Estimates of olivine–basaltic melt electrical conductivity using a digital rock physics approach
Estimates of olivine–basaltic melt electrical conductivity using a digital rock physics approach
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使用数字岩石物理方法估计橄榄石玄武岩熔体电导率
DOI：
10.1016/j.epsl.2015.10.004
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发表时间：
2015
影响因子：
5.3
通讯作者：
Zhu, Wen-lu
中科院分区：
文献类型：
--
作者：
Miller, Kevin J.;Montési, Laurent G.J.;Zhu, Wen-lu
从大地电磁层析成像(MT)推断的快速扩张的大洋中脊下的熔体含量估计在0.01到0.10之间。这种差异在很大程度上可能是因为缺乏对颗粒尺度熔体几何形状如何影响部分熔融岩石的整体电导率的了解，特别是在低熔体分数时。我们通过模拟实验获得的部分熔融几何形状中的电流，计算了熔体分数从0.02到0.20的橄榄石-玄武岩聚集体的整体电导率。在固体-介质活塞-圆柱体装置中热压圣卡洛斯橄榄石和高铝玄武岩，合成了橄榄石-玄武岩集合体。实验装药的运行条件为1.5 Gpa和1350°C。完成后，对装药进行淬火和取芯。使用同步辐射X射线微型计算机断层扫描(μ-CT)对样品进行成像。由此得到的熔体分布的高分辨率三维(3-D)图像构成了一个数字岩石样品，在其上进行了数值模拟以估计材料的性质。为了计算整体电导率，我们通过求解电流连续性方程来模拟直流测量，假设橄榄石和熔体的电导率。欧姆定律的应用得出了部分熔融区的体电导率。名义干燥材料的体电导值符合幂定律关系σBulk=Cσ熔体ϕm，拟合参数m=1.3±0.3，C=0.66±0.0 6。在相同的部分熔体几何形状上进行了层流流动模拟，以获得渗透率，并比较了电流和流体在相同熔体几何形状上的各自流动路径。结果表明，流体的流动路径与电流的路径不同。电气弯曲度低于流体流动弯曲度。将模拟结果与已有的实验数据进行了比较，并讨论了挥发分和熔体膜对部分熔融岩石电导率的潜在影响。
Estimates of melt content beneath fast-spreading mid-ocean ridges inferred from magnetotelluric tomography (MT) vary between 0.01 and 0.10. Much of this variation may stem from a lack of understanding of how the grain-scale melt geometry influences the bulk electrical conductivity of a partially molten rock, especially at low melt fraction. We compute bulk electrical conductivity of olivine–basalt aggregates over 0.02 to 0.20 melt fraction by simulating electric current in experimentally obtained partially molten geometries. Olivine–basalt aggregates were synthesized by hot-pressing San Carlos olivine and high-alumina basalt in a solid–medium piston-cylinder apparatus. Run conditions for experimental charges were 1.5 GPa and 1350° C. Upon completion, charges were quenched and cored. Samples were imaged using synchrotron X-ray micro-computed tomography (μ-CT). The resulting high-resolution, 3-dimensional (3-D) image of the melt distribution constitutes a digital rock sample, on which numerical simulations were conducted to estimate material properties. To compute bulk electrical conductivity, we simulated a direct current measurement by solving the current continuity equation, assuming electrical conductivities for olivine and melt. An application of Ohm's Law yields the bulk electrical conductivity of the partially molten region. The bulk electrical conductivity values for nominally dry materials follow a power-law relationship σ bulk= C σ melt ϕ m with fit parameters m= 1.3±0.3 and C= 0.66±0.06. Laminar fluid flow simulations were conducted on the same partially molten geometries to obtain permeability, and the respective pathways for electrical current and fluid flow over the same melt geometry were compared. Our results indicate that the pathways for flow fluid are different from those for electric current. Electrical tortuosity is lower than fluid flow tortuosity. The simulation results are compared to existing experimental data, and the potential influence of volatiles and melt films on electrical conductivity of partially molten rocks is discussed.