俯冲带中主要含水矿物的电导率实验研究及其对高导异常成因的启示

Numerous field magnetotelluric results from various subduction zones indicate that the extensive presence of anomalies of electrical conductivity in the backarc area (>50 km) and forearc near trench area (40 km). In these regions, electrical conductivity is characterized by the high value of ~10-1.5−1 S/m. The highly conductivity anomalies are often interpreted as the fluid accumulation from the progressive dehydration of subducting slab, partial melt or the hydrous minerals. As with all geophysical techniques, magnetotelluric has a non-unique inverse problem, therefore, the interpretation for the highly conductivity anomalies in the mantle wedge is still in debate. However, laboratory-based electrical conductivity results on geomaterials can provide a significant and independent constraint on the inversion results of magnetotelluric, ultimately the accurate information of the Earth interior can be obtained. .The electrical conductivities of serpentine (12.3 wt % H2O) and amphibole (2.1 wt %) will be measured under the simulated subduction context by virtue of the alternating current impedance spectroscopy technique as well as high-temperature and high-pressure devices. The electrical conductivity data before and after dehydration reaction and temperature of dehydration will be obtained. In combination with petrology model and thermal structure in subduction zone, the laboratory-based electrical conductivity−depth profile will be established, which will compare with geophysically observed conductivity profiles in certain specific subduction zones in order to explain the origin of highly conductivity anomalies in mantle wedge.

大量的野外大地电磁测深结果显示，在俯冲带背弧深部（>50 km）和近海沟前弧浅部（<40 km）分别都广泛地存在高导异常，其电导率值在10-1.5−1 S/m左右，这些高导异常通常被认为是水流体、部分熔融或含水蚀变矿物的富集引起。由于地球物理反演结果的非唯一性，使得对地幔楔高导异常的解释到目前为止一直没有一个统一的认识。电导率的实验研究可以约束野外数据的反演结果，从而获得更加精准的地球内部信息。借助目前先进的交流阻抗谱技术和高温高压设备，在模拟俯冲带环境下开展主要含水矿物蛇纹石(含12.3 wt% H2O)和角闪石(2.1 wt% H2O) 电导率的实验研究，获得脱水前后的电导率数据以及脱水温度。利用这些数据，结合俯冲带岩石学模型和热结构，建立起实验基于的电导率−深度剖面及确定含水矿物的脱水深度，从而对野外大地电磁测深数据进行约束，最终对地幔楔浅部和深部的高导异常成因提供准确的信息。

利用交流阻抗谱技术和高温高压设备，在模拟俯冲带温度和压力的环境下开展了主要俯冲带主要含水矿物蛇纹石(含12.3 wt% H2O)和角闪石(2.1 wt% H2O) 电导率的实验研究，获得了表征蛇纹石和角山石微观导电机制的活化焓、指前因子、活化能及活化体积等物性参数。建立起了这些含水矿物的电导率随温度和压力变化的函数关系。根据其电导率在高温高压下的变化情况，确定了这些含水矿物的脱水或者氧化脱氢的温度。利用这些数据，结合俯冲带和中下地壳岩石学模型和热结构，建立起基于实验电导率−深度剖面及确定含水矿物在俯冲带和中下地壳的脱水或脱氢深度。最终我们的实验结果与野外大地电磁测深数据进行整合，对中下地壳、俯冲带与地幔楔界面以及地幔楔浅部和深部的高导异常提供了新的合理解释，确定了这些含水矿物对地球深部高导异常的贡献。该系列研究成果解决了地球深部高导异常区的水源问题，揭示了含水矿物对地球内部挥发分的贡献及对氧化还原环境的影响。