Electrical conductivity of amphibolite at high temperature and high pressure and its geophysical implications
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摘要:
中下地壳和俯冲带区域的高电导率异常（0.01~1 S·m-1）可能与地球内部的特定物质及其变化有关.斜长角闪岩是中下地壳以及俯冲带区域的重要组成之一，高温高压下斜长角闪岩的电导率研究对认识电导率异常具有重要意义.本研究采用交流阻抗谱法，在0.5，1.0，1.5 GPa和473~1073 K条件下测量了天然斜长角闪岩样品的复阻抗，实验结果表明压力对斜长角闪岩的电导率影响非常小，而温度对于电导率影响非常显著，其电导率在1073 K可以达到10-0.5 S·m-1；实验获得的活化能值为52.21 kJ·mol-1，推断其导电机制可能为小极化子传导（Fe2+的氧化）主导.结合本实验获得的结果与大陆岩石圈和俯冲带的温度结构，我们计算得到相应的电性结构剖面，并与三种不同构造背景下的大陆岩石圈（克拉通、大陆裂谷和活动造山带）和俯冲带区域的电磁剖面结构进行了对比研究，结果发现斜长角闪岩可以解释大陆裂谷和活动造山带构造背景下的莫霍面附近的高电导率异常现象，同时可能是导致较热的俯冲带区域（例如卡斯卡迪地区）高电导率异常现象的原因.
Abstract:High electrical conductivity anomalies (0.01~1 S·m-1) at continental middle-lower crust as well as subduction zones at different depths detected by Magnetotelluric (MT) are often associated with certain chemical compositions and their changes. Amphibolite is one of the most important components of the continental lower crust and subduction regions. The study of electrical conductivity of amphibolite at different pressures and temperatures is important to provide reasonable explanation for the high electrical conductivity anomalies in subduction zone. In this study, we measured the complex impedances of natural amphibolite at 0.5, 1.0 and 1.5 GPa and 473~1073 K. Our results indicated that pressure has a very weak effect on the electrical conductivity of amphibolite, while temperature strongly influences conductivity of amphibolite. The electrical conductivity of amphibolite is increased to 10-0.5 S·m-1 at 1073 K. The activation energy derived is 52.21 kJ·mol-1, implying that the conduction mechanism was small polaron conduction (the electron hopping between Fe2+ and Fe3+). By combining our experimental results with geothermal models, we construct the conductivity models and compare the electrical structure profiles of the continental lithosphere from three different tectonic settings and subduction zones. We conclude that the electrical conductivity of amphibolite could be responsible for the anomalous high conductivities of lower crust beneath continental rifts and mobile orogenic belts and the hot subduction zone.
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Key words:
- Amphibolite /
- Electrical conductivity /
- Pressure /
- Middle-lower crust /
- Subduction zone
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图 1
电导率实验前后斜长角闪岩样品扫描电子显微镜图片
Figure 1.
SEM images of the amphibolite sample before and after conductivity measurements
图 2
斜长角闪岩电导率实验测量装置图
Figure 2.
Sample assembly for measurements of electrical conductivity of amphibolite
图 3
压力为1.0 GPa时不同温度条件下的斜长角闪岩样品阻抗谱图
Figure 3.
Complex impedance spectra of amphibolite at different temperatures at 1.0 GPa
图 4
(a) 1.0 GPa, 473~1073 K条件下，斜长角闪岩样品不同的升降温阶段的log σ-10000/T关系图; (b) 0.5, 1.0, 1.5 GPa, 473~1073 K条件下斜长角闪岩样品log σ-10000/T数据图
Figure 4.
(a) Logarithm of electrical conductivity versus reciprocal temperature for amphibolite at different heating and cooling cycles at pressure of 1.0 GPa and temperature range of 473~1073 K; (b) Logarithm of electrical conductivity versus reciprocal temperature for amphibolite at pressures of 0.5, 1.0, 1.5 GPa and temperature range of 473~1073 K
图 5
实验前后斜长角闪岩的傅里叶红外光谱图
Figure 5.
FTIR spectra for amphibolite before and after experiments
图 6
本研究测得的斜长角闪岩电导率与前人研究的对比
Figure 6.
Comparison of electrical conductivity of amphibolite of this study with previous studies
图 7
不同铁含量的角闪岩(石)在673 K的电导率
Figure 7.
Electrical conductivity of amphibolite (amphibole) with various iron content at 673 K
图 8
(a) 斜长角闪岩电导率的实验室测量值与不同地温梯度结合得到的电磁结构剖面，及其与元古代克拉通，毛里塔尼亚构造带和格里高利裂谷电磁结构剖面对比图；(b)俯冲带区域不同地温梯度条件下，斜长角闪岩的电导率与不同俯冲带地区的大地电磁观测数据对比图
Figure 8.
(a) Comparison of laboratory-based conductivity profiles obtained from this study with Archean craton, Mauritanides belt and Gregorian rift; (b) Comparison of laboratory-based conductivity profiles obtained from this study with subduction regions with different geothermal gradients
表 1
斜长角闪岩电子探针分析(wt.%)
Table 1.
Electron microprobe analysis of amphibolite
Pl Am SiO2 66.76 43.24 TiO2 0.02 1.14 Al2O3 21.11 9.99 Cr2O3 0.01 0.06 FeO 0.06 17.92 MnO 0.01 0.26 MgO 0.00 9.23 CaO 1.75 11.24 Na2O 10.14 1.41 K2O 0.22 0.87 总量 100.08 95.36 注：Pl斜长石，Am角闪石. 
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表 2
斜长角闪岩电导率拟合所得参数表
Table 2.
The fitting parameters for electrical conductivity of amphibolite
P(GPa) T/K log10σ0/(S·m-1) ΔU/(kJ·mol-1) ΔV/(cm3·mol-1) 0.5，1.0，1.5 473~1073 1.93±0.09 52.21±2.61 0.33 ±0.02
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