Vesiculation dynamics – Part 2: Decompression-induced H<sub>2</sub>O vesicle growth, onset, and progression of coalescence
<p>Supersaturation of H<span class="inline-formula"><sub>2</sub></span>O during magma ascent can drive efficient degassing through the formation of fluid vesicles within phonolitic melt, with high vesicle number densities (VNDs) ranging from a logVND of 4 to 6...
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Copernicus Publications
2025-07-01
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| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/37/413/2025/ejm-37-413-2025.pdf |
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| author | P. L. Marks M. Nowak |
| author_facet | P. L. Marks M. Nowak |
| author_sort | P. L. Marks |
| collection | DOAJ |
| description | <p>Supersaturation of H<span class="inline-formula"><sub>2</sub></span>O during magma ascent can drive efficient degassing through the formation of fluid vesicles within phonolitic melt, with high vesicle number densities (VNDs) ranging from a logVND of 4 to 6 mm<span class="inline-formula"><sup>−3</sup></span>. Rapid vesicle formation and growth reduces magma density and may significantly influence eruption style. However, the behavior of vesicle-bearing melt under continued decompression remains poorly understood. This study aims to investigate vesicle growth as well as the onset and progression of vesicle coalescence during decompression in the well-characterized phonolitic Lower Laacher See melt.</p>
<p>Synthetic phonolite melts were hydrated in an internally heated gas pressure vessel at 200 MPa with 5.7 wt % or 5.0 wt % H<span class="inline-formula"><sub>2</sub></span>O, establishing conditions of saturation and slight undersaturation prior to decompression. The hydrated melts were continuously decompressed at a superliquidus temperature of 1323 K with rates ranging from 0.064 to 1.7 MPa s<span class="inline-formula"><sup>−1</sup></span>, reaching final pressures between 70 and 30 MPa before being rapidly quenched to vesiculated glass.</p>
<p>As expected, our results indicate that the pressure range for uniform vesicle formation differs between initially H<span class="inline-formula"><sub>2</sub></span>O saturated and undersaturated melts, with coalescence starting at higher pressures in saturated melts. The onset of coalescence is also influenced by the decompression rate, occurring at higher pressures with slower decompression rates. The coalescence process transforms the quenched opaque samples with maximum VND, characterized by nebula of small vesicles sized between 2 and 16 <span class="inline-formula">µm</span> into transparent samples with larger vesicles ranging from 37 <span class="inline-formula">µm</span> up to <span class="inline-formula">∼500</span> <span class="inline-formula">µm</span> and VNDs reduced by orders of magnitude, depending on decompression rate, final pressure, and initial H<span class="inline-formula"><sub>2</sub></span>O concentration. The sizes of these coalesced vesicles are inversely related to the decompression rate, with larger vesicle sizes occurring at slower decompression rates.</p>
<p>While the maximum logVNDs of initially formed vesicle volumes remain constant across different decompression rates, with high values of 5.1 to 5.7, compatible with the theory of spinodal decomposition, the logVNDs of sample volumes with coalesced vesicles drop markedly by 1.2 to 4.1 log units. The lowest logVNDs, between 0.5 and 0.8, occur at the slowest decompression rate of 0.064 MPa s<span class="inline-formula"><sup>−1</sup></span>, while the highest logVNDs, between 3.1 and 3.7, are observed at the fastest decompression rate of 1.7 MPa s<span class="inline-formula"><sup>−1</sup></span>. This coalescence driven pattern mimics the expected decompression rate dependence of initial VND in cases of vesicle nucleation.</p>
<p>These experimental results suggest that significant vesicle coalescence can occur in phonolitic magmas even at relatively fast ascent rates, influencing eruption dynamics. In the case of open-system outgassing, vesicle coalescence creates channel structures with high permeability, allowing fluid to percolate and escape at the top of the magma column. In contrast, in a closed system, vesicles can accumulate as foam with closed porosity at the top of the magma chamber, forming large, gas-pressurized pockets. This buildup may lead to significant disruptions and violent, explosive eruptions. During volcanic activity, transitions between these two systems may occur.</p> |
| format | Article |
| id | doaj-art-86b585215daa4a5f87bd25df72ca71d2 |
| institution | Kabale University |
| issn | 0935-1221 1617-4011 |
| language | English |
| publishDate | 2025-07-01 |
| publisher | Copernicus Publications |
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| series | European Journal of Mineralogy |
| spelling | doaj-art-86b585215daa4a5f87bd25df72ca71d22025-08-20T03:31:26ZengCopernicus PublicationsEuropean Journal of Mineralogy0935-12211617-40112025-07-013741343510.5194/ejm-37-413-2025Vesiculation dynamics – Part 2: Decompression-induced H<sub>2</sub>O vesicle growth, onset, and progression of coalescenceP. L. Marks0M. Nowak1Department of Geosciences, Eberhard Karls University of Tübingen, Tübingen 72074, GermanyDepartment of Geosciences, Eberhard Karls University of Tübingen, Tübingen 72074, Germany<p>Supersaturation of H<span class="inline-formula"><sub>2</sub></span>O during magma ascent can drive efficient degassing through the formation of fluid vesicles within phonolitic melt, with high vesicle number densities (VNDs) ranging from a logVND of 4 to 6 mm<span class="inline-formula"><sup>−3</sup></span>. Rapid vesicle formation and growth reduces magma density and may significantly influence eruption style. However, the behavior of vesicle-bearing melt under continued decompression remains poorly understood. This study aims to investigate vesicle growth as well as the onset and progression of vesicle coalescence during decompression in the well-characterized phonolitic Lower Laacher See melt.</p> <p>Synthetic phonolite melts were hydrated in an internally heated gas pressure vessel at 200 MPa with 5.7 wt % or 5.0 wt % H<span class="inline-formula"><sub>2</sub></span>O, establishing conditions of saturation and slight undersaturation prior to decompression. The hydrated melts were continuously decompressed at a superliquidus temperature of 1323 K with rates ranging from 0.064 to 1.7 MPa s<span class="inline-formula"><sup>−1</sup></span>, reaching final pressures between 70 and 30 MPa before being rapidly quenched to vesiculated glass.</p> <p>As expected, our results indicate that the pressure range for uniform vesicle formation differs between initially H<span class="inline-formula"><sub>2</sub></span>O saturated and undersaturated melts, with coalescence starting at higher pressures in saturated melts. The onset of coalescence is also influenced by the decompression rate, occurring at higher pressures with slower decompression rates. The coalescence process transforms the quenched opaque samples with maximum VND, characterized by nebula of small vesicles sized between 2 and 16 <span class="inline-formula">µm</span> into transparent samples with larger vesicles ranging from 37 <span class="inline-formula">µm</span> up to <span class="inline-formula">∼500</span> <span class="inline-formula">µm</span> and VNDs reduced by orders of magnitude, depending on decompression rate, final pressure, and initial H<span class="inline-formula"><sub>2</sub></span>O concentration. The sizes of these coalesced vesicles are inversely related to the decompression rate, with larger vesicle sizes occurring at slower decompression rates.</p> <p>While the maximum logVNDs of initially formed vesicle volumes remain constant across different decompression rates, with high values of 5.1 to 5.7, compatible with the theory of spinodal decomposition, the logVNDs of sample volumes with coalesced vesicles drop markedly by 1.2 to 4.1 log units. The lowest logVNDs, between 0.5 and 0.8, occur at the slowest decompression rate of 0.064 MPa s<span class="inline-formula"><sup>−1</sup></span>, while the highest logVNDs, between 3.1 and 3.7, are observed at the fastest decompression rate of 1.7 MPa s<span class="inline-formula"><sup>−1</sup></span>. This coalescence driven pattern mimics the expected decompression rate dependence of initial VND in cases of vesicle nucleation.</p> <p>These experimental results suggest that significant vesicle coalescence can occur in phonolitic magmas even at relatively fast ascent rates, influencing eruption dynamics. In the case of open-system outgassing, vesicle coalescence creates channel structures with high permeability, allowing fluid to percolate and escape at the top of the magma column. In contrast, in a closed system, vesicles can accumulate as foam with closed porosity at the top of the magma chamber, forming large, gas-pressurized pockets. This buildup may lead to significant disruptions and violent, explosive eruptions. During volcanic activity, transitions between these two systems may occur.</p>https://ejm.copernicus.org/articles/37/413/2025/ejm-37-413-2025.pdf |
| spellingShingle | P. L. Marks M. Nowak Vesiculation dynamics – Part 2: Decompression-induced H<sub>2</sub>O vesicle growth, onset, and progression of coalescence European Journal of Mineralogy |
| title | Vesiculation dynamics – Part 2: Decompression-induced H<sub>2</sub>O vesicle growth, onset, and progression of coalescence |
| title_full | Vesiculation dynamics – Part 2: Decompression-induced H<sub>2</sub>O vesicle growth, onset, and progression of coalescence |
| title_fullStr | Vesiculation dynamics – Part 2: Decompression-induced H<sub>2</sub>O vesicle growth, onset, and progression of coalescence |
| title_full_unstemmed | Vesiculation dynamics – Part 2: Decompression-induced H<sub>2</sub>O vesicle growth, onset, and progression of coalescence |
| title_short | Vesiculation dynamics – Part 2: Decompression-induced H<sub>2</sub>O vesicle growth, onset, and progression of coalescence |
| title_sort | vesiculation dynamics part 2 decompression induced h sub 2 sub o vesicle growth onset and progression of coalescence |
| url | https://ejm.copernicus.org/articles/37/413/2025/ejm-37-413-2025.pdf |
| work_keys_str_mv | AT plmarks vesiculationdynamicspart2decompressioninducedhsub2subovesiclegrowthonsetandprogressionofcoalescence AT mnowak vesiculationdynamicspart2decompressioninducedhsub2subovesiclegrowthonsetandprogressionofcoalescence |