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

岩浆脱气的效率取决于水在熔体中的扩散和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分过饱和，启动了气泡的形成，并在气泡中积累了过量的挥发物(即H2O)。另一方面，熔体中水分含量的减少会强烈地降低熔体的粘度。因此，特别是在硅质岩浆中，可能会发生岩浆碎裂。在这种岩浆中，水的整体扩散系数不仅取决于水分子的迁移性，还取决于羟基和H2O分子之间的相互转化速率。在描述水在硅质熔体中的扩散率的模型中，通常的假设是在熔体中建立了水物种的局部平衡。然而，这一假设只有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，岩浆的脱气效率朝着低温和短时间尺度(快速减压)强烈下降，原因很简单，因为结构松弛(熔体粘度)太慢，不利于水物种之间的相互转化。因此，在这种情况下应用已发表的水扩散方程可能会严重高估水的释放。为了验证这一假设，我们将在含有1-4wt%溶解水的英安玻璃的玻璃化转变范围内进行脱水实验。微拉曼光谱将允许使用深度剖面法以高空间分辨率测量浓度-距离剖面。玻璃表面附近的水分含量将提供关于静止水物种比例的信息。此外，对测量轮廓的分析将得到水扩散系数的浓度依赖关系的信息，这反映了熔体松弛对脱气动力学的影响。所获得的数据对改进低温和/或短时间尺度下的水扩散方程具有重要意义。这对岩浆脱气和碎裂的模拟有一定的意义。

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.