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Violence and volume: an experimental study of CO2 driven eruptions of Colli Albani

dc.contributor.advisorWebb, Sharon Prof. Dr.
dc.contributor.authorKleest, Christin
dc.titleViolence and volume: an experimental study of CO2 driven eruptions of Colli Albanide
dc.contributor.refereeWebb, Sharon Prof. Dr.
dc.description.abstractengThis thesis presents the first rheological investigations of melts from the Colli Albani Volcanic District (CAVD) near Rome (Italy). Despite their silica-poor (< 45 wt%) and ultrapotassic composition – indicating a low viscosity melt –, the volcanic events unexpectedly showed mainly an explosive eruptive style emitting large volumes of pyroclastic material. Such eruptions usually occur in highly viscous melts. The magma chamber of the CAVD is seated in a thick carbonate sequence getting assimilated by the melt and suppling the magma with CaO and a CO2-rich fluid. Viscosity and calorimetric measurements are performed with synthetic melts with the compositions of the two largest eruptions of the CAVD: the Pozzolane Rosse (PR) and the Pozzolane Nere (PN) event. To study the rheologic PR melt evolution during magma storage and ascent, 7 wt% CaO were subtracted from the erupted PR composition (PR-CaO) representing the melt before the contamination by the carbonate wall rock. Since the pyroclastic products of the PR event contain mainly leucite crystals, 4 mol% KAlSi2O6 are removed from the PR composition to investigate the viscosity of the residual melt (PR-Leu). The compositions of the melts range from foiditic (PR and PR-CaO) to tephritic (PR-Leu) to tephri-phonolitic (PN) up to a further phonolitic melt with the composition from the “white pumice” of the AD 79 Vesuvius eruption (WPVe). Three series of measurements are done: to get an overview to the rheological features in general, investigated glasses are synthesised in air and investigated. Since the CAVD melts are Fe-rich, further melts are made with the half amount (PR-0.5Fe, PR-Leu-0.5Fe and PN-0.5Fe) and with 30 wt% (PR-0.7Fe) of the original Fe content and Fe-free (PR-Fe and PN-Fe) to avoid crystallisation effects during measurements. Fe2+ was replaced by Mg2+ and Fe3+ was replaced by Al3+. To address the enrichment of the melts with a CO2-rich fluid, the rheology of PR-0.5Fe and PR-Leu-0.5Fe, PN-0.5Fe as well as the WPVe melts made in an internally heated pressure vessel are investigated as a function of CO2 concentrations up to ~0.50 wt% CO2. A third series of viscosity measurements is done with two different Fe ratios in the melts affecting their viscosities. The PR-0.5Fe, PR-Leu-0.5Fe, PN-0.5Fe and the WPVe melts are re-equilibrated in a gas mixing furnace with H2/CO2 controlling the oxygen fugacities of 10^(-4.00) and 10^(-7.00) bar and hence the Fe speciation. Viscosity and calorimetric measurements were done in the glass transition range Tg from 600 - 820 °C and viscosities range from ~10^8.5 to ~10^13.5 Pa s. The measurements indicate a viscosity decrease during PR melt evolution: the contamination of the carbonate host rock lowers the PR melt viscosity due to an enrichment in CaO. The viscosity of the residual melt PR-Leu decreases as a consequence of the depletion in SiO2 and Al2O3 caused by leucite crystallisation. An enrichment of CO2 affects the viscosities and calorimetric Tg slightly to negligibly. However, disintegrating and foaming of the samples during measurements before reaching Tg partly prevent reliable statements for PR-0.5Fe and PN-0.5Fe. In contrast, a CO2 content of ~0.07 wt% and ~0.50 wt% decreases Tg12 by 14 °C and 5°C for the WPVe and PR-Leu-0.5Fe melt, respectively. An increase of Fe2+/Fetot from 0.42 to 0.76 for the CAVD melts results in a decrease in Tg12 from 5 °C, 30 °C and up to 50 °C for PR-Leu-0.5Fe, PR-0.5Fe and PN-0.5Fe, respectively, with the highest drop in Tg12 for the most polymerised melt. Although all melts investigated in the presented thesis are nominally anhydrous (< 0.5 wt% H2O), they contain up to 0.23 wt% H2O. It is shown that these small amounts of H2O have a stronger decreasing effect on the viscosities than an increase in CO2 or Fe2+/Fetot investigated in this study. Regarding the PR eruption, it is assumed to have a low viscosity residual liquid melt after the crystallisation of up to 60 vol% leucite. The entry of the CO2-rich fluid exceeds the CO2 solubility of the melt resulting in bubble formation and buoyancy of the resulting magma. Both processes  the former one leads to a dramatic increase of the magma viscosity and the latter one builds up pressure within the magma  are consequently assumed to feed the explosivity of the PR eruption and the high volume of erupted
dc.contributor.coRefereeWilke, Max Prof. Dr.
dc.contributor.thirdRefereeHoltz, Francois Prof. Dr. rer. nat.
dc.contributor.thirdRefereeBehrens, Harald apl. Prof. Dr.
dc.contributor.thirdRefereeSchmidt, Burkhard Dr.
dc.contributor.thirdRefereeTechmer, Kirsten Dr.
dc.subject.engColli Albanide
dc.subject.engglass transitionde
dc.subject.engcarbon dioxidede
dc.subject.engiron ratiode
dc.affiliation.instituteFakultät für Geowissenschaften und Geographiede
dc.subject.gokfullGeologische Wissenschaften (PPN62504584X)de

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