Rheology of magmas with bimodal crystal size and shape distributions: Insights from analog experiments
Rheology of magmas with bimodal crystal size and shape distributions: Insights from analog experiments
复制标题
DOI：
10.1029/2011gc003606
复制
发表时间：
2011-07-28
影响因子：
3.5
通讯作者：
Mader, H. M.
中科院分区：
文献类型：
--
作者：
Cimarelli, C.;Costa, A.;Mader, H. M.
火山管道中的岩浆通常含有与先前存在的斑晶有关的微粒岩，这通常由火山岩的结构所指示。在这项研究中，我们提出了两个不同的实验，调查这些双分散系统的流动行为。在第一个实验中，使用旋转流变方法来确定在无气泡牛顿液体（硅酸盐熔体）中由较小的扁长颗粒（微晶）和较大的等长颗粒（斑晶）组成的单分散和多分散悬浮液的流变性。我们的数据表明，增加相对比例的长细晶等长斑晶在岩浆中的总颗粒含量不变，可以增加相对粘度高达三个数量级。因此，在岩浆中的颗粒的流变效应不能通过假设一个单分散的粒子群来模拟。我们提出了一个新的模型，使用内插参数的基础上的相对比例的小颗粒和大颗粒，并产生了相当大的改进，适合的数据比以前的模型。在第二个系列的实验中，我们调查的纹理所产生的剪切双峰悬浮液逐渐固化环氧树脂在一个同心圆筒设置。由此产生的纹理显示扁长颗粒与流线对齐，并且在组织良好的弦中发现了球形颗粒，在高剪切区域存在球形耗尽的剪切带。这些观察结果可以解释所测量的剪切变稀和屈服应力行为的变化与固体分数和颗粒纵横比的增加。岩浆流动的影响进行了讨论，流变学的结果和纹理的意见进行了比较与观察自然样品。
Magmas in volcanic conduits commonly contain microlites in association with preexisting phenocrysts, as often indicated by volcanic rock textures. In this study, we present two different experiments that investigate the flow behavior of these bidisperse systems. In the first experiments, rotational rheometric methods are used to determine the rheology of monodisperse and polydisperse suspensions consisting of smaller, prolate particles (microlites) and larger, equant particles (phenocrysts) in a bubble-free Newtonian liquid (silicate melt). Our data show that increasing the relative proportion of prolate microlites to equant phenocrysts in a magma at constant total particle content can increase the relative viscosity by up to three orders of magnitude. Consequently, the rheological effect of particles in magmas cannot be modeled by assuming a monodisperse population of particles. We propose a new model that uses interpolated parameters based on the relative proportions of small and large particles and produces a considerably improved fit to the data than earlier models. In a second series of experiments we investigate the textures produced by shearing bimodal suspensions in gradually solidifying epoxy resin in a concentric cylinder setup. The resulting textures show the prolate particles are aligned with the flow lines and spherical particles are found in well-organized strings, with sphere-depleted shear bands in high-shear regions. These observations may explain the measured variation in the shear thinning and yield stress behavior with increasing solid fraction and particle aspect ratio. The implications for magma flow are discussed, and rheological results and textural observations are compared with observations on natural samples.