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Microscale textural heterogeneity and tip-streaming instability in alkaline magmas: Evidence in tube pumices from Campi Flegrei, Italy
Journal of Volcanology and Geothermal Research ( IF 2.9 ) Pub Date : 2021-02-18 , DOI: 10.1016/j.jvolgeores.2021.107200
Monica Piochi , Margherita Polacci , Fabio Arzilli , Guido Ventura

We present a textural investigation of compositionally homogeneous alkaline tube pumices from the plinian fallouts of the Agnano Monte Spina eruption (ca. 4.1 ka), Campi Flegrei, Italy. The pumice clasts present strong textural complexity from hand specimen to microscopic scale of inspection. The clasts are characterized by large vesicle tubes that extend up to several millimetres along the clast main axis. They also exhibit small and elongated vesicles arranged in stretched bands around textural domains of larger, sub-spherical vesicles associated with pyroxene, feldspar, and biotite phenocrysts. Clasts, vesicle tubes, vesicle deformation, major axes of pyroxene phenocrysts and crystals with size ≤100 μm align in the same direction. In addition, we observe chains of gradually smaller stretched vesicles likely generated from break-up of a larger parent vesicle (i.e., tip-streaming phenomenon). We use image analysis to quantify the heterogeneity of vesicle sizes and shapes. Then, we deduce the Capillary number (Ca) of the each single vesicle, which represents the ratio between the deforming viscous stress on a bubble and the restoring stress supplied by surface tension, and obtain the spatial distribution of Ca values within pumice clasts. As Ca depends on strain-rate and viscosity, the spatial distribution of Ca values provides us with a snapshot of the sub-millimetre scale variation in magma strain-rate and rheology before fragmentation. Ca in highly stretched vesicles is close to values (0.45 to 0.6) required for bubble-breakup induced by tip-streaming. Instead, low Ca values (0.3 to 0.45) are calculated for large sub-spherical vesicles mostly distributed around phenocrysts. We suggest that sub-millimetre heterogeneities in vesicle sizes and shapes recorded in tube pumices are caused by a combination of tip-streaming, heterogeneous bubble nucleation and second-boiling processes prior to fragmentation. The heterogeneous spatial distribution of Ca values may be the result of local micrometric shear localisation and consequent deformation upon break-up experienced by magma while ascending in the conduit. This deformation suggests that a variation of magma strain-rate and viscosity occurred before fragmentation at the clast microscale and this is likely to affect magma dynamics and fragmentation. Our results claim for future experimental and numerical modelling studies that investigate and consider the effect of such micrometric heterogeneities on magma rheology and flow condition variations inside volcanic conduits.



中文翻译:

碱性岩浆中微观尺度的质地异质性和尖端流不稳定性:来自意大利Campi Flegrei的管脓袋的证据

我们对来自意大利Campi Flegrei的Agnano Monte Spina火山喷发(约4.1 ka)的普林岩屑组成的均质碱性管粉刺进行了质地调查。浮石碎屑呈现出从手标本到检查的微观尺度的强烈纹理复杂性。夹层的特征在于大的囊泡管沿夹层主轴线延伸至几毫米。它们还表现出小而细长的囊泡,这些囊泡围绕着与辉石,长石和黑云母隐晶球相关的较大的亚球形囊泡的纹理区域围绕伸展带排列。碎屑,囊泡管,囊泡变形,辉石ox的长轴和尺寸≤100μm的晶体排列在同一方向上。此外,我们观察到可能由较大的亲本囊泡破裂产生的逐渐变细的囊泡链(即,尖端流现象)。我们使用图像分析来量化囊泡大小和形状的异质性。然后,我们推导毛细管数(Ca)表示气泡上的变形粘性应力与表面张力所提供的恢复应力之间的比率,并获得浮石碎屑中Ca值的空间分布。由于Ca取决于应变率和黏度,因此Ca值的空间分布为我们提供了破碎前岩浆应变率和流变性的亚毫米级变化的快照。高拉伸囊泡中的Ca接近由尖端流引起的气泡破裂所需的值(0.45至0.6)。相反,低钙对于主要分布在表晶周围的较大的亚球形囊泡,可计算出值(0.3至0.45)。我们建议在管腔内记录的囊泡大小和形状中的亚毫米异质性是由碎裂之前的尖端流,异质气泡成核和二次沸腾过程共同引起的。Ca的异质空间分布值可能是局部测微剪切局部化的结果,是岩浆在管道中上升时经历的破裂时变形的结果。这种变形表明岩浆应变率和黏度的变化在碎裂微观尺度发生之前就发生了,这很可能影响岩浆动力学和破碎。我们的结果要求进行进一步的实验和数值模型研究,以研究和考虑此类微观异质性对火山岩流变学和火山管道内流动条件变化的影响。

更新日期:2021-02-28
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