Quantifying degassing-driven crystal growth in basaltic lavas

熔岩流研究的一个主要目的是发展精确的流动模型，用于预测淹没地区，并估计熔岩在停止之前会前进多远。熔岩流是由晶体、液体和气泡混合而成的复杂流体，在流动过程中，它们冷却并失去挥发性物质（主要是水和二氧化碳），这些挥发性物质最初在地表下的高压下溶解在熔体中。冷却和脱气都会导致熔体结晶，从而对流动推进产生重要影响。冷却是结晶的主要驱动力，但它的影响主要局限于热边界层，在那里它是形成表面地壳和横向分层的一个完整过程。相比之下，脱气并不局限于边界层，而是发生在整个流动过程中，有可能影响整个体流变。尽管在当前的熔岩流模型中考虑了冷却驱动结晶的影响，但由于脱气导致的晶体生长尚未得到充分的量化，因此无法将其纳入模型。在最近的实验室实验中，我们首次能够同时测量脱气和结晶，我们建议通过使用热阶显微镜直接检查晶体的生长来进一步研究。这将提供晶体大小、生长速率和形态方面的数据，以量化脱气对熔岩整体结晶的贡献。最终，这些结果将允许在数值熔岩流模型中考虑脱气引起的结晶。

A primary aim of lava flow research is the development of accurate flow models that can be used to forecast areas of inundation, and to estimate how far lavas will advance before stopping. Lava flows are complex fluids comprising mixtures of crystals, liquid and gas bubbles and, as they flow, they cool and lose volatile species (mainly water and carbon dioxide) that were initially dissolved in the melt at high pressure beneath the surface. Both cooling and degassing lead to crystallisation of the liquid melt, and thus have significant influence on flow advance. Cooling is a major driver of crystallisation, but its effects are mainly restricted to the thermal boundary layers, where it is an integral process in the formation of surface crust and lateral levées. In contrast, degassing is not restricted to boundary layers and occurs throughout flows, with the potential to affect the entire bulk rheology. Although the effects of cooling-driven crystallisation are accounted for in the current generation of lava flow models, crystal growth due to degassing has not yet been sufficiently quantified to allow its incorporation into models. In recent laboratory experiments, we have been able to simultaneously measure degassing and crystallisation for the first time, and we propose to further this research by examining the growth of crystals directly using hot stage microscopy. This will provide the data on crystal sizes, growth rates and morphologies necessary to quantify the contribution of degassing to the overall crystallisation of lavas. Ultimately, these results will allow degassing-induced crystallisation to be accounted for in numerical lava flow models.