New insights into the interplay between water diffusion and viscosity in magma fragmentation

关于岩浆破碎中水扩散和粘度之间相互作用的新见解

• 批准号: 428916619
• 负责人: Professor Dr. Harald Behrens
• 金额: --
• 依托单位: Institut für Mineralogie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2019
• 资助国家: 德国
• 起止时间: 2018-12-31 至 2020-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/428916619?language=en
• 关键词: New; insights; into; interplay; between

The efficiency of magma degassing depends on the interplay of water diffusion and viscous flow in the melt. Volatile oversaturation in the melt generated during magma ascent initiates bubble formation with accumulation of excess volatiles (i.e. H2O) in the bubbles. The decrease of the water content in the melt, on the other hand, strongly reduces the melt viscosity. As a consequence, magma fragmentation may occur in particular in silicic magmas.In such magmas, the bulk water diffusivity is determined by the mobility of water molecules but also by the interconversion rate between OH groups and H2O molecules. In models describing water diffusivity in silicic melts, the common assumption has been that local equilibrium of water species is established in the melt. However, this assumption holds only at high temperature far beyond the glass transition and at sufficiently long time scales. Our hypothesis is that the efficiency of magma degassing strongly decreases towards low temperature and short time scales (rapid decompression), simply because structural relaxation (melt viscosity) is too slow for water species interconversion. Thus, application of published water diffusion equation at such conditions may strongly overestimates the release of water.To test this hypothesis, we will perform dehydration experiments in the range of glass transition with dacitic glasses containing 1 - 4 wt% dissolved water. Micro Raman spectroscopy will allow to measure concentration-distance profiles with high spatial resolution using a depth profiling modus. Water contents near the glass surface will provide information on the fraction of immobile water species. Furthermore, the analysis of the measured profiles will yield information about the concentration dependence of water diffusivity, which reflects the influence of melt relaxation on degassing kinetics. The obtained data are of interest to improve water diffusion equations at low temperature and/or at short time scales. This has implications in modelling of magma degassing and fragmentation.

岩浆脱气的有效性取决于水扩散和熔体粘性流动的相互作用。在岩浆上升过程中产生的熔体中的挥发性过饱和引发气泡形成，气泡中积累了过量的挥发性物质（即H2O）。另一方面，熔体中水含量的降低强烈地降低了熔体粘度。因此，岩浆破碎可能会发生，特别是在岩浆中，在这样的岩浆中，整体水扩散率是由水分子的流动性，但也由OH基团和H2O分子之间的相互转化率。在描述水扩散系数的模型中，通常的假设是在熔体中建立了水物种的局部平衡。然而，这一假设仅在远远超过玻璃化转变的高温和足够长的时间尺度下才成立。我们的假设是，岩浆脱气的效率强烈降低对低温和短时间尺度（快速减压），仅仅是因为结构松弛（熔体粘度）是太慢的水物种相互转换。因此，在这样的条件下，公开的水扩散方程的应用可能会大大高估水的释放。为了验证这一假设，我们将进行脱水实验的范围内的玻璃化转变的英安质玻璃含有1 - 4重量%的溶解水。微拉曼光谱学将允许使用深度剖析模式以高空间分辨率测量浓度-距离分布。玻璃表面附近的水含量将提供关于不动水物种的分数的信息。此外，所测得的配置文件的分析将产生有关的水扩散系数的浓度依赖性的信息，这反映了熔体弛豫脱气动力学的影响。所获得的数据是感兴趣的，以改善水扩散方程在低温和/或在短的时间尺度。这对岩浆脱气和碎裂的建模有影响。