摘要
石榴子石作为下地壳与上地幔的组成成分之一,其对于下地壳、与板块俯冲有关的超高压岩石(如榴辉岩)及上地幔的变形行为有重要影响。尤其是石榴子石作为俯冲洋壳和地幔转换带中重要的组成矿物,其流变学性质的认识是理解俯冲板块和地幔转换带地球动力学过程的关键。高温高压实验是认识地球深部组成物质物理性质及地球内部动力学过程的重要途径之一。但是,长期以来由于高温高压变形实验仪器实验技术受限,国际上对石榴子石流变学性质的研究主要集中于变形显微结构分析与低压变形实验模拟,就其高温高压条件下变形行为的实验研究很少。目前,水对石榴子石流变学性质的具体影响如何更是缺乏含水条件下石榴子石高温高压变形实验的定量研究,这在一定程度上限制了我们对富水环境下俯冲洋壳的变形行为及其相关地球动力学问题(如,拆离作用和深俯冲作用等)的认识。为探究高温高压条件下水对石榴子石矿物流变学性质的影响,本论文主要利用配合有同步辐射源的Deformation-DIA(简称D-DIA)高压实验装置对大别—苏鲁超高压变质带超高压榴辉岩中石榴子石进行了含水条件下的一系列高温高压流变实验研究工作。该工作在美国纽约布鲁克海文国家实验室(BNL)国家同步辐射光源中心(NSLS)完成。围绕这一研究工作,论文主要包括以下三个方面的研究内容:含水条件下石榴子石的高温高压三轴压缩变形实验;变形实验后石榴子石样品中含水量的测试与分析;变形实验后石榴子石样品显微结构的观察。(1)本论文利用结合同步辐射X射线衍射技术的D-DIA装置共对7个含水石榴子石样品完成了13组不同温度(1223≤T≤1423K)与压力(1.6≤P≤5.6GPa)条件下的流变学实验。石榴子石样品的高温高压变形过程中,水源是由样品组合装置中的滑石通过高温脱水反应提供。在每组变形实验过程中,石榴子石样品以某一固定的应变速率(0.64×104-3.7×10-5s-1)完成竖直轴向总体应变量为6-30%不等的变形。D-DIA装置是最近十年来兴起的一种新型高温高压变形实验设备,通常可实现的最高压力为15GPa和温度约为2000K。同步辐射X射线为高温高压变形实验过程中样品所受应力和样品应变量及应变速率的分析提供了原位(in situ)测量分析的手段。样品变形过程中,同步辐射X射线通过样品后,不同方位的探测器可以同时接受样品晶面的x射线衍射信号,从而产生样品晶面x射线衍射图谱,并且,CCD相机可以拍摄下不同时间间隔内样品的X射线照片。样品所受差异应力和样品所产生应变量是分别通过同步辐射X射线衍射图谱和X射线照片进行确定:通过对样品X射线衍射图谱数据的分析可获得样品弹性应变量,根据胡克定律可进一步确定样品所受差异应力大小;通过对一系列不同时间间隔内样品x射线照片中样品竖直轴向长度的测量,可以获得样品长度与其对应变形累计时间之间的关系,根据样品初始长度可以计算出每一变形时间点的样品应变量。对已获得的样品应变量与其对应变形实验时间的数据对可以进行一一投图。在应变量—变形时间关系图中,可以对每一实验变形稳定阶段(P,T和σ保持恒定)的数据点进行线性拟合,由直线斜率可求得该变形阶段对应的样品应变速率ε。(2)利用傅利叶变换红外光谱仪(FTIR)对变形实验后石榴子石样品中含水量进行了红外光谱测试,并根据Paterson公式对含水量大小进行了计算。石榴子石样品的红外光谱显示,其位于3600cm-1附近的红外吸收峰峰位随压力增加而发生移动,从3570cm-1附近移至3625cm-1附近。并且,在相对低压(1.6-2.9GPa)条件下,石榴子石样品红外吸收峰强度随压力增加而增加,而在相对高压(4.9-5.6GPa)条件下,石榴子石样品红外吸收峰强度随压力增加而减小。同时,每一个石榴子石样品中不同测试点的红外吸收光谱形状都很类似,而吸收峰强度大小在同一样品不同测试点却略有不同。利用Paterson公式对石榴子石样品的含水量计算结果表明,同一样品中不同测试点的含水量明显不同。因此,论文中给出了石榴子石样品中含水量范围及平均值。对每一个石榴子石样品,通过分析至少5个不同的FTIR光谱来确定其平均含水量。对不同实验样品中含水量的比较发现,在P=1.6-2.9GPa范围,石榴子石样品中的平均含水量随压力增大从7440H/106Si增加至21000H/106Si,但是在P=4.9-5.6GPa范围,其随压力增大逐渐降低至~3000H/106Si。为测量实验中水的饱和程度,石榴子石样品组合装置中放置了圣卡洛斯橄榄石样品。因为就橄榄石中水溶解度与温度、压力及水逸度之间的关系前人已经进行了详细、系统的研究。我们同样利用Paterson公式对橄榄石样品中含水量进行了计算。结果显示,其水溶解度随水逸度(fH2O)增加而呈现出近线性增加关系,这与前人对橄榄石矿物单晶水溶解度研究中得出的结论非常一致。基于我们橄榄石样品中水溶解度与水逸度(和压力)之间的系统性依赖关系,推断本论文中的变形实验是在水饱和条件下完成的。其中,fH2O是利用Pitzer and Sterner(1994)中的方法根据我们变形实验中的温压条件进行计算获得。重要的是,对于石榴子石样品,其含水量与实验压力之间的关系显示,随着压力从1.6GPa增至2.9GPa,石榴子石含水量随水逸度(其与压力呈正相关)增加而增加,而后在压力为4.9GPa和5.6GPa附近的高水逸度条件下,石榴子石含水量却明显降低。这一实验结果第一次证实了前人对高压下合成镁铝榴石单晶样品实验中所获得的水溶解度随压力变化规律的相关研究成果。(3)利用扫描电镜(SEM)及电子背散射衍射技术(EBSD)对变形实验后石榴子石样品的显微结构进行了观察与分析。显微照片显示,样品中颗粒边界明显呈不平整的锯齿状,指示在变形过程中发生了颗粒边界迁移。大量细小颗粒围绕较大颗粒的现象表明变形过程中样品发生了动态重结晶作用,使得大颗粒周围产生了许多细小重结晶颗粒。另外,样品颗粒明显沿竖直轴向压缩方向发生了压扁变形,呈扁平状。基于样品的显微结构特征,我们推断实验中石榴子石的变形机制以位错蠕变为主导并伴有动态重结晶作用。样品组构特征的EBSD测量结果表明,变形石榴子石样品的结晶学优选方位(CPOs)并不明显,样品中各晶体呈现出随机不定向性的分布特点,这与前人对天然变形石榴子石及实验变形石榴子石的EBSD研究结果一致。根据我们实验变形中石榴子石样品伴有动态重结晶作用,并结合前人的研究认识,石榴子石晶体呈立方对称、具有66组可能的滑移系且在塑性变形中多组滑移系可同时启动,这些可以很好地解释以位错蠕变为主导变形机制的含水石榴子石样品未发育明显结晶学优选方位(CPOs)的问题。综合以上三个方面的研究结果与分析,我们对含水石榴子石样品确立了水饱和条件下的流变律:其中ε是应变速率(单位为s-1),σ是差异应力(单位为MPa),fH2O是水逸度(单位为MPa),R是气体常数,P和T分别是压力(单位为MPa)和温度(单位为K)。这是目前对石榴子石多晶流变学性质研究中获得的第一个含水条件下的流变律。与前人实验结果的对比显示,在P=2GPa,T=1473K时,位错蠕变机制下,对于给定的应力,水饱和情况下的石榴子石蠕变速率要比不含水条件下的结果高出近2个数量级。这表明,在一定温压范围内,水的确对石榴子石蠕变强度具有明显的弱化作用,从而为解释一些天然岩石中含水石榴子石的变形现象提供了定量实验方面的证据。同时,该流变律的建立为俯冲洋壳的流变学性质提供了重要的实验约束资料。在富水环境中,在上地幔浅部的相对低压条件下,由于石榴子石中含水量随压力增加而增加,水的作用使得俯冲板块中富石榴子石层的黏度明显降低;而随着深度的逐渐增加,在某一深度(对应于某特定压力,例如,水饱和条件下对应深部与压力分别约为100km和3.5GPa)处,富石榴子石层的黏度将开始转变为随压力增加而逐渐增大,这是因为当压力达到一定数值时,压力通过含水条件下相对较大的活化体积数值(V*=28×10-6m3/mol)对含水石榴子石蠕变强度的直接增强作用将开始明显大于压力通过水溶解度对石榴子石蠕变强度的间接削弱作用,从而最终导致含水石榴子石蠕变强度随压力的增加而增强。因此,即使俯冲大洋岩石圈处于高富水条件下的地幔转换带环境中,俯冲板块富石榴子石层仍可与其下伏俯冲板块之间存在较高的黏度差而发生拆离,而后滞留于地幔俯冲带附近处。这或许对理解一些俯冲带地区地幔转换带410km地震不连续面附近存在的地震波分裂现象及一些地区地幔转换带底部660km附近出现的地震波低波速异常现象具有一定启示意义。同时,本论文还简要介绍了博士学习阶段对另外两个方面实验的初步尝试性研究工作。一是利用明尼苏达大学Paterson流变仪,尝试对天然含水钙铁榴石完成了2个热压实验和5个变形实验。虽然实验方法还有待进一步改进,但是该实验尝试的主要收获有:a)认识到氧逸度缓冲剂对钙铁榴石稳定性的重要作用;b)每组实验的实验结果均显示应力指数n≈3,指示含水钙铁榴石样品变形主导变形机制为位错蠕变。二是在无同步辐射光源条件下,利用明尼苏达大学D-DIA装置对上下叠置的超高压榴辉岩中石榴子石矿物与圣卡洛斯橄榄石矿物同时进行了初步简单剪切试验。实验样品中应变标示体的形态特征直观显示,不含水条件下石榴子石样品的流变强度明显大于橄榄石样品的流变强度。利用D-DIA装置进行简单剪切实验的可行性,为接下来利用配合同步辐射X射线技术的D-DIA装置对含水条件下石榴子石矿物与橄榄石矿物流变强度差进行比较的简单剪切实验提供了可能。
Although garnet is a relatively minor component of Earth's crust and upper mantle, it may significantly affect the deformation of the lower continental crust, of ultra-high pressure rocks associated with subduction, such as eclogites, and of the upper mantle. Especially, as garnet is an important constituent mineral in subducted slabs and in the transition zone, knowledge of rheological properties of garnet is essential to understanding the geodynamic response of these important regions of Earth's interior. However, relatively little is currently known about the creep strength of garnet at high pressure. Although several studies have been made to constrain the rheological properties of garnet or garnet analogs, experimental results mainly focused on deformation micro structures or mechanical data at lower pressures. Quantitative experimental investigations of the creep strength of garnet at mantle conditions have been hindered due to technical limitations. To date, few deformation experiments of garnet at high temperatures and pressures have been carried out, especially studies that quantify the effect of water on the creep behavior of garnet are very limited. As a consequence, the deformation behavior of subducted slabs and its related geodynamics processes (e.g., delamination, deep subduction) in a water-rich enviroment remain poorly understood.To explore experimentally the influence of water on the rheological properties of garnet at high temperatures and pressures, deformation experiments of garnet under hydrous conditions at at high temperatures and pressures have been carried out in this thesis. The garnet samples were fabricated from crushed eclogite rocks collected from Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt of China. The deformation apparatus is deformation-DIA (D-DIA) coupled with the synchrotron X-ray source available at beam line X17B2of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, USA. The research work in this thesis mainly includes triaxial compressive creep experiments of hydrous garnet at high temperatures and high pressures, measurements of water content of deformed garnet samples and preliminary scanning electronic microscope (SEM) and electron back-scattered diffraction (EBSD) studies on their deformation microstructures.(1) Thirteen deformation experiments were carried out on seven samples at pressures of1.6to5.6GPa and temperatures of1223to1423K using a deformation-DIA apparatus coupled with synchrotron X-rays. The garnet samples were deformed at constant strain rates ranging from0.64to3.7×10-5s-1to total axial strains of6to30%. Dehydration of a talc sleeve in the sample assembly provided a water source for establishing a hydrous environment during the experiment. Synchrotron X-rays provided the means of in situ determining the strain (and strain rate), stress and pressure of the sample.(2) Infrared spectra were obtained for the doubly polished samples after deformation experiments by a Bruker Tensor37Fourier transform infrared (FTIR) spectrometer equipped with a Hyperion2000super-microscope. Water concentrations in the samples were determined based on Paterson's calibration.For garnet samples, the absorption peak that occurs at~3600cm-1shifts from3570to3625cm-1with increasing pressure. The band intensity of infrared absorption for garnet samples increases with increasing pressure at lower pressures (1.6-2.9GPa), and then decreases at higher pressures (4.9-5.6GPa). The shape of the infrared spectra for each sample is similar from one region to the next, although the height of the absorption peak varies from one point to the next wihin a sample.The water concentrations calculated from Paterson's calibration for garnet indicate that, the water content in each garnet sample varies from one point to another within a same sample. Therefore, for garnet, we report the range of water content and the average value. Average water content was determined from at least5FTIR spectra for each sample. From a comparison of water content in our samples, it is clear that the average water content in garnet increases from7440to21000H/106Si as pressure increases from1.6to2.9GPa, but then decreases to~3000H/106Si as pressure increases to4.9and5.6GPa.To measure the degree of water saturation in each experiment, a San Carlos olivine sample was stacked with a garnet sample during the preparation of each sample assembly. Water solubility for olivine as a function of temperature, pressure and water fugacity has previously been experimentally determined, while little is known about water solubility in garnets of our composition. For our olivine samples, the approximately linear increase in water solubility with increasing water fugacity is in good agreement with that reported for single crystals. Based on this systematic dependence of water solubility on water fugacity (and pressure) in our olivine samples, we conclude that our deformation experiments were carried out under water-saturated conditions. Water fugacity was calculated based on the equation of state for water at experimental P-T conditions.More importantly, the water content in garnet increases with increasing water fugacity as pressure increases from1.6to2.9GPa, but then decreases even with higher water fugacity at pressures of4.9and5.6GPa. This result is very similar to that of previous study on the water solubility of the synthetic pyrope samples.Both SEM and EBSD techniques were used for microstructural and crystallographic analyses of garnet samples after deformation experiments. From the SEM images, grain boundaries are distinctly serrated, indicating that grain boundary migration was active during deformation. A population of smaller grains surrounding larger grains suggests that dynamic recrystallization was active during deformation. Furthermore, it is evident that individual grains shortened in the direction of the compression. Our observations of these deformation microstructures suggest that the dominant deformation mechanism of our garnet samples is dislocation glide assisted by dynamic recrystallization. Finally, based on electron backscattered diffraction analyses, we note that our samples exhibit a weak crystallographic preferred orientation (CPO). Because garnet has a cubic crystal structure with66possible slip systems available for plastic deformation, garnets deformed by dislocation creep in nature and in the laboratory typically exhibit similarly weak CPOs.A fit of our experimental results on the creep of garnet samples to a power law yields where ε is the axial strain rate in s-1, σ is the differential stress in MPa,fH2O is the water fugacity in MPa, R is the gas constant, and P and T are pressure in MPa and temperature in K, respectively. This flow law demonstrates strong dependencies of strain rate on water fugacity with a water fugacity exponent of r=1and on pressure with an activation volume of V*=28×10-6m3/mol. A comparison of our creep results with those from previous studies on the dislocation creep of garnet at high pressure was made. At a pressure of2GPa and temperature of1473K, for a given stress under water-saturated conditions, it is clear that garnet creeps~2orders of magnitude faster under hydrous conditions than under anhydrous conditions, demonstrating the significant effect of water on the creep strength of garnet aggregates in the dislocation creep regime. Based on the results of our present study, relative to other garnet-bearing rocks, the lower creep strength of garnet in some naturally deformed garnet-rich rocks from the shallow upper mantle reported in previous studies can be explained by the role of water weakening. Importantly, the flow law for garnet samples deformed in the dislocation creep regime under hydrous condition in our study provides an important constraint on the viscosity of the garnet-rich layer of the subducted oceanic lithosphere in a subduction zone and in the transition zone, where rocks are likely enriched in water. In a water-rich environment, the viscosity of a garnet-rich layer in a subducting slab is lowered by the presence of water in the shallow upper mantle, because garnet becomes weaker with increasing water content for the increasing depth. However, garnet then becomes stronger with further increase in depth. This transition occurs at about the pressure at which water solubility begins to decrease with increasing pressure. Furthermore, at greater depths, the viscosity will be high because the direct effect of pressure on the creep through thermally activated processes outweighs the indirect effect on the creep through water solubility. Thus, in the deeper part of the upper mantle, the process of delamination of the subducting oceanic crust from the decending slabs should occur because of the high viscosity contrast, even if the the subducted crust is in the water-enriched mantle transition zone. The delamination and stagnation of the subducted oceanic crust around the mantle transition zone may provide a clue to understanding the observed feature that410-km discontinuity is split into two discontinuities in some subduction zones and the deflection of the slow velocity anomalies imaged by seismic tomography at the660km discontinuity in some locations that close to the subducted plates.In addition, preliminary results from deformation experiments at lower pressures in a Paterson apparatus and higher pressures in a D-DIA apparatus without a synchrotron radiation source have been briefly included in this thesis. Firstly, we attempted to carry out two hot-pressing runs and four triaxial compressive experiments and one compression-extension test on hydrous andradite using the gas-medium (Paterson) apparatus at the rock deformation laboratory in the University of Minnesota. From this work, we reached two main points:a) the oxygen buffer is important for the stability of andradite; b) the mechanical data from each deformation experiment yield a stress exponent n≈3, suggesting that dislocation creep is the dominant deformation mechanism for the andradite samples. Secondly, under anhydrous conditions, two simple shear deformation experiments on garnet samples and San Carlos olivine samples were completed with a D-DIA apparatus without X-ray beam at the rock deformation laboratory in the Uinversity of Minnesota. The strain markers in the deformed samples, rotated farther in the olivine sample than in the associated garnet sample, indicating the creep strength of garnet aggregates is larger than that of the olivine aggregates under anhydrous conditions.