Melt degassing triggered by magma injection?

<p>Explosive eruptions of silicic magmas depend mainly on the amount and the degassing behavior of soluble volatile components like H<sub>2</sub>O and CO<sub>2</sub>. The injection of a hot mafic magma into a cooler volatile-rich rhyolitic magma chamber might initiate mingling and mixing processes at the interface of the two melt reservoirs (Paredes-Marino et al. 2017). An accompanying increase in temperature and a buoyant ascent of the H<sub>2</sub>O-saturated rhyolitic melt may cause a sufficiently high decrease in solubility at pressures < 300 MPa (e.g. Holtz et al. 1995) to trigger vesicle formation. Furthermore, the interface between different melt compositions might act as a site for enhanced vesicle formation. To test this hypothesis, bimodal decompression experiments were conducted. Basaltic and rhyolitic compositions similar to the Askja eruption 1875 in Iceland (Sparks and Sigurdsson 1977) were used for this purpose. For the preparation of the experiments, rhyolitic and basaltic glass cylinders were molten and hydrated separately in an internally heated argon pressure vessel with H<sub>2</sub>O excess at 200 MPa and 1523 K for 96–168 h and then isobarically quenched with 16 K∙s<sup>‑1</sup>. The hydrated glass samples were cut perpendicular to the cylinder axis. The cylinder faces were polished to enable a perfect contact of the rhyolite cylinder with the basalt cylinder. An additional decompression experiment with two contacted hydrated rhyolite cylinders was conducted as a reference to test the experimental setup.</p><p>Each pair of cylinders was heated isobarically with 25 K·s<sup>-1</sup> to 1348 K at 210 MPa and equilibrated for 10 min. To simulate the magma ascent, three bimodal samples and the reference sample were decompressed with rates of 0.17 MPa∙s<sup>-1 </sup>or 1.7 MPa∙s<sup>-1</sup> to the final pressure of 100 MPa and then quenched with 44 K∙s<sup>-1</sup>. H<sub>2</sub>O vesicle number and spatial distribution as well as the H<sub>2</sub>O contents in the decompressed samples were analysed by microscope, quantitative BSE image analysis, and FTIR-spectroscopy, respectively.</p><p>All decompression experiments resulted in vesiculated samples. In the rhyolite reference experiment, the H<sub>2</sub>O vesicles are homogeneously distributed within the whole sample. The former interface of the cylinders is no longer visible. This confirms that the former contact plane of the cylinders does not influence the degassing behaviour during decompression.</p><p>Optical examination and electron microprobe analysis of oxide diffusion profiles of the decompressed bimodal samples expose the development of a hybrid melt zone between the rhyolite and the partially crystallized basalt, documenting mixing processes during the decompression experiments (Petri 2020). The hybrid zone in the rhyolitic compositional dominated region is decorated with an enhanced number of H<sub>2</sub>O vesicles compared to the rhyolitic and basaltic glass volumes. This suggests that the injection of a basaltic melt into a rhyolitic melt reservoir may lead to significantly enhanced homogeneous H<sub>2</sub>O vesicle formation in the hybrid zone and, therefore, enhanced degassing with the concomitant triggering of explosive eruptions.</p><p> </p><p>Holtz F. et al. (1995) American Mineralogist 80: 84-108.</p><p>Paredes-Marino J. et al. (2017) Scientific Reports 7: 16897.</p><p>Petri P. (2020) Master thesis University of Tübingen.</p><p>Sparks S.R.J. and Sigurdsson H. (1977) Nature 267: 315-318.</p>

Door-Triggering Mechanism for Large-Scale Rapid-Decompression Experiments

For large-scale rapid-decompression experiments, a new door-triggering mechanism is proposed for a 750 mm diameter pressure relief channel. Quick opening of the door is realized by utilizing a spring-based release mechanism to instantly convert large amounts of elastic potential energy into kinetic energy. To counteract the significant inertial effect of the high-speed door on the chamber, a flywheel-based cushioning mechanism is designed to absorb the kinetic energy of the door after opening. This carefully designed mechanism consists of the closing mechanism, energy storage unit, locking/releasing mechanism, and cushioning mechanism. Kinetic models are established to analyze the dynamic properties. Simulation results reveal that it takes approximately 280 ms for the door to open from 0° to 90°. This work can provide insights for the development of large-scale rapid-decompression equipment in the future.

Caldera-forming eruptions of mushy magma modulated by feedbacks between ascent rate, gas retention/loss and bubble/crystal framework interaction

Caldera-forming eruptions of mushy silicic magma are among the most catastrophic natural events on Earth. In such magmas, crystals form an interlocking framework when their content reaches critical thresholds, resulting in the dramatic increase in viscous resistance to flow. Here, we propose a new mechanism for the ascent of mushy magma based on microstructural observations of crystal-rich silicic pumices and lavas from the Central Andes and decompression experiments. Microstructural data include spherical vesicles and jigsaw-puzzle association of broken crystals in pumices, whereas there is limited breakage of crystals in lavas. These observations insinuate that shearing of magma during ascent was limited. Decompression experiments reveal contrasting interaction between growing gas bubbles and the crystal framework in crystal-rich magma. Under slow decompression typical of effusive eruptions, gas extraction is promoted, whereas under rapid decompression, bubbles are retained and the crystal framework collapses. This feedback between decompression rate, retention of gas bubbles, and integrity of the crystal framework leads to strong non-linearity between magma decompression rate and eruption explosivity. We extend these findings to caldera-forming eruptions of crystal-rich magma where large overpressures are induced by caldera-collapse, resulting in magma plug-flow, rapid decompression facilitated by shear-localization at conduit margins, and explosive eruption.