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.

岩浆脱气的效率取决于熔体中水扩散和粘性流的相互作用。岩浆上升过程中熔体中产生的挥发性过饱和引发气泡形成，气泡中积累了过量的挥发物（即H2O）。另一方面，熔体中水含量的降低会强烈降低熔体粘度。因此，岩浆碎裂可能会发生，特别是在硅质岩浆中。在此类岩浆中，大量水扩散率由水分子的迁移率决定，但也由 OH 基团和 H2O 分子之间的相互转化率决定。在描述硅熔体中水扩散率的模型中，常见的假设是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出玻璃化转变温度的高温和足够长的时间尺度下成立。我们的假设是，岩浆脱气效率在低温和短时间尺度（快速减压）下急剧下降，这仅仅是因为结构松弛（熔体粘度）对于水物种相互转化来说太慢。因此，在这种条件下应用已发表的水扩散方程可能会严重高估水的释放。为了检验这一假设，我们将使用含有 1 - 4 wt% 溶解水的英安石玻璃在玻璃化转变范围内进行脱水实验。显微拉曼光谱将允许使用深度分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有关固定水物种比例的信息。此外，对测量曲线的分析将产生有关水扩散率浓度依赖性的信息，这反映了熔体弛豫对脱气动力学的影响。所获得的数据对于改进低温和/或短时间尺度下的水扩散方程很有意义。这对岩浆脱气和破碎的建模具有影响。