Impact of CO2 on alkali-rich explosive volcanism

Assbichler, Donjá (2020): Impact of CO2 on alkali-rich explosive volcanism: case studies from Saray Volcano, Iran and Laacher See Volcano, Germany. Dissertation, LMU München: Fakultät für Geowissenschaften

Abstract

Saray Volcano, Northwest Iran, and Laacher See Volcano, Germany, erupted alkaline, potassic magmas, including carbonatite-bearing magmatic cumulates with trachytic to phonolitic composition. In addition, both contain felsic ejecta consisting mainly of sanidine. Aim of this thesis was to study the petrogenesis of the sanidine-rich rocks with focus on the role of CO2 and to figure out the tipping point that initiated their explosive eruption. Whole-rock composition was analyzed with XRF and ICP-MS. Particular attention was set on the microtexture of the investigated samples. Polarizing light microscope and BSE-images as well as microchemical analysis (EMPA) were used to decipher the relationships and mineral reactions of the mineral assemblage. Photoluminescence helped to study zircon textures and generations. Microchemical analysis of volatile-bearing minerals and Raman spectroscopy of fluid inclusions and of volatile-bearing minerals were performed in order to constrain the reaction conditions of the investigated samples. Of specific interest are “live” magmatic cumulates or bombs as they are directly extracted from their formation site in the magma chamber or the margin of it. In these samples, the reaction conditions and mineral relationships are immediately quenched during ejection due to rapid cooling. Special feature of the trachyte from Saray Volcano are sanidine megacrysts with up to 10 cm in size, embedded in a glass-rich to microlitic groundmass. Since Saray is a poorly studied volcano, all eruption products, trachytic lavas and bombs as well as lamprophyric dykes were sampled in order to understand their relationship. Thermobarometric calculations showed that the trachyte of Saray Volcano crystallized at a temperature of 980-1135°C and a pressure 11-16 kbar, which translates into a depth of ~ 40 km. Multiple resorption and crystallization events are reflected in pseudo-oscillatory zoning of sanidine due to varying Ba content. It was caused by continuous percolation of mantle derived CO2, which transferred heat into the magma chamber and contributed to the temperature fluctuating around liquidus. This way, formation of sanidine megacrysts up to 10 cm was enabled by the suppression of groundmass nucleation. The influx of a CO2-loaden lamprophyre into the magma chamber marks the tipping point for the eruption. The source of lamprophyre was estimated to be at a depth > 70 km. Based on the mantle xenolith cargo in lamprophyre, the ascent rate estimation yielded at least 3 m/s. Consequently, the contact time between trachyte and lamprophyre was very short and the interaction was limited to transfer of CO2 including heat, carbonatite and Fe-Ti-Ba-F-phlogopite (phl-II) from lamprophyre into trachyte, and sanidine and hydrous, Mg-rich phlogopite megacrysts from trachyte into lamprophyre. The fast ascent and eruption quenched the mineral reactions, providing perfect conditions to study the partitioning behavior of Ba and F between the two types of phlogopite and sanidine in the different chemical environment of these two melts. Carbonate + quartz pseudomorphs after diopside prove an unusual high X(CO2) > 0.9 in the top of the trachytic magma chamber and in felsic minette. The systematic of the microtextures in different samples enabled to decipher the chronology of the eruption event triggered by the accumulation of CO2. At Laacher See Volcano, focus was set on the formation of sanidinites, which are cumulates from the magma chamber roof. These felsic cumulates are holocrystalline and rich in cavities, which are typically filled with carbonate and a variety of euhedral crystals, specifically high field strength elements (HFSE) containing minerals. Laacher See Volcano turned out to be a par excellence example to study carbothermal processes occurring in the volatile-rich part of the magma chamber. Cavities and minerals crystallizing in the open spaces document the existence of a free gas phase in the top of the magma chamber. The systematics of the different types of sanidinite documented in the change from haüyne to nosean reflect the respective fluid composition in the upper part of the magma chamber. The sodalite minerals haüyne and nosean are key indicators of changed reaction conditions. This study also showed that CO2 leaches and extracts preferentially divalent elements and REE leading with time to a “purification” of specific minerals such as zircon. Carbothermal vapor depositions of carbonates fill part of the cavities. The formation of a gas-tight caprock by densification of the porous crystallized magma chamber roof enabled the formation of a CO2-fluid cap in the rigid part of the magma chamber and thus the required conditions to generate a critical overpressure in the magma chamber. In both cases, CO2 played a key role, although the impact of CO2 depends on external factors, such as temperature and depth of the magma chamber. At Saray Volcano, CO2 percolation enabled the growth of sanidine megacrysts whereas at Laacher See Volcano, carbothermal processes are recorded in the cumulates of the magma chamber roof. In both cases, however, the tipping point of the eruption was caused by reaching a critical overpressure due to CO2 accumulation. As a result, the brecciation by wall rock failure triggered the explosive expansion of CO2.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften
• Fakultäten: Fakultät für Geowissenschaften
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 22. Januar 2020
• 1. Bericht­erstatter:in: Kunzmann, Thomas