Experimental investigation of homogeneous vesicle formation and growth in hydrous silicate melts during decompression
Final Report Abstract
In this project, silicate melt degassing during decompression (e.g., magma ascent) was investigated experimentally at high temperature and pressure. Within the frame of this project, mainly the homogeneous vesicle formation in hydrous phonolitic melt with white pumice composition of the AD79 Vesuvius eruption was investigated. A high vesicle number density (~105 mm^-3) independent from decompression rate was found. This contrasts nucleation theory that is commonly used to describe vesicle formation in silicate melts and is applicable for, e.g., rhyolitic melt composition. Based on these new experimental data, spinodal decomposition was proposed as an alternative phase separation mechanism in phonolitic melts. This spontaneous phase separation may cause sudden melt degassing, triggering explosive eruptions, even at low decompression rates. While the decompression rate does not influence the vesicle number density in phonolitic melt, it was shown by further experimental suites that the vesicle number density varies by one order of magnitude with initially dissolved H2O concentration (3.3–6.3 wt%). Additionally, a change in degassing evolution at 5.6 wt% initial H2O concentration was found. At lower concentration, the textures observed in natural volcanic products can represent the initially formed vesicles. At higher concentration or if heterogeneous nucleation on crystals is involved, vesicle textures are likely obscured by coalescence or secondary vesicle formation. Further experiments on rhyolitic and peralkaline rhyolitic melts were performed to investigate the influence of melt composition on the phase separation mechanism. The samples are currently being analyzed and interpreted. They will be helpful to understand the difference in degassing behavior between rhyolitic and phonolitic melts. Besides the investigation of melt degassing, the experimental problem of vesicles shrinkage during cooling was investigated during this project. The experimental results and calculations show that vesicles can significantly shrink during cooling, resulting in a glass porosity less than half the melt porosity. Vesicle shrinkage is governed by the decrease in molar volume of H2O and the resorption of H2O from fluid vesicles back into the melt during cooling. Consequently, the glass porosity and H2O concentration in the glass do not necessarily represent the melt porosity and the H2O concentration in the melt before cooling. This poses a problem in the interpretation of degassing processes derived from quenched decompression experiments. Especially the comparison of experimental samples with natural volcanic products can lead to misinterpretations when melt porosity is directly derived from glass porosity. Therefore, new methods for the analysis and interpretation of vesicle shrinkage and H2O resorption are developed. An example is the application of attenuated total reflection FTIR spectroscopy coupled to a focal plane array detector, which enables high spatial resolution mapping of H2O in glass. With this method it was possible to detect increased H2O concentrations in the glass around vesicles that formed by resorption of H2O during cooling. The new methods are useful for future studies to correctly apply experimental results to natural volcanic processes.
Publications
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(2018) Message in a bottle: Spontaneous phase separation of hydrous Vesuvius melt even at low decompression rates. EPSL 501, 192-201
Allabar A and Nowak M
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(2020) High spatial resolution analysis of H2O in silicate glass using attenuated total reflection FTIR spectroscopy coupled with a focal plane array detector. Chemical Geology 556, 119833
Allabar A and Nowak M
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(2020) In situ observation of the percolation threshold in multiphase magma analogues. Bulletin of Volcanology 82, 32
Colombier M, Wadsworth FB, Scheu B, Vasseur J, Dobson JK, Cáceres F, Allabar A, Marone F, Schlepütz CM, Dingwell DB
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(2020) Quantifying microstructural evolution in moving magma. Frontiers in Earth Science 8, 287
Dobson KJ, Allabar A, Bretagne E, Coumans J, Cassidy M, Cimarelli C, Coats R, Connolley T, Courtois L, Dingwell DB, Di Genova D, Fernando B, Fife JL, Fyfe F, Gehne S, Jones T, Kendrick JE, Kinvig H, Kolzenburg S, Lavallée Y, Liu E, Llewellin EW, Madden-Nadeau A, Madi K, Marone F, Morgan C, Oppenheimer J, Ploszajski A, Reid G, Schauroth J, Schlepütz CM, Sellick C, Vasseur J, von Aulock WF, Wadsworth FB, Wiesmaier S, Wanelik K
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(2020) The effect of initial H2O concentration on decompression-induced phase separation and degassing of hydrous phonolitic melt. Contributions to Mineralogy and Petrology 175, 22
Allabar A, Salis Gross E, Nowak M
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(2020) Vesicle shrinkage in hydrous phonolitic melt during cooling. Contributions to Mineralogy and Petrology 175, 21
Allabar A, Dobson, JK, Bauer CC, Nowak M
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(2021) Viscosity of Palmas-type magmas of the Paraná Magmatic Province (Rio Grande do Sul State, Brazil): implications for high-temperature silicic volcanism. Chem. Geol. 560, 119981
Giordano D, Vona A, Gonzalez-Garcia D, Allabar A, Kolzenburg S, Polo L, De Assis Janasi V, Behrens H, De Campos C, De Cristofaro SP, Guimaraes LF; Nowak M, Müller D, Günther A, Masotta M, Roverato M, Romano C, Dingwell DB
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(2021). Retrieving dissolved H2O content from micro-Raman spectroscopy on nanolitized silicic glasses: Application to volcanic products of the Paraná Magmatic Province, Brazil. Chemical Geology, 120058
González-García D, Giordano D, Allabar A, Andrade FRD, Polo LA, Janasi VA, Lucchetti, ACF, Hess KU, De Campos C, Dingwell DB
