中地壳的水-岩作用对相关的地球物理性质影响

Impact of water rock interactions in the middle crust conditions upon the Geophysical features.

对于地壳的地球物理详细探测发现中地壳普遍存在着高导层和低速层。高温高压下的矿物(岩石)-流体反应的化学动力学实验和水的性质研究可以提供我们认识地壳的地球物理探测结果的新信息。通常,中地壳大致位于15至25km的深度范围。高导层和低速层普遍存在于中地壳。各地的地壳厚度不同,但是中地壳和高导-低速层的深度范围十分相似。中地壳的顶界温度处于300℃,底界为450℃～500℃范围。为了认识中地壳的高导低速层的起因,作者重点做了模拟中地壳条件下水-岩相互作用实验,同时,分析了水在近临界区至超临界区的突变性质。矿物(钠长石、透辉石、阳起石)与水,玄武岩与水反应的化学动力学实验是使用流体通过叠层反应器的开放体系在25℃～400℃和22MPa下完成的。与水反应的矿物或岩石里的各个元素溶解到溶液里的释放速率一般不一样。硅酸盐矿物的硅的最大释放速率是在300℃。其它元素,如Na、K、Mg、Ca、Fe、Al等释放速率在<300℃时都高于硅的释放速率,在>300℃时硅的释放速率要高于其它元素的释放速率。在水中的硅的大释放速率会导致硅酸盐矿物格架解体。作者假定地壳里普遍存在水占1%(体积)。这时,地壳里会发生水-岩相互作用。中地壳的流体处于300℃～500℃。在由亚临界态进入超临界区的演化过程中,在跨越临界温度时水的性质发生剧烈变化,如密度、介电常数等热力学参数。这一变化会引起水/岩相互作用的反应动力学涨落,会导致中地壳岩层的硅的淋失,硅酸盐矿物解体,岩石崩塌。同时,在临界区水的性质强烈变化还有水的电导率、迁移性质、等,这会影响中地壳一些物理性质,如中地壳高导层和低速层的出现。

Detailed geophysical study of earth crust found the high conductance layer and low velocity layer distributed in the middle crust. Lots of kinetic experiments of silicate mineral( rock) -water reactions at high temperatures and high pressures combined study of water properties from sub-critical to super-critical states provide new message for us to understand deep geophysical investigations. Middle crust is located at 15 - 25km depth, where high conductance layer and low velocity layer distributed. Even though, the thickness of earth crust is different everywhere, the high conductance and low velocity layers in the Middle crust often locate at the same depth. Usually, the temperature of the top of the middle-crust is 300 degrees C. The temperature of its lower boundary is about 500 degrees C. In order to simulate the water-rock interactions in the middle crust, the authors performed the kinetic experiments of minerals (albite, actinolite, diopside) and basalt in water, and also studied the variations of water properties during the transfer from subcritical to critical states of water. Kinetic experiments of minerals and basalt in water were carried out by using packed bed reactor in the temperature range of 25 to 400 degrees C and at 22MPa. Experiments found that the release rates for different elements in minerals and in rock are different. The maximum release rate of Si always occurs at 300 degrees C and at 22MPa. At T < 300 degrees C, release rates for Na, Mg, Ca, Fe, Al are higher than them of Si. But at T > 300 degrees C, the release rate of Si is higher than others. As release rate of Si behaves the maximum value, silicate framework of minerals will break. Assumed 1% (volume) water distributed in the crust, thus water-rock interactions occur. Water in the middle crust is present in a sub-critical state to the critical state region, behaving a strong variation of water proper-ties. The variations of water properties in the middle crust will induce the fluctuation in kinetics of water-rock interaction, i. e, the fluctuation in the release rates of Si will lead the silicate framework of minerals broken, strong eluviation of Si, and then rock collapse. In this case, water distributed in the middle crust will show the strong variations of water proper-ties in the critical region and affect the geophysical features of rock layer, such as high conductance layer and low velocity layer.