Timescales of mingling in shallow magmatic reservoirs

Arrival of magma from depth into shallow reservoirs has been documented as one of the possible processes leading to eruption. Magma intruding and rising to the surface interacts with the already emplaced, degassed magmas residing at shallower depths, leaving chemical signatures in the erupted products. We performed two-dimensional numerical simulations of the arrival of gas-rich magmas into shallow reservoirs. We solve the fluid dynamics for the two interacting magmas, evaluating the space–time evolution of the physical properties of the mixture. Convection and mingling develop quickly into the chamber and feeding conduit/dyke, leading on longer timescales to a density stratification with the lighter, gas-richer magma, mixed with different proportions of the resident magma, rising to the top of the chamber due to buoyancy. Over timescales of hours, the magmas in the reservoir appear to have mingled throughout, and convective patterns become harder to identify. Our simulations have been performed changing the geometry of the shallow reservoir and the gas content of the initial end-member magmas. Horizontally elongated magma chambers, as well as higher density contrasts between the two magmas, cause faster ascent velocities and also increase the mixing efficiency.

深部岩浆（magma）抵达浅部岩浆房（shallow reservoirs），被认为是触发火山喷发（eruption）的潜在过程之一。侵入并上升至地表的岩浆，会与赋存于更浅深度的已就位脱气岩浆（degassed magmas）发生相互作用，并在喷发产物中留下化学特征（chemical signatures）。
我们针对富气岩浆（gas-rich magmas）注入浅部岩浆房的过程开展了二维数值模拟（two-dimensional numerical simulations）：通过求解两种相互作用岩浆的流体动力学（fluid dynamics）控制方程，我们评估了混合体系物理性质的时空演化（space–time evolution）规律。
对流（convection）与混合（mingling）过程会在岩浆房及补给通道/岩墙（feeding conduit/dyke）中快速发育；在更长时间尺度下，体系将形成密度分层（density stratification）结构：更轻、富气的岩浆与不同比例的驻留岩浆混合后，因浮力（buoyancy）作用上升至岩浆房顶部。
在小时级时间尺度内，岩浆房内的岩浆似乎已完全混合，对流模式也难以辨识。
本研究通过改变浅部岩浆房的几何形态（geometry）与初始端元岩浆（initial end-member magmas）的含气量开展了系列模拟试验。结果表明，水平伸长状的岩浆房以及两种岩浆间更大的密度差（density contrasts），会提升岩浆的上升速度（ascent velocities）并增强混合效率（mixing efficiency）。