高温高压下不同成分斜长石电导率的实验研究及地球物理意义

Laboratory electrical conductivity data of minerals and rocks can provide important constraints on the inversion results obtained from the field magnetotelluric data. Magnetotelluric surveys have revealed that a high conductivity zone (HCZ) exists in many places worldwide in the middle to lower continental crust, whereas there is still no generally accepted explanation for the origin of HCZ until now. The reason is that many factors such as temperature, pressure, mineralogical constituents and mineralogical composition, can affect the electrical conductivity of rocks, however, it is still controversial that which of these factors mainly control the electrical properties of rocks under the conditions of the middle to lower continental crust. Consequently, it is the first most important step to establish the relationship between the electrical conductivity of the major rock-forming minerals and these factors. Plagioclase, which is a solid solution series in feldspar family, is widespread and common in the different categories of crustal rocks in the Earth's crust. To some extent, the electrical conductivity of plagioclase has an important influence on the electrical properties of rocks in the Earth's crust. Therefore, in this project it will be the first time to systematically study on the quantitative relationship between the electrical conductivity of plagioclase and temperature, pressure and chemical composition by using the impedance spectroscopy technique and large volume high-pressure apparatus. Meanwhile, combining with the previous conductivity data of other crustal minerals, the electrical conductivity of plagioclase-bearing rocks in the Earth's crust will be established in order to construct the laboratory-based conductivity-depth profile of the Earth's crust. Furthermore, the effect of mineralogical compositions and mineralogical constituents on the conductivity-depth profile will be studied in detail so as to possibly provide a new evidence for origin of the HCZ existing in the middle to lower continental crust.

矿物岩石电导率的实验数据可以约束野外电磁观测数据的反演结果。大地电磁测深结果显示大陆中、下地壳普遍存在着异常高导层。目前对高导层的起源一直没有统一的认识，其原因是影响岩石电导率变化的因素有很多，比如温度、压力、矿物组成、矿物化学成分等，具体哪些影响因素对岩石的电性起主导作用，一直都存有争议。因此，建立地壳主要造岩矿物的电导率与各影响因素之间的函数关系是最基础也是最重要的一步。斜长石作为长石矿物的一个类质同象系列，广泛地存在于地壳各类岩石中，它的电导率在很大程度上影响着地壳岩石的电学性质。本项目拟借助交流阻抗谱测量技术和高温高压设备，首次系统地开展斜长石电导率与温度、压力和化学成分之间定量关系的研究，并结合其他矿物电导率数据建立含斜长石地壳岩石的电导率，最终构建地壳电导率-深度剖面并探究矿物组分与矿物化学成分对其影响，从而可能为中、下地壳高导层的起因提供一个新的解释。

大地电磁测深结果显示大陆中、下地壳普遍存在着异常高导层。目前，对高导层的起源一直没有统一的认识，其原因是影响岩石电导率变化的因素有很多，比如温度、压力、矿物组成、矿物化学成分等，具体哪些影响因素对岩石的电性起主导作用，一直都存有争议。长石是地壳中分布最广泛、最重要的造岩矿物，而斜长石是长石族矿物的一个类质同象系列，存在于地壳各类岩石中，因此它的电导率对地壳岩石的电学性质有重要的影响。到目前为止，斜长石的电导率还没有进行过系统地研究，本项目首次系统地开展了斜长石电导率与温度、压力和成分之间的研究。为了获得一系列的实验样品，首先利用固相法获得了钙长石合成的最佳温度和反应时间，从而得到了高纯度的钙长石。之后，在多面顶高压设备上合成了一系列不同成分的斜长石样品，并对所有实验样品进行了成分和微观形貌分析。通过对实验样品电导率的测量，结果获得了端员组分钙长石在高温高压下的电导率数据，并根据其电导率数据，利用Nernst-Einstein方程，计算出了钙离子在钙长石中的扩散系数，该成果填补了此类数据的空白，为实验结果提供重要的约束。此外，通过分析不同成分斜长石电导率的实验数据，发现斜长石电导率受温度和样品中钠离子含量影响较大，压力对其电导率影响较小。根据斜长石电导率对温度、压力和成分的依赖关系，建立了斜长石电导率与这些参数变化之间的定量函数关系模型。由此，我们推断中、下地壳高导层可能与含斜长石的地壳岩石中钠离子的含量有一定的关系，钠离子的增加可能会引起地壳高导异常。除了完成该项目设计的实验－高温高压条件下温度、压力和化学成分的斜长石电导率研究工作以外，特此增加了在高温高压条件下干的钾长石、不同组分的花岗岩、含水的玄武岩与安山岩、含水的辉长岩、干的榴辉岩等一系列矿物岩石体系的电导率实验。