上地幔的电导率--高温高压下地幔岩石电导率的实验研究

In situ laboratory measurements of the electrical conductivity of the major mantle minerals and rocks under high-pressure and high-temperature conditions in combination with the geophysically inferred conductivity-depth profiles of the mantle can provide some constraints on the structure and composition in the Earth’s interior. A lot of experimental studies have been conducted on olivine, pyroxene and garnet which are important constituent minerals in the Earth’s upper mantle. However, little research has been investigated on mantle rocks such as lherzolite which is typical rock of upper mantle. In addition, knowledge of pressure dependence on the electrical conductivity of minerals and rocks is also important for the construction of conductivity- depth profile. Some researchers have measured the pressure dependence of electrical conductivity of the olivine. However, the activation volume has not been constrained well even though they considered hopping conduction as a dominant conduction mechanism in iron-poor olivine. In situ laboratory measurements of electrical conductivity for hydrous olivine have been carried out for estimating water content in the Earth’s upper mantle. However, there is controversy over reports whether the hydrous olivine is responsible for the high conductivity at the top of the asthenosphere. So the electrical conductivity of rocks should be investigated. .Pyrolite and piclogite are synthetic rock composition conceived by Ringwood and Anderson, respectively, representing two kinds of mantle mineralogical model with slightly more obvious differences between the mineral proportions. MORB is main component in oceanic crust. In this proposal, we determined electrical conductivities of anhydrous and hydrous pyrolite, piclogite and MORB under high-pressure up to 14GPa and high-temperature up to 1673K in upper mantle. The influence of silicate melt and carbonate melt on electrical conductivity of rocks will be studies. The activation volume of rocks will be determined and reference conductivity-depth profile will be constructed. Further, the water content will be estimated in asthenosphere and the cause of high conductivity will be discussed. This project can provide some orginal data. The experimental results will help us to understand the electrical and thermal state, water content and cause of high conductivity in the Earth’s interior.

高温高压电导率实验研究是透视地球内部结构与物质组成的一个重要窗口。百年来进行了大量地幔矿物电导率的研究，但对地幔岩石的研究却不足；进行了压力对地幔矿物电导率影响的研究，但获得的激活体积值却有正有负；进行了含水橄榄石电导率的测量，但仍存在干湿软流圈地幔之争。本项目将在上地幔的温度、压力条件下，系统地研究干、湿pyrolite地幔岩、piclogite苦橄质榴辉岩（代表了两类地幔矿物模型的假想岩石）和MORB大洋中脊玄武岩（洋壳的主要物质组成）的电导率随温度、压力的变化特征，以及硅酸盐熔体和碳酸盐熔体对地幔岩石电导率的影响，从而约束岩石的激活体积，构建电导率-深度剖面；推测软流圈地幔的水含量；探讨高导异常现象的成因等。整个研究对人们认识地幔的物质组成和矿物相变；地幔的电性结构和热结构；以及洋壳俯冲和高导异常成因等地幔动力学问题有重大意义。

为了建立具有普遍适用性的上地幔电性结构, 探讨地幔上部（60-240km）的高导成因，厘清碳酸盐熔体在金伯利岩岩浆形成过程中所起的作用，我们采用交流阻抗谱方法，利用Solarton 1260阻抗／增益相位分析仪和Kawai 1000t压机，在高压4-14GPa、873-1673K的条件下，测量了不含水的地幔岩的电导率；利用六面顶压机，在低压1.0-3.0GPa、673-1873K的条件下，分别测量了含少量碳酸钠的纯橄榄岩的电导率和含高比例碳酸钠、碳酸钙和大洋中脊玄武岩（MORB）的地幔岩样品的电导率，实验结果显示：（1） 岩石的电导率随温度升高而大幅度的增大； 在较大的温度范围内地幔岩的导电机制发生了变化，中低温时为小极化子导电，此时激活焓约为0.94，激活体积约为0.11±0.92cm3/mol，高温时为和镁空穴相关的离子导电， 此时激活焓约为1.6-3.17eV，激活体积为6.75±7.43cm3/mol。（2）在温度低于1023K时，含Na2CO3地幔岩样品的电导率明显高于含同比重 CaCO3和MORB的；温度达到1023K 时，含Na2CO3地幔岩样品开始熔融，但在之后的200K 温度区间内，该部分熔融样品的电导率随温度的增加几乎不发生变化。（3）极低熔体比例下（F<1wt%），含碳酸钠橄榄石的电导率比纯橄榄石的高1-2个数量级，熔体连通指数n约为1.25，其在橄榄石中的连通方式与高比例熔体下的不同。利用我们的实验结果，我们构建了Fennoscandian地盾上地幔的一维电导率剖面；解释了Slave克拉通中部 Lac de Gras地区约80-120km深处电导率约0.1S/m的异常高导现象，并推测该地区碳酸盐的熔体比例小于２wt%；解释了São Francisco克拉通下部100km深处电导率约0.01S/m的高导现象，推测该地碳酸盐的熔体比例约为0.27-0.44wt%，可能有306-498ppm的C。本研究厘定了（1）在小极化子导电的情况下，压力对电导率的影响不大；（2）少量的碳酸盐熔体（F<1wt%）可以长期存在于地幔中，且极大地增强了橄榄石的电导率。