Abstract Volcanic ash and pyroclasts can weld when deposited hot by pyroclastic density currents, in near-vent fall deposits, or in fractures in volcano interiors. Welding progressively decreases the permeability of the particle packs, influencing a range of magmatic and volcanic processes, including magma outgassing, which is an important control on eruption dynamics. Consequently, there is a need for a quantitative model for permeability evolution during welding of ash and pyroclasts under the range of conditions encountered in nature. Here we present in situ experiments in which hydrous, crystal-free, glassy pyroclasts are imaged via x-ray tomography during welding at high temperature. For each 3D dataset acquired, we determine the porosity, Darcian gas permeability, specific surface area, and pore connectivity. We find that all of these quantities decrease as a critical percolation threshold is approached. We develop a constitutive mathematical model for the evolution of permeability in welding volcanic systems based on percolation theory, and validate the model against our experimental data. Importantly, our model accounts for polydispersivity of the grainsize in the particle pack, the pressures acting on the pack, and changes in particle viscosity arising from degassing of dissolved H2O during welding. Our model is theoretically grounded and has no fitting parameters, hence it should be valid across all magma compositions. The model can be used to predict whether a cooling pyroclast pack will have sufficient time to weld and to degas, the scenarios under which a final deposit will retain a permeable network, the timescales over which sealing occurs, and whether a welded deposit will have disequilibrium or equilibrium H2O content. A user-friendly implementation of the model is provided.

中文翻译：

---

火山熔焊渗透率演化模型

摘要 火山灰和火山碎屑在被火山碎屑密度流、靠近喷口的坠落沉积物或火山内部的裂缝热沉积时可以焊接。焊接会逐渐降低粒子群的渗透性，影响一系列岩浆和火山过程，包括岩浆释气，这是对喷发动力学的重要控制。因此，在自然界遇到的各种条件下，灰烬和火山碎屑焊接过程中需要一个渗透性演变的定量模型。在这里，我们展示了在高温焊接过程中通过 X 射线断层扫描对含水、无晶体、玻璃状火山碎屑进行成像的原位实验。对于获取的每个 3D 数据集，我们确定孔隙度、达西气体渗透率、比表面积和孔隙连通性。我们发现所有这些量随着接近临界渗透阈值而减少。我们基于渗流理论开发了焊接火山系统渗透率演变的本构数学模型，并根据我们的实验数据验证了该模型。重要的是，我们的模型考虑了颗粒堆中颗粒大小的多分散性、作用在颗粒上的压力以及焊接过程中溶解的 H2O 脱气引起的颗粒粘度变化。我们的模型在理论上是有根据的，没有拟合参数，因此它应该适用于所有岩浆成分。该模型可用于预测冷却的火山碎屑堆是否有足够的时间进行焊接和脱气，最终沉积物将保持可渗透网络的情况，发生密封的时间尺度，以及焊接熔敷物是否具有不平衡或平衡 H2O 含量。提供了一个用户友好的模型实现。