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; 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.

岩浆脱气的效率取决于熔体中水扩散和粘性流的相互作用。在岩浆上升期间产生的熔体中的挥发性过度饱和启动气泡形成，气泡中过量挥发物（即H2O）的积累。另一方面，熔体中水含量的降低会大大降低熔体粘度。因此，岩浆碎片可能特别是在硅质岩浆中发生。在这样的岩浆中，大量的水扩散率取决于水分子的迁移率，也取决于OH基团和H2O分子之间的互转换速率。在描述硅质熔体中水扩散率的模型中，共同的假设是在熔体中建立了水物种的局部平衡。但是，此假设仅在高温下远远超出玻璃过渡和足够长的时间尺度。我们的假设是，岩浆脱气的效率大大降低了低温和短时间尺度（快速减压），仅仅是因为结构弛豫（融化粘度）对于水物种互换而言太慢了。因此，在这种条件下发布的水扩散方程的应用可能会强烈高估水的释放。为了检验这一假设，我们将使用含有1-4 wt％溶解水的Dacitic玻璃的玻璃转变范围进行脱水实验。微拉曼光谱法将允许使用深度分析模式测量具有高空间分辨率的浓度距离曲线。玻璃表面附近的水内容物将提供有关固定水物种比例的信息。此外，对测得的曲线的分析将产生有关水扩散率浓度依赖性的信息，这反映了熔体松弛对脱气动力学的影响。所获得的数据是在低温和/或短时间尺度上改善水扩散方程的感兴趣。这对岩浆脱气和碎片化的建模具有影响。