深部流体对岩石物性的影响及其地震地质意义

【作者】 杜建国；

【导师】 刘丛强；

【作者基本信息】 中国科学院研究生院（地球化学研究所） ， 地球化学， 2003， 博士

【摘要】 中国是地震多发的国家之一，强震常给人民生命财产造成巨大损失。但是，目前人类对地震孕育、发生的机理认识尚浅，进行地震短临预测的水平很低，远不能满足减轻地震灾害的社会需求。因此，有必要对中-强震孕育过程、发生机理以及流体地震前兆等方面进行深入研究。 在总结深部流体、高温高压实验研究和流体在地震预测研究进展的基础上，利用高温高压实验技术测定了辉长岩和斜长岩高温高压下弹性波速和电导率，结合实验产物的岩组学和微区化学分析结果探讨了孕震介质高温高压下物理性质变化的机制；采用气体地球化学方法研究了四川西部三个地震带温泉气体的来源及其与地震活动的关系。研究表明从震源介质演化和流体地震异常特征方面探讨地震成因和地震预测是个有效的途径。 高压下不同类型岩石的波速随温度的升高逐渐降低，但是具体变化特征存在差异。在1.0 GPa下，辉长岩的纵波速度从室温到800℃下降4％，而从800℃到1100℃下降幅度为16.4％；但是，在2.0 GPa下，纵波速度从室温到850℃时下降3.6％，从850℃到1000℃下降4.6％。然而，在1.0 GPa下，斜长岩纵波速度（V_P）当温度低于680℃时，斜长岩的纵波速度缓慢下降，V_P从室温到680℃降低了2.2％；当温度高于680℃时，纵波速度开始大幅度下降，V_P从680℃到880℃下降了7.6％。高温高压下矿物脱水、熔融导致岩石密度、结构变化，形成新的颗粒边界相，从而使岩石纵波速度开始大幅度下降。 高压下辉长岩和斜长岩电导率值随温度的升高而升高。在1.2GPa和较低温度条件下，岩石导电性质变化不大，颗粒内部电子传导方式站主导地位。在1.0GPa、410-750℃的温压条件下，测得的斜长岩的电导率值较低，分布在10^-7S/m到10^-5S/m之间。辉长岩在680℃以下，导电机制以颗粒内部传导机制为主，在680℃以上，开始出现颗粒边缘传导机制。岩石在固体状态下，颗粒边缘传导会降低岩石的总电阻率。 辉长岩和斜长岩的纵波速度和电导率的就位测量表明，岩石中的含水矿物脱水导致其P波速度开始大幅度下降时，岩石的电传导机制和电导率值却未发生突变。这是由于脱水程度和水的连通程度较低，所以在这样的实验条件下矿物脱水未使电导率急剧增大，也未改变岩石的电传导机制。地球内部低速层和高导层的形成与演化也可能具有类似的特征。在一定的条件下，通过含水矿物脱水可以形成地球内部的低速层，但不一定同时形成高导层，低速层是否同时具有高导的特征取决于矿物脱水程度和区域内自由水的连通程度。 稳定同位素地球化学特征表明四川西部鲜水河地震带、龙门山地震带和安宁河地震带温泉中的氦具有多源性，温泉的氦、碳同位素组成在不同的地震带存在明显的差异。在鲜水河地震带、龙门山地震带和安宁河地震带温泉的³He/⁴He值分别在0.48-2.59R_A、0.22-1.07R_A和0.01-0.82R_A的范围内。康定地区温泉中的氦有相一当部分来源于地幔。然而，采自鲜水河断裂带西北段甘孜（No.1，2）、泸定（No.9）和安宁河地震带石棉南—普格（No.12，13，21-25）与龙门山地震带的温泉样品具有很低的³He/⁴He值，表明这些温泉的氦主要来自地壳。当然，温泉气中也不能排除大气氦的混入的可能。 四川西部温泉中二氧化碳的δ¹³C值分布在-17.09‰—-3.34‰的范围内，游离气样中的CO₂比水溶气样中的CO₂富集重碳同位素¹³C。不同的构造带内温泉二氧化碳具有不同的碳同位素组成。鲜水河断裂带内温泉二氧化碳主要来源于地慢，地慢的二氧化碳与氦一起向地表运移。 高的’He/4He和6”c值，地热异常是四川西部地震带的显著特征。研究区不同构造带温泉气体的主要来源不同，意味着不同构造带的热能的来源和供给速率也不同。四川西部地热区热储的主要能量源为上地慢热和地壳热。地下流体中的氦与二氧化碳是反映地震活动的有效指标。 深部流体是导致地震的重要因素。地慢流体向上逸散，同时带入地壳大量的热。当地慢流体在地壳内局部富集时，地体内产生的空隙压力和热应力能使流体富集区的围岩破裂产生地震。火山、地热区的地震与中、深源地震可以用这种孕震机制来解释。 然而，目前我们对地球内部的认识还是很肤浅的。合理地解释地震的成因和利用气体地球化学方法实现短临地震预测尚需开展大量工作。更多还原

【Abstract】 It is necessary to study the genetic mechanics of strong earthquake and the fluid precursors for the shocks because our knowledge about earthquake is so poor that can not meet the social needs of mitigating disaster of strong earthquakes that frequently occurred in China.The advances of the role of deep-earth fluids in earthquake generation, high temperature and high pressure experiments with rocks and the earthquake prediction based on fluid precursors were reviewed in the dissertation. The compression wave velocities (VP) and conductivity (σ) of gabbro and plagioclasite under high temperature and high pressure were measured with the ultrasonic pulse transmission method and impedance spectra technique, respectively. The original and unloaded samples were chemically measured and analyzed with the electron microprobe instrument and observed under the microscope. The mechanisms of VP and σ variations of the rocks were discussed based on the data of the experiments and analysis. In addition, the origins of gases in the hot springs and seismological activity in West Sichuan Province were mentioned in the viewpoint of stable isotopic geochemistry.VP values of the rocks under high pressure decreased with the increase of temperature. However, VP values of different rocks decreased in the different ways. VP values of the gabbro at the pressure of 1.0 GPa reduced by 4% from the room temperature to 800℃, but reduced severely by 16.4% from 800C to 1100C. VP values of the gabbro at 2.0 GPa reduced 3.6% from room temperature to 850℃, and decreased 4.6% from 850℃ to 1000℃. Otherwise, VP values of plagioclasite at 1.0 GPa decreased by 2.2% from room temperature to 680℃, and decreased sharply by 7.6% from 680℃ to 880℃. The VP value decrease results from mineral dehydration, rock partially-melting under the experiment conditions.The values of conductivity of the rocks at the pressures of 1 and 2 GPa increased with temperature increasing. Under the conditions of 1GP and 410-750℃, the conductivity values of plagioclasite are in the range of 10-7 S/m to 10-5 S/m, and the complex impedance of plagioclasite collected from 12Hz to 105Hz shows only one semicircular impedance arc that represents the conductive mechanism of the grain interior conduction. The experiment data indicated that conductive mechanism of gabbro at temperature less than 680 ℃ was the grain interior conduction, and that the grain boundary conduction occurred at the temperature higher than 680 ℃, which resulted in the increase of conductivity of the rocks.The data of the microscope observation and microprobe analyses argue that no high conductivity grid of interconnection hydrous fluids in the rock was formed during the dehydration, which hints that the formation and evolution of the high conductivity zone in the crust have a close relationship with the amount of the fluids and the interconnection of the fluids as well.The phenomena that the conductivity values of the experiment rocks changed little when the VP values began to decreasing sharply indicated that the grade of minerals dehydration was so lower that the mineral grain boundary and structure of the rock samples changed severely, but did not change the conductive mechanism, which would be the feature of the early stage of a lower-velocity-layer evolution in lithosphere.The stable isotopic geochemical characteristics of helium and carbon of the hot springs indicate that the helium and carbon dioxide in the three earthquake zones of Xianshuihe,Anninghe and Longmenshan in West Sichuan Province have the multiple origins. 3He/4He ratios of the hot springs in Xianshuihe, Longmenshan and Anninghe earthquake zones have the ranges of 0.48-2.59RA, 0.22-1.07 RAand 0.01-0.82 RA, respectively. Higher 3He/4He ratios of the hot springs in the Kangding district demonstrate that the helium is partially derived from the mantle, and the helium of the hot springs in the Aninghe and Longmenshan earthquake zones are predominately derived from the crust with mixing of atmospheric helium. 13C values of carbo更多还原