EGU21-10227
https://doi.org/10.5194/egusphere-egu21-10227
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

An image-based technique to determine the freezing temperature Tf of vesicle volumes in decompressed, synthetic melt samples

Dennis Eul1, Anja Allabar2, and Marcus Nowak1
Dennis Eul et al.
  • 1Department of Geosciences, Eberhard Karls University, Tübingen, Germany (dennis.eul@student.uni-tuebingen.de)
  • 2Department of Mineralogy, Georg-August University, Göttingen, Germany

The non-in-situ analysis of H2O degassing of silicate melt at high temperature and pressure is conducted using synthetic, decompressed melt samples quenched to glass. Interpretations regarding the degassing behavior are based on the number of H2O filled vesicles and the porosity of the vitrified samples. These properties of the glass samples may not represent the vesiculation at experiment temperature Texp and target pressure Pfinal. Even at high quench rates q, a decrease of vesicle volumes during cooling occurs, facilitated by resorption of H2O fluid back into the melt (McIntosh et al., 2014) and by the decrease of molar volume of H2O (Marxer et al., 2015) in the vesicles. This vesicle shrinkage introduces uncertainty regarding the true q-dependent “freezing” temperature Tf, at which shrinkage stops, represented by the vesiculated glass sample. While often neglected, knowledge of Tf is useful for improved sample interpretation.

McIntosh et al. (2015) developed a computer tomography (CT) based method to determine Tf. This approach infers Tf from the volume fraction of liquid H2O in vesicles (whose volumes are comprised of a liquid and a gaseous H2O phase) which decreases for increasing Tf.

Using their theoretical foundations, we developed a simple, transmitted light microscopy (TLM) image-based approach for the determination of this intra-vesicle phase ratio, applying two different model calculations: 1) Approximation of phase boundaries using polynomial functions. 2) Calculation of total vesicle and gas-phase volumes from ellipsoid axes measurements, approximating the vesicle and gas-phase volumes with symmetrical spheroids. In our analyzed hydrous, haplogranitic samples, we found mean Tf’s up to ~250 to ~300 K lower than Texp, at which quench was initiated, for q’s of ~40 and ~90 K/s. These values are close to the estimated Tf’s obtained using an independent glass porosity equation (Gardner et al., 1999). The large scatter of volume fractions and thus Tf for individual vesicles cannot be attributed to our image-based approach as data obtained from phonolitic samples using the CT method (Allabar et al., 2020) depict a similar scatter. At present, no correlation of Tf with vesicle size or position within the sample could be made. The method is, for the range of vesicle sizes investigated (20 to 50 µm in diameter), limited to liquid volume fractions larger than ~10 vol% as a distinction between phases is limited by optical resolution.

Nevertheless, our TLM based approach provides a simple, readily available method to constrain Tf of vitrified vesiculated samples which significantly improves the quality and comparability of derived interpretations. Our method uses standard polished sections for FTIR, making it even applicable to already existing samples.

 

 

Allabar, A. et al. (2020), Contrib. Mineral. Petrol, 175, 21, 1-19

Gardner, J.E., Hilton, M. and Carroll, M.R. (1999), Earth Planet. Sci. Lett, 168, 201-218

Marxer, H., Bellucci, P. and Nowak, M. (2015), J. Volcanol. Geotherm. Res, 297, 109-124

McIntosh, I.M. et al. (2014), Earth Planet. Sci. Lett, 401, 1-11

McIntosh et al. (2015): ‘Practical’ glass transition temperatures of vesicular glasses: a combined FTIR-XRCT approach. Abstract, 10th Silicate melt workshop. La Petite Pierre, France

How to cite: Eul, D., Allabar, A., and Nowak, M.: An image-based technique to determine the freezing temperature Tf of vesicle volumes in decompressed, synthetic melt samples, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10227, https://doi.org/10.5194/egusphere-egu21-10227, 2021.

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