石榴辉石岩的电导率及对岩浆底侵的约束

The electrical conductivity of garnet pyroxenite:Implications for magmatic underplating

河北汉诺坝新生代玄武岩携带的石榴辉石岩为岩浆底侵于上地幔顶部40～45km形成的堆晶岩，是壳幔过渡带的典型样品。本文使用S01artron1260阻抗分析仪，测量1．2GPa和380～900℃下石榴辉石岩WD958的电阻率。样品电导率（d）与温度的关系遵守Arrhenius方程：d=exp（=AH／kT），其中T是样品的绝对温度，k是Boltzmann常数，指前因子GO为97．5S／m，活化焓AH为1．27eV。使用傅立叶红外光谱仪测定样品中单斜辉石的平均含水量为117×10-6 H2O，橄榄石无水，石榴子石的含水量凶蚀变无法测定。使用Hashin—Shtrikman平均方法计算样品WD958的电导率，发现样品可看作由含水单斜辉石、无水石榴子石和无水橄榄石组成的高阻集合体，电导率同时受小极化予导电和质子导电的影响。如果原位的石榴辉石岩的矿物含水量保持平衡，其电导率将提高1个数量级，质子导电成为主控导电机制。石榴辉石岩在高温的壳一幔过渡带（～1000℃）表现为高导层，而在正常地温梯度上，与尖晶石二辉橄榄岩的电导率近似，为高阻层。闪此，在岩浆底侵作用下，电性上的壳一幔边界将随着温度和含水量的变化而改变。

Garnet pyroxenite xenoliths from the Hannuoba Cenozoic basalt were formed by magmatic underplat- ing in the uppermost mantle （40--45 kin）, and represent the crust-mantle transition zone. The electrical con- ductivity of sintered garnet pyroxenite WD958 was measured at 1.2 GPa and 380 -- 900℃, using a Solartron 1260 Phase-Gain Analyzer. The temperature dependence of electrical conductivity （a） can be fitted by an Arrhe- nius equation： a - o0 exp（ - AH/kT）, where T is in Kelvin and k is the Dohzmann constant. Values of the pre- exponential factor （a0） and activation enthalpy of electric conductivity （AH） of sample WD958 are 97.5 S/m and 1.27 eV, respectively. The water contents of minerals were analyzed using the Fourier transform infrared spectrometry. The average water content in clinopyroxene is 117 ×10-6 H2O, whereas olivine is very dry （〈 1 ×10-6H20） and the water content in garnet cannot be determined due to alteration. The laboratory-derived electrical conductivity of mantle minerals shows that the calculated conductivity using the Hashin-Shtrikman av- erage can match the measured values by assuming a mixture of hydrogen-bearing clinopyroxene, dry garnet and dry olivine. This demonstrates the contribution of both small polaron conduction and proton conduction mecha- nisms to the bulk conductivity of garnet pyroxenite, and the sample can be regarded as a resistive matrix with non-interconnected conductive inclusions. If the water partition equilibrium between minerals is preserved at the in situ depth （40--45 km）, the electrical conductivity of garnet pyroxenite will be enhanced by 1 order magni- tude and the proton conduction mechanism becomes predominant. For the lithosphere with a high geothermal gradient, the temperature at the Moho depth could reach 1 000 ℃ and garnet pyroxenite is characterized by high conductivity. In contrast, under normal geothermal gradients, garnet pyroxenite shows conductivity as low as spinel lherzolite. During magmatic underplating, therefore, the electrical crust-mantle boundary will vary with temperature and water concentration.