高温高压下碳酸盐熔体对地幔岩电导率的影响

The influences of carbonate melt on the electrical conductivity for pyrolite at high temperature and high pressure

为了观测含碳酸盐地幔岩部分熔融过程中电导率的变化,厘清碳酸盐熔体在金伯利岩岩浆形成过程中所起的作用,并探讨Slave克拉通中部Lac de Gras地区约80～120km深处的高导成因,我们利用DS 3600t六面顶压机和Solartron 1260阻抗/增益-相位分析仪在1.0～3.0GPa、673～1873K温压条件下分别测量了含碳酸钠(Na_2CO_3)、碳酸钙(CaCO_3)和大洋中脊玄武岩(MORB)的地幔岩样品的电导率.实验结果表明,地幔岩样品的电导率主要受到温度和组分的影响,而压力对其影响较小.在温度低于1023K时,含Na_2CO_3地幔岩样品的电导率明显高于含同比重CaCO_3和MORB的;温度达到1023K时,含Na_2CO_3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.这一现象或许揭示:地幔深部的碳酸质岩浆在快速上升过程中会同化吸收岩石圈地幔中的斜方辉石(Opx),进而形成金伯利岩岩浆,期间岩浆的电导率几乎不发生变化.含CaCO_3和MORB的地幔岩样品分别在1723K和1423K开始熔融,其部分熔融样品的电导率随温度的增加而快速增加.依据前人的研究结果和我们的实验结果,我们认为可以用含碳酸盐的部分熔融样品来解释Slave克拉通中部Lac de Gras地区约80～120km深处的异常高导现象,并推测熔体中碳酸盐的熔体比例小于2wt.%.

In order to obtain the electrical conductivity of partially molten pyrolite with carbonate and make clear the causes of formation of kimberlite magma and high conductivity zone at depth of about 80~120 km of Lac de Gras area in Central Slave Craton, we measured the electrical conductivity on pyrolite with carbonate melt at pressure of 1.0~3.0 GPa, temperature of 673~1873 K and frequency ranges of 10 -1 ~10 6 Hz using Solartron-1260 impedance/Gain-Phase analyzer with DS 3600 t multi-anvil apparatus. The experimental results show that electrical conductivity of partial melting samples is mainly affected by temperature and compositions. The electrical conductivities of samples containing Na 2CO 3 are significantly higher than those of other samples containing CaCO 3 and MORB below 1023 K. The electrical conductivities of partially molten Na 2CO 3-bearing pyrolite hardly increase when increasing temperature at 1073~1273 K. It reveals that carbonated silicate melt has constant electrical conductivity during the formation of kimberlite magma. According to previous experimental data, the high conductivity phenomenon in Lac de Gras area in Central Slave Craton at about 80~120 km may be explained by partially molten pyrolite containing carbonated silicate melt. It is speculated that less than 2 wt.% carbonate melts occur in the kimberlite magma.