Vesicle shrinkage in hydrous phonolitic melt during cooling

Allabar, A.; Dobson, K.J.; Bauer, C.C.; Nowak, M.

Abstract

The ascent of hydrous magma prior to volcanic eruptions is largely driven by the formation of H2O vesicles and their subsequent growth upon further decompression. Porosity controls buoyancy as well as vesicle coalescence and percolation, and is important when identifying the differences between equilibrium or disequilibrium degassing from textural analysis of eruptive products. Decompression experiments are routinely used to simulate magma ascent. Samples exposed to high temperature (T) and pressure (P) are decompressed and rapidly cooled to ambient T for analysis. During cooling, fluid vesicles may shrink due to decrease of the molar volume of H2O and by resorption of H2O back into the melt driven by solubility increase with decreasing T at P < 300 MPa. Here, we quantify the extent to which vesicles shrink during cooling, using a series of decompression experiments with hydrous phonolitic melt (5.3–3.3 wt% H2O, T between 1323 and 1373 K, decompressed from 200 to 110–20 MPa). Most samples degassed at near-equilibrium conditions during decompression. However, the porosities of quenched samples are significantly lower than expected equilibrium porosities prior to cooling. At a cooling rate of 44 K·s−1, the fictive temperature Tf, where vesicle shrinkage stops, is up to 200 K above the glass transition temperature (Tg), Furthermore, decreasing cooling rate enhances vesicles shrinkage. We assess the implications of these findings on previous experimental degassing studies using phonolitic melt, and highlight the importance of correctly interpreting experimental porosity data, before any comparison to natural volcanic ejecta can be attempted.

Citation

Allabar, A., Dobson, K., Bauer, C., & Nowak, M. (2020). Vesicle shrinkage in hydrous phonolitic melt during cooling. Contributions to Mineralogy and Petrology, 175(3), Article 21. https://doi.org/10.1007/s00410-020-1658-3

• Journal Article Type: Article
• Acceptance Date: Jan 16, 2020
• Online Publication Date: Feb 12, 2020
• Publication Date: 2020
• Deposit Date: Feb 26, 2020
• Publicly Available Date: Feb 26, 2020
• Journal: Contributions to Mineralogy and Petrology
• Print ISSN: 0010-7999
• Electronic ISSN: 1432-0967
• Publisher: Springer
• Peer Reviewed: Peer Reviewed
• Volume: 175
• Issue: 3
• Article Number: 21
• DOI: https://doi.org/10.1007/s00410-020-1658-3
• Public URL: https://durham-repository.worktribe.com/output/1276538