Melt migration in crystal mushes by viscous fingering: insights from high-temperature, high-pressure experiments (DATA)

BSE mosaics of mushes and experimental products.

Abstract: "We conducted experiments to study melt migration in crystal-rich mushes, with application to magma ascent within transcrustal magma reservoirs. Mushes with crystal volume fractions of 0.59 to 0.83 were prepared by hot-pressing crushed borosilicate glass mixed with different amounts quartz sand particles. Each experimental sample comprises stacked disks of mush and soda-lime glass, a proxy for crystal-free magma. Samples were subjected to confining pressures of 100 to 300 MPa and a temperature of 900°C (above the glass transition temperatures of the borosilicate and soda-lime glasses) for up to 6 h. The bottom and circumference of the mush and soda lime disks experience the confining pressure, but the top of the mush disks are at room pressure, resulting in a pore-pressure gradient across the mush layer. Following cooling and decompression, we determined the area fraction and morphology of soda-lime melt that migrated into the mush layer during experiments. Melt fraction is more strongly correlated to crystal fraction than pore-pressure gradient, increasing with crystal fraction before sharply decreasing as crystal fractions exceed 0.8. This change at 0.8 coincides with the transition from crystals in the mush moving during soda-lime migration to crystals forming a continuous rigid network. In our experiments, melt migration occurred by viscous fingering, but near the mobile-to-rigid transition, melt migration is enhanced by additional capillary action. Our results indicate that magma migration may peak when rigid mushes “unlock” to become mobile. This transition may mark an increase in magma migration, a potential precursor to volcanic unrest and eruption."

Imaging: "Transverse sections cut from the top and/or bottom of the vacuum hot-pressed mushes were polished, carbon-coated, and imaged in BSE mode using the JEOL JXA-8530FPlus Electron Probe Microanalyzer (EPMA) at UMN (15 kV, 10 nA). About ten 50x magnification images were taken per sample and then compiled into BSE mosaics using Affinity Designer. The different compositions of the borosilicate glass and crystalline materials are distinguishable by greyscale in BSE images. [...] Following each experiment, sample assemblies were cut longitudinally along the cylindrical axis to produce sections for microstructural analysis. Scored samples (pHi-19s, Int-20s, Lo-21s) were cut again to produce sections tangential to the sample cylinder. Cut sections were vacuum impregnated with EpoFix resin and hand-polished on diamond lapping film from 30 to 0.5 μm grit. Polished and carbon-coated samples were imaged in BSE mode in the EPMA at UMN (15 kV, 10 nA). The different compositions of the soda-lime glass, borosilicate glass, and crystalline materials are distinguishable by greyscale in BSE images. Twenty to forty 50x magnification images were taken per sample and then compiled into sample-scale BSE mosaics using Affinity Designer."

BSE 熔融岩屑与实验样品的镶嵌图像。

摘要：本研究旨在探究富含晶体的熔融岩屑中的熔融迁移现象，并将其应用于地壳下熔浆储库中熔浆上升过程。通过热压法将硼硅酸盐玻璃与不同比例的石英砂颗粒混合，制备了晶体积分数为 0.59 至 0.83 的熔融岩屑样品。每个实验样品由熔融岩屑与钠钙玻璃（无晶体熔浆的代用品）的叠层圆盘组成。样品在 100 至 300 MPa 的围压和 900°C（高于硼硅酸盐和钠钙玻璃的玻璃化转变温度）下，持续加热至 6 小时。熔融岩屑和钠钙玻璃圆盘的底部及边缘承受围压，而熔融岩屑圆盘顶部处于常压，从而在熔融岩屑层中形成孔隙压力梯度。冷却和减压后，我们确定了实验过程中迁移至熔融岩屑层的钠钙熔融物的面积分数和形态。熔融分数与晶体积分数的相关性高于孔隙压力梯度，随着晶体积分数的增加而增大，但当晶体积分数超过 0.8 时，则急剧下降。这一变化与熔融岩屑层中的晶体在钠钙熔融迁移过程中移动转变为形成连续刚性网络的转变相一致。在我们的实验中，熔融迁移是通过粘性指状结构发生的，但在移动到刚性转变附近时，熔融迁移受到额外的毛细作用增强。我们的结果表明，当刚性熔融岩屑‘解锁’变为流动状态时，熔浆迁移可能达到峰值。这种转变可能标志着熔浆迁移的增加，这可能是火山活动不稳定和喷发的前兆。

成像：从真空热压熔融岩屑的顶部和/或底部切取横截面，经过抛光、碳涂层后，在明尼苏达大学（UMN）的 JEOL JXA-8530FPlus 电子探针微分析仪（EPMA）下以 BSE 模式成像（15 kV，10 nA）。每个样品拍摄约十张 50 倍放大图像，然后使用 Affinity Designer 编制成 BSE 镶嵌图像。BSE 图像中的不同成分（硼硅酸盐玻璃和结晶材料）可通过灰度区分。在每个实验之后，样品组件沿圆柱轴纵向切割以制备用于显微结构分析的截面。打分样品（pHi-19s，Int-20s，Lo-21s）再次切割以产生与样品圆柱相切的截面。切割的截面用 EpoFix 树脂真空浸渍，并在从 30 到 0.5 μm 粒度的不锈钢抛光带上手工抛光。抛光并碳涂层的样品在 EPMA 下以 BSE 模式成像（15 kV，10 nA）。BSE 图像中的不同成分（钠钙玻璃、硼硅酸盐玻璃和结晶材料）可通过灰度区分。每个样品拍摄 20 至 40 张 50 倍放大图像，然后使用 Affinity Designer 编制成样品级 BSE 镶嵌图像。