高温高压下地幔岩和苦橄质榴辉岩的电导率实验

为了探讨地幔岩模型和苦橄质榴辉岩模型在上地幔存在的合理性,建立上地幔的电性结构,本文利用YJ-3000t紧装式六面顶压机和Solartron IS-1260阻抗/增益-相位分析仪,在1.0~4.0GPa、700~1150℃的条件下,采用交流阻抗谱法(频率范围10-1~106 Hz)分别测量了地幔岩和苦橄质榴辉岩的电导率.实验结果表明:随着温度的升高,地幔岩和苦橄质榴辉岩的电导率大幅增加;随着压力的增大,地幔岩的电导率略有增加,活化体积ΔV为-4.73cm3·mol-1,而苦橄质榴辉岩的电导率几乎没有变化,活化体积ΔV为-0.11cm3·mol-1;在电性方面,用苦橄质榴辉岩来表示深部的物质较为合理,地幔岩解释浅部可能更恰当,但浅部物质的分布不均匀,电导率随深度的变化主要受控于温度的影响,其次才是成分.

Experimental study on electrical conductivity of pyrolite and piclogite at high temperature and high pressure

Measurements of electrical conductivity for rocks and minerals at high temperature and high pressure provide a powerful tool to acquire information about chemical and mineralogical composition of the Earth's interior. Based on physical properties of minerals and rocks such as density, and elastic-wave velocity, Ringwood and Anderson & Bass proposed two competing mineralogical models of the mantle:pyrolite model and piclogite model. In order to investigate electrical structure of the upper mantle and the rationality of these two models, the electrical conductivities were measured using YJ-3000t multi-anvil apparatus and Solartron-1260 impedance/Gain-Phase analyzer under conditions of 1.0~4.0 GPa, 700~1150℃ and 10^-1~10⁶ Hz frequency ranges.#br#The experimental results show that the electrical conductivities significantly increase with temperature. The linear relationship between Lgσ and 1/T indicates that they follow an Arrhenius formula and the small polaron model is the dominant conduction mechanism. The pre-exponential factor σ₀ and the activation energy ΔU have been obtained in the setting range. They are respectively 68.08~248.09 S·m^-1 and 1.14 eV for pyrolite; 17.99~77.46 S·m^-1 and 1.01 eV for piclogite. With increasing pressure, the electrical conductivities of pyrolite slightly increase, while those of piclogite almost do not change. If pressure effects on conductivity of pyrolite and piclogite are expressed as the activation volume ΔV in the Arrhenius equation, they yield values of order of magnitude -4.73 cm³·mol^-1 and -0.11 cm³·mol^-1, respectively. So far, researchers have not yet formed a unified understanding about the reason of positive and negative ΔV. The conductivity-depth profiles have been established by using our experimental results and geothermal gradients. Comparing our results with the previous electromagnetic data beneath the different regions in the mantle, the reasonability of the pyrolite model and the piclogite model are discussed. In the aspect of conductivity, the piclogite model is more reasonable for explaining deep geophysical field data than the pyrolite model. The conductivities of shallow part of upper mantle are primarily controlled by the temperature,followed by composition.