Petford Nick, Mirhadizadeh Seyed
Faculty of Arts, Science and Technology, University of Northampton, Northampton, UK.
R Soc Open Sci. 2017 May 31;4(5):161083. doi: 10.1098/rsos.161083. eCollection 2017 May.
The McMurdo Dry Valleys magmatic system, Antarctica, provides a world-class example of pervasive lateral magma flow on a continental scale. The lowermost intrusion (Basement Sill) offers detailed sections through the now frozen particle microstructure of a congested magma slurry. We simulated the flow regime in two and three dimensions using numerical models built on a finite-element mesh derived from field data. The model captures the flow behaviour of the Basement Sill magma over a viscosity range of 1-10 Pa s where the higher end (greater than or equal to 10 Pa s) corresponds to a magmatic slurry with crystal fractions varying between 30 and 70%. A novel feature of the model is the discovery of transient, low viscosity (less than or equal to 50 Pa s) high Reynolds number eddies formed along undulating contacts at the floor and roof of the intrusion. Numerical tracing of particle orbits implies crystals trapped in eddies segregate according to their mass density. Recovered shear strain rates (10-10 s) at viscosities equating to high particle concentrations (around more than 40%) in the Sill interior point to shear-thinning as an explanation for some types of magmatic layering there. Model transport rates for the Sill magmas imply a maximum emplacement time of 10 years, consistent with geochemical evidence for long-range lateral flow. It is a theoretically possibility that fast-flowing magma on a continental scale will be susceptible to planetary-scale rotational forces.
南极洲麦克默多干谷岩浆系统是大陆尺度上广泛侧向岩浆流动的世界级范例。最底部的侵入体（基底岩床）呈现了如今已冻结的拥堵岩浆浆体颗粒微观结构的详细剖面。我们利用基于实地数据得出的有限元网格构建的数值模型，对二维和三维的流动状态进行了模拟。该模型捕捉了基底岩床岩浆在1 - 10帕斯卡·秒的粘度范围内的流动行为，其中较高粘度端（大于或等于10帕斯卡·秒）对应晶体含量在30%至70%之间变化的岩浆浆体。该模型的一个新特点是发现了沿着侵入体底部和顶部起伏接触面形成的瞬态、低粘度（小于或等于50帕斯卡·秒）高雷诺数涡旋。颗粒轨道的数值追踪表明，被困在涡旋中的晶体根据其质量密度进行分离。在与岩床内部高颗粒浓度（约40%以上）相当的粘度下恢复的剪切应变率（10 - 10秒）表明，剪切变稀是那里某些类型岩浆分层的一种解释。岩床岩浆的模型输运速率意味着最大侵位时间为10年，这与远距离侧向流动的地球化学证据一致。从理论上讲，大陆尺度上快速流动的岩浆有可能受到行星尺度旋转力的影响。
