西藏南部蛇绿岩套电导率研究

大地电磁(MT)资料显示,青藏高原地壳及地幔中普遍存在着高导层.作为大陆造山带中古洋盆岩石圈残片,蛇绿岩套的电导率测量可为了解古洋盆地区地壳及地幔的电性结构提供极其有用的信息.本研究中,我们在压力为1 GPa或3 GPa下,用交流阻抗谱法测量了采自西藏南部地区的蚀变辉长岩、玄武岩、角闪橄榄岩及方辉橄榄岩四个样品的阻抗谱,并进一步得出样品的电导率,不同样品电导率与温度之间的关系满足Arrhenius关系式.在实验温度范围内,蛇绿岩套电导率的对数logσ位于-6.0~-0.5 S/m之间,且随着温度的增高,不同样品电导率增大约4~5.5个量级.样品在未脱水的情况下,低温段的活化焓变化范围在0.4~0.6 eV之间,高温段的活化焓变化范围为1.7~2.6 eV之间.同时,我们研究了样品中结构水含量及铁含量对实验电导率的影响,验证了样品电导率与铁含量之间呈正比关系.当对样品结构水含量进行归一化后,相同温度下各样品的电导率随铁含量的增加而增大,而对样品铁含量归一化后,相同温度下各样品的电导率随样品中水含量的增加而增大.将实验电导率与藏南地区大地电磁结果进行了对比,发现本研究中各样品高温段实验电导率结果均落在大地电磁结果范围内.

The electrical conductivity of ophiolite in Southern Tibet

Recent magnetotelluric (MT) field measurements indicate the existence of high conductivity zones (HCZ) in the crust and upper mantle of Tibetan Plateau. As ocean basin lithosphere fragments in continental orogenic belts, the electrical conductivity of ophiolites may provide useful information to understand the electrical structure of the crust and the mantle. Using the samples of altered gabbro, basalt, hornblende peridotite and harzburgite collected from Yarlung Zangbo ophiolite zone in south Tibet, we detected the electrical conductivities of ophiolites by using the impedance spectrum method at 1 or 3 GPa and different temperatures respectively. The results show that the logarithm of electrical conductivity of ophiolites varies from -6 to -0.5 S/m, the conductivity increases by 4~5.5 orders of magnitude with the increase of experimental temperature. Before dehydration, the activation enthalpies are 0.4~0.6 eV for different samples at low temperature and 1.7~2.6 eV at high temperature. Meanwhile, we studied the effect of the water content and the iron content on the experimental conductivity, our result verified the positive correlation between the conductivity of the samples and the water content or iron content. While we normalized the water content or iron content of samples, the relationship of conductivity was consistent with the iron content or water content of the samples. The results were applied to Earth's interior and compared with the magnetotelluric observations, our model is consistent with the electrical conductivity structures derived from geophysical observations.