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.
Highlights
Volcanic eruptions are driven by magma density decrease caused by the exsolution of volatiles, mainly H2O (e.g., Gonnermann and Manga 2007). H2O supersaturation of the melt can be induced by a pressure (P) decrease and causes formation of vesicles, which grow by both pressure related equation of state (EOS) expansion and continuous diffusion of H 2O from the melt into the fluid phase (e.g., Sparks 1978)
In the samples of this study, we observed heterogeneously nucleated fringe vesicles attached to the capsule walls, a vesicle free drainage zone, and a finely vesiculated central volume formed by homogeneous phase separation as reported elsewhere (e.g., Iacono-Marziano et al 2007; Allabar and Nowak 2018; Allabar et al 2020)
While we can extract semi-quantitative information that constrains whether melt degassing is occurring in equilibrium or disequilibrium, we highlight the need for improved understanding of EOS- and resorption shrinkage through both in-situ decompression experiments and numerical modelling
Summary
Volcanic eruptions are driven by magma density decrease caused by the exsolution of volatiles, mainly H2O (e.g., Gonnermann and Manga 2007). H2O supersaturation of the melt can be induced by a pressure (P) decrease and causes formation of vesicles, which grow by both pressure related equation of state (EOS) expansion and continuous diffusion of H 2O from the melt into the fluid phase (e.g., Sparks 1978). Volcanic eruptions are driven by magma density decrease caused by the exsolution of volatiles, mainly H2O (e.g., Gonnermann and Manga 2007). H2O supersaturation of the melt can be induced by a pressure (P) decrease and causes formation of vesicles, which grow by both pressure related equation of state (EOS) expansion and continuous diffusion of H 2O from the melt into the fluid phase (e.g., Sparks 1978). The porosity of a magma is a key parameter influencing the buoyancy and driving the acceleration of magma during ascent. The porosity of decompressed silicate melts subsequently quenched to glass has been used to investigate vesicle growth and coalescence as well as the evolution of permeability or percolation (Giachetti et al 2019; Lindoo et al 2016). Porosity has been used to distinguish between equilibrium or disequilibrium degassing by comparing the glass porosity (Φglass) or the residual H2O concentration in the glass (cH2Oglass) with
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