Abstract In the last few decades, advanced monitoring networks have been extended to the main active volcanoes, providing warnings for variations in volcano dynamics. However, one of the main tasks of modern volcanology is the correct interpretation of surface-monitored signals in terms of magma transfer through the Earth's crust. In this frame, it is crucial to investigate decompression-induced magma degassing as it controls magma ascent towards the surface and, in case of eruption, the eruptive style and the atmospheric dispersal of tephra and gases. Understanding the degassing behaviour is particularly intriguing in the case of poorly explored evolved alkaline magmas. In fact, these melts frequently feed hazardous, highly explosive volcanoes (e.g., Campi Flegrei, Somma-Vesuvius, Colli Albani, Tambora, Azores and Canary Islands), despite their low viscosity that usually promotes effusive and/or weakly explosive eruptions. Decompression experiments, together with numerical models, are powerful tools to examine magma degassing behaviour and constrain field observations from natural eruptive products and monitoring signals. These approaches have been recently applied to evolved alkaline melts, yet numerous open questions remain. To cast new light on the degassing dynamics of evolved alkaline magmas, in this study we present new results from decompression experiments, as well as a critical review of previous experimental works. We achieved a comprehensive dataset of key petrological parameters (i.e., 3D textural data for bubbles and microlites using X-ray computed microtomography, glass volatile contents and nanolite occurrence) from experimental samples obtained through high temperature-high pressure isothermal decompression experiments on trachytic alkaline melts at super-liquidus temperature. We explored systematically a range of final pressures (from 200 to 25 MPa), decompression rates (from 0.01 to 1 MPa s−1), and volatile (H2O and CO2) contents. On these grounds, we integrated coherently literature data from decompression experiments on evolved alkaline (trachytic and phonolitic) melts under various conditions, with the aim to fully constrain the degassing mechanisms and timescales in these magmas. Finally, we simulated numerically the experimental conditions to evaluate strengths and weaknesses in decrypting degassing mechanisms and timescale from field observations. Our results highlight that bubble formation in evolved alkaline melts is primarily controlled by the initial volatile (H2O and CO2) content during magma storage. In these melts, bubble nucleation needs low supersaturation pressures (≤ 50–112 MPa for homogeneous nucleation, ≤ 13–25 MPa for heterogeneous nucleation), resulting in high bubble number density (~ 1012–1016 m−3), efficient volatile exsolution and thus in severe rheological changes. Moreover, the bubble number density is amplified in CO2-rich melts (mole fraction XCO2 ≥ 0.5), in which continuous bubble nucleation predominates on growth. These conditions typically lead to highly explosive eruptions. However, moving towards slower decompression rates (≤ 10−1 MPa s−1) and H2O-rich melts, permeable outgassing and inertial fragmentation occur, promoting weakly explosive eruptions. Finally, our findings suggest that the exhaustion of CO2 at deep levels, and the consequent transition to a H2O-dominated degassing, can crucially enhance magma vesiculation and ascent. In a hazard perspective, these constraints allow to postulate that time-depth variations of unrest signals could be significantly weaker/shorter (e.g., minor gas emissions and short-term seismicity) during major eruptions than in small-scale events.

中文翻译：

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演化碱性岩浆中的脱气动力学：岩石学、实验和理论见解

摘要 在过去的几十年里，先进的监测网络已经扩展到主要的活火山，为火山动力学的变化提供预警。然而，现代火山学的主要任务之一是根据通过地壳的岩浆转移来正确解释地表监测信号。在这个框架中，研究减压引起的岩浆脱气是至关重要的，因为它控制着岩浆上升到地表，以及在喷发的情况下，火山灰和气体的喷发方式和大气扩散。在对演化的碱性岩浆探索不足的情况下，了解脱气行为特别有趣。事实上，这些熔体经常为危险的、高度爆炸性的火山（例如 Campi Flegrei、Somma-Vesuvius、Colli Albani、Tambora、Azores 和 Canary Islands）提供能量，尽管它们的粘度低，通常会促进喷发和/或微弱的爆发性喷发。减压实验与数值模型一起，是检查岩浆脱气行为和限制自然喷发产物和监测信号的现场观测的有力工具。这些方法最近已应用于进化的碱性熔体，但仍有许多悬而未决的问题。为了对演化出的碱性岩浆的脱气动力学有新的认识，在这项研究中，我们提出了减压实验的新结果，以及对以前实验工作的批判性回顾。我们获得了关键岩石学参数的综合数据集（即使用 X 射线计算机显微断层扫描的气泡和微晶石的 3D 纹理数据，在超液相线温度下对粗面碱性熔体进行高温-高压等温减压实验获得的实验样品中的玻璃挥发物含量和纳米晶发生率。我们系统地探索了一系列最终压力（从 200 到 25 MPa）、减压速率（从 0.01 到 1 MPa s-1）和挥发物（H2O 和 CO2）含量。基于这些理由，我们整合了来自不同条件下演化的碱性（粗面和音质）熔体的减压实验的相关文献数据，目的是充分限制这些岩浆中的脱气机制和时间尺度。最后，我们对实验条件进行了数值模拟，以评估从现场观察中解密脱气机制和时间尺度的优缺点。我们的结果强调，演化出的碱性熔体中气泡的形成主要受岩浆储存过程中初始挥发性（H2O 和 CO2）含量的控制。在这些熔体中，气泡成核需要低过饱和压力（均相成核≤ 50-112 MPa，异质成核≤ 13-25 MPa），从而产生高气泡数密度（~1012-1016 m-3）、有效的挥发性脱溶和因此在剧烈的流变变化中。此外，在富含 CO2 的熔体（摩尔分数 XCO2 ≥ 0.5）中气泡数密度被放大，其中连续气泡成核在生长中占主导地位。这些条件通常会导致高度爆炸性的喷发。然而，朝着更慢的减压速率（≤ 10-1 MPa s-1）和富含 H2O 的熔体移动，会发生可渗透的除气和惯性碎裂，促进微弱的爆发性喷发。最后，我们的研究结果表明，深层次二氧化碳的枯竭，以及随之而来的以 H2O 为主的脱气的转变，可以至关重要地增强岩浆囊泡和上升。从灾害的角度来看，这些约束允许假设在大爆发期间，与小规模事件相比，动荡信号的时间深度变化可能明显更弱/更短（例如，少量气体排放和短期地震活动）。