Experimental Conductivity of Partial Melt Granite at High Temperature and Pressure
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摘要: 大地电磁测深结果显示青藏高原中上地壳存在高导层，而花岗岩是地壳岩石的主要组成部分，在地壳演化过程中发挥着重要作用。在高温高压下开展花岗岩部分熔融时电导率实验对认识青藏高原地壳电性结构及地壳演化过程具有重要意义。在0.5～2.0 GPa压力、773～1 373 K温度条件下测量花岗岩的电导率。实验结果表明：在温度为773～1 223 K时，样品的活化焓为1.01～1.09 eV；在温度为1 223～1 373 K时，样品的活化焓为2.16～2.97 eV。不同温度段内活化焓的变化可能与花岗岩样品中黑云母的脱水熔融有关，推断花岗岩部分熔融时导电机制为离子导电，Na+起主导作用。将实验测得的电导率与西藏高导层地壳温度背景结合发现：在973～1 223 K范围内实验电导率值在0.016～0.310 S/m范围内，与大地电磁测深数据吻合较好，表明西藏地壳高导层的成因与花岗岩部分熔融关系较为密切。Abstract: Magnetotelluric (MT) surveys reveal that high conductivity layer appear in the upper crust beneath Tibet. Granite is the main rocks composed of upper crust, playing an important role in the process of crustal evolution. Electrical conductivity of granite during partial melting is of great significance to understanding the conductivity structure of Tibetan Plateau crust and the crustal evolution process. Electrical conductivity of granite collected from the Tibet was conducted under the conditions of 0.5－2.0 GPa and 773－1 373 K. The activation enthalpies of 1.01－1.09 eV and 2.16－2.97 eV are derived from 773 to 1 223 K and from 1 223 to 1 373 K, respectively. The change of activation enthalpy in different temperature zones may be related to the partial melting of granite induced by the biotite dehydration. Combining the experimental results and geothermal gradient of Tibet, we found that the experimental conductivity values fell between 0.016 S/m and 0.310 S/m in the temperature range of 973－1 223 K, which was in good agreement with the magnetotelluric sounding data. This may indicate that there is a close relationship between the genesis of the high conductivity layer and the partial melting of granite.
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Key words:
- high temperature and high pressure /
- conductivity /
- granite /
- partial melting
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图 1 样品高压电导率测量组装示意图
Figure 1. Sample assembly for high pressure conductivity measurement
图 2 压强1.0 GPa、温度773 ～1 373 K条件下花岗岩样品相角和模随频率的变化
Figure 2. Respectively changes of phase angle and modulus of impedance as the function of frequency for granite at 1.0 GPa and 773－1 373 K
图 3 1.0 GPa、773～1 373 K条件下样品的复阻抗谱
Figure 3. Complex impedance spectra of the samples at 1.0 GPa and 773－1 373 K
图 4 0.5～2.0 GPa压强下电导率随温度的变化
Figure 4. Electrical conductivity as the function of temperatures at 0.5－2.0 GPa
图 5 高压实验前（a）、后（b）样品的扫描电镜图像
Figure 5. Images of scanning electron microscope for the samples before (a) and after (b) the experiment
图 6 花岗岩电导率实验结果对比
Figure 6. Comparison of the conductivity results in this work with previous data
图 7 基于实验室电导率模型与大地电磁结果对比
Figure 7. Comparison of laboratory-based conductivity profile established with the result of the upper crust derived from MT conductivity model
表 1 花岗岩样品的矿物成分及含量
Table 1. Compositions of the minerals in granite
Compounds Content/% Quartz K-feldspar Albite Biotite Na2O 0.008 0.774 10.887 0.319 MgO 0.002 0 0 2.694 Al2O3 0.113 18.771 22.019 20.368 SiO2 97.196 67.117 65.805 40.506 SrO 0.288 0.639 0.240 0 K2O 0.026 15.175 0.144 8.342 CaO 0.016 0.003 1.550 0.084 MnO 0.023 0.072 0 0.470 FeO 0.087 0.096 0.016 24.901 TiO2 0.069 0.103 0 2.147 Total 97.828 102.750 100.661 99.831 
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表 2 不同压力下花岗岩样品电导率的Arrhenius关系拟合参数
Table 2. Fitting parameters of Arrhenius relationship of the conductivity of granite samples under different pressures
p/GPa T/K lg $\sigma $0/(S·m−1) ΔH/eV 0.5 773－1 223 3.53 ± 0.08 1.01 ± 0.01 1 223－1 373 11.66 ± 1.00 2.97 ± 0.26 1.0 773－1 223 3.82 ± 0.09 1.09 ± 0.02 1 223－1 373 8.38 ± 0.63 2.16 ± 0.16 2.0 773－1 223 3.64 ± 0.09 1.06 ± 0.02 1 223－1 373 8.22 ± 0.59 2.21 ± 0.13
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