Frictional melt homogenisation during fault slip: Geochemical, textural and rheological fingerprints
Frictional melt homogenisation during fault slip: Geochemical, textural and rheological fingerprints
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DOI：
10.1016/j.gca.2019.04.010
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发表时间：
2019-06
影响因子：
5
通讯作者：
Paul A. Wallace;Sarah H. De Angelis;A. Hornby;J. Kendrick;S. Clesham;Felix W. von Aulock;Amy Hughes;J. Utley;T. Hirose;Donald Bruce Dingwell;Y. Lavallée
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文献类型：
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作者：
Paul A. Wallace;Sarah H. De Angelis;A. Hornby;J. Kendrick;S. Clesham;Felix W. von Aulock;Amy Hughes;J. Utley;T. Hirose;Donald Bruce Dingwell;Y. Lavallée
火山环境通常代表结构活跃的设置，其中应变局部化可以促进断层，摩擦变形，以及随后的熔融沿着断层面。这种摩擦熔化是一种不平衡过程，由单个矿物相的选择性熔化和火山玻璃在其玻璃化转变时的软化引发，作为对快速摩擦加热的响应。在断层运动期间，在断层面表面上形成的薄熔融层可以急剧加速或终止滑动。摩擦熔体的物理和化学性质的全面理解是必要的滑动机制的全面评估，摩擦流变学取决于选择性熔融矿物和玻璃相的贡献，以及悬浮在摩擦熔体中的restite碎片的物理效应。在这里，我们实验研究的影响，主岩矿物学上的成分和纹理的演变摩擦熔体在滑动。高速旋转剪切（HVR）实验是在受控的火山相关同震条件下进行的（1 m s-1滑动速率和1 MPa正应力）使用三个具有对比矿物组合的中间穹状熔岩，从断层摩擦明显的火山系统中取样：（1）含角闪石的安山岩（Soufrière Hills火山，蒙特塞拉特）;（2）贫角闪石的英安岩（危地马拉的Bagaguito圆顶复合体）;和（3）无角闪石的安山岩（Volcán de Colima，墨西哥）。对于每个样品，五个HVR实验在摩擦熔体演化的不同阶段终止，即：（1）在开始熔化时，（2）在形成稳态熔体层时，和（3）在稳态条件下滑动5 m、（4）10 m和（5）15 m之后。通过双扩散对流在摩擦熔体层内的选择性单相熔体的渐进混合和均质化证明了熔体组合物对滑移行为的控制。角闪石优先熔融，导致持续较低的剪切应力（小于角闪石贫样品）和明显的剪切弱化过程中的角闪石含熔岩的摩擦熔融。结果突出了矿物组合对断层滑动的影响，包括导管流动过程，这可能会影响喷发的风格，和快速颗粒流的流出距离。
Volcanic environments often represent structurally active settings where strain localisation can promote faulting, frictional deformation, and subsequent melting along fault planes. Such frictional melting is thermodynamically a disequilibrium process initiated by selective melting of individual mineral phases and softening of volcanic glass at its glass transition as a response to rapid frictional heating. The formation of a thin melt layer on a fault plane surface can drastically accelerate or terminate slip during fault motion. A comprehensive understanding of the physical and chemical properties of the frictional melt is required for a full assessment of slip mechanisms, as frictional rheology depends on the contributions from selectively melted mineral and glass phases as well as the physical effects of restite fragments suspended in the frictional melt. Here, we experimentally investigate the impact of host-rock mineralogy on the compositional and textural evolution of a frictional melt during slip. High-velocity rotary shear (HVR) experiments were performed under controlled, volcanically relevant, coseismic conditions (1 m s−1slip rate and 1 MPa normal stress) using three intermediate dome lavas with contrasting mineral assemblages, sampled from volcanic systems where fault friction is evident: (1) an amphibole-bearing andesite (Soufrière Hills Volcano, Montserrat); (2) an amphibole-poor dacite (Santiaguito dome complex, Guatemala); and (3) an amphibole-free andesite (Volcán de Colima, Mexico). For each sample, five HVR experiments were terminated at different stages of frictional melt evolution, namely: (1) at the onset of melting, (2) upon formation of a steady-state melt layer, and (3) after 5 m, (4) 10 m, and (5) 15 m of slip at steady-state conditions. Progressive mixing and homogenisation of selective, single-phase melts within the frictional melt layer through double-diffusion convection demonstrates the control of melt composition on slip behaviour. Amphiboles melted preferentially, leading to consistently lower shear stress (∼1 MPa less than amphibole-poor samples) and pronounced shear-weakening during the frictional melting of amphibole-bearing lavas. The results highlight the implications of mineral assemblages on fault slip, including conduit flow processes, which may influence the style of eruptions, and run-out distances of rapid granular flows.