pressure to shed light on the storage conditions of those melts in the Earth’s crust. Synthetic melts with leucititic (SULm) and phonolitic (VES79) compositions from eruptions of the Colli Albani and Mt. Somma-Vesuvius were used. Solubility experiments were carried out in an internally heated pressure vessel (IHPV) at 1250 °C and pressures between 50 and 300 MPa for fluid compositions ranging from pure CO2 to pure H2O. It was found that the highly depolymerized SULm melt is capable of dissolving very large amounts of CO2 (up to 8500 ppm at 300 MPa) that is five times higher than in the polymerized VES79 composition at similar pressures. At the same time H2O solubility is fairly similar in both compositions throughout the studied pressure range. In the second part we determined the diffusivity of CO2 in dry and hydrous leucititic melt. The transport properties of volatiles in silicate melts are important parameters that control bubble growth during magma ascent and thus the effectivity of volatile degassing and eruptive behaviour. We observed that H2O has a strong accelerating effect on CO2 diffusion, that exceeds by far the effect of anhydrous melt composition. CO2 diffuses slower than other volatile components such as halogens and H2O in leucititic melt, enabling a fractionation of volatiles during magma ascent and degassing. In the third part we experimentally investigated the decompression induced volatile exsolution of mixed CO2-H2O bearing leucitite melts that occurs during magma ascent through the crust. In an IHPV at 1250 °C, the pressure was decreased from 200 MPa to about 150, 100, 50 and 30 MPa at a fast, constant decompression rate of dP/dt= 1 MPa/s for sample series with XH2O = 0.0, 0.5, 1.0. After rapidly quenching the samples to room temperature, optical analysis was done to obtain parameters important for interpretation of the melt degassing behaviour such as vesicle number density (VND) and porosity. We demonstrate that the exsolution mechanisms of H2O strongly differ from those of CO2. In contrast to the single homogeneous nucleation event of H2O bubbles in purely hydrous melts, CO2 bearing melts tend to continuously nucleate new bubbles after surpassing the critical supersaturation pressure. In CO2-H2O bearing melts we found that CO2 degasses exclusively at higher pressure before H2O exsolves from the melt, rapidly decreasing the volatile supersaturation at low final pressure. We propose that this causes increased magma ascent rates at shallow depths (where H2O exsolution starts) leading to enhanced volcanic explosivity.
Projektbezogene Publikationen (Auswahl)
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CO2–H2O solubility in K-rich phonolitic and leucititic melts. Contributions to Mineralogy and Petrology, 174(6).
Schanofski, Maximilian; Fanara, Sara & Schmidt, Burkhard C.
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CO2 diffusion in dry and hydrous leucititic melt. European Journal of Mineralogy, 35(1), 117-132.
Koch, Lennart & Schmidt, Burkhard C.
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CO2 quantification in silicate glasses using µ-ATR FTIR spectroscopy. American Mineralogist, 108(7), 1346-1356.
Schanofski, Maximilian; Koch, Lennart & Schmidt, Burkhard C.
