【作者】 盛英明；

【导师】 夏群科；

【作者基本信息】 中国科学技术大学 ， 地球化学， 2006， 博士

【摘要】 高压-超高压变质流体的示踪一直是地学界关注的焦点和前沿领域，也一直是大陆造山带动力学研究的薄弱环节，大陆板块深俯冲到一定深度后（～90-110 km），即使没有含水矿物的存在，超高压岩石中的名义上无水矿物（nominally anhydrous minerals，简为NAMs，即理想化学式中不含H的矿物，如石榴石、辉石、金红石等）仍然可以携带一定量的水，成为俯冲板块中水的重要载体，是示踪超高压变质流体的重要途径。 本文以大别山地区典型的超高压榴辉岩为主要研究对象，选取了一些典型产地：南大别超高压变质带的碧溪岭、双河、五庙、毛屋、新店、三祖寺和北大别变质杂岩带的黄尾、百丈崖以及宿松低温变质杂岩带的黄镇等地的～70个新鲜榴辉岩样品，运用显微傅立叶变换红外光谱（FTIR）和电子探针（EMP）技术详细观察了榴辉岩的主要构成矿物（石榴石、绿辉石、金红石等NAMs）中结构水的存在与否、可能的赋存方式以及水含量在不同尺度上的分布情况，近2000个FTIR光谱分析点的结果发现这些矿物中都含有或多或少的以缺陷形式存在的结构水，这些微量水的存在必然对整个变质体系的物理化学性质产生显著的影响。 在典型的OH红外吸收区域(3000-3800cm^-1)，石榴石的吸收峰出现在3610-3630 cm^-1、3560-3580cm^-1和3400-3420 cm^-1，前两组峰对应OH形式存在的结构水，而3400-3420 cm^-1峰可能是由显微流体包裹体中的H₂O造成的，结构OH含量与主要化学组成之间不存在任何相关性，表明主要元素不是控制石榴石容纳OH的主要因素，与典型水合钙铝榴石的红外光谱的比较发现，SiO₄^4-<->（O₄H₄）^4-取代可能是最主要的赋存方式。石榴石的水含量（表示为H₂O wt．，下同）范围为14-1915 ppm，多数集中在200-800 ppm，而南大别和北大别石榴石水含量的变化范围基本一致。绿辉石的OH吸收峰出现在～3450 cm^-1、～3520 cm^-1和～3620 cm^-1，都是结构OH，结构水含量与M2空位含量表现出大致的正相关，因此主要的取代方式可能是OH占据O2氧离子位置并直接指向M2空位。绿辉石的结构水含量的变化范围为115-695 ppm，黄镇低温榴辉岩中绿辉石的水含量总体上要高于南大别。金红石的OH吸收峰以3280 cm^-1和3295 cm)（-1）为主，个别颗粒出现～3360 cm^-1较弱且宽的峰，结构水同样以OH的形式赋存，Ti³⁺+H⁺<->Ti⁴和Fe³⁺+H⁺<->Ti⁴⁺取代可能是H进入金红石结构的重要方式，水含量的变化范围为161-854 ppm。由构成矿物的百分含量及其结构水含量计算的榴辉岩全岩水含量为220-750 ppm。以上结果表明即使大陆板块深俯冲到超过200 km的地幔深部，石榴石、绿辉石和金红石等NAMs仍然可以携带数百ppm的水，构成壳幔之间水循环的重要途径。 无论是石榴石、辉石还是金红石，结构水含量都表现出手标本尺度上的明显不均一性。此外，石榴石的水含量在cm尺度上表现出明显的变化。这样的不均一分布不但说明了超高压变质阶段自由流体的小尺度活动性，而且表明板块的俯冲-折返过程都是十分快速的。石榴石和绿辉石部分颗粒的水含量表现出核高边低的分布特征，暗示了板块折返的早期阶段，由于压力的骤降，导致这些矿物中的部分结构水可能会扩散出来，释放出来的水构成早期退变质流体的重要来源之一。榴辉岩中石英脉两侧的矿物水含量向脉体方向的近似递减，暗示了形成这些高压石英脉的流体可能来源于名义上无水矿物中结构水的减压出溶，与矿物颗粒水含量核高边低的分布特征所暗示的地球动力学意义是一致的。因此，通过详细观察名义上无水矿物中水含量在不同尺度上的分布特征可以为超高压变质流体活动和板块俯冲-折返动力学过程提供一定的制约条件。 采用常规分析方法（真空抽取Zn还原法）获得了大别山绿辉石的氢同位素组成为-67～-109.6‰，绿辉石的δD值落在大别-苏鲁超高压榴辉岩和片麻岩中多硅白云母（可能的超高压矿物）的δD值范围更多还原

【Abstract】 Investigation of fluid activity during the UHP metamorphism is one of the most important topics in studying geodynamics of orogenic belt, but little is known up to the present. When the continental crust subducted to certain mantle depth （>100 km）, even if no hydrous minerals existed in the plate, the nominally anhydrous minerals （NAMs, namely not H-bearing in the ideal chemical formula, such as garnet, pyroxene, rutile, quartz/coesite） in UHP rocks can contain trace amount of water, therefore become the main carriers to transport water into the mantle, and new ways to trace the UHP metamorphic fluid activity.In this thesis, ～70 fresh eclogite samples were collected from several representative localities in the Dabie orogen: Bixiling, Shuanghe, Wumiao, Maowu, Xindian and Sanzusi in South Dabie; Huangwei and Baizhangya in North Dabie; Huangzhen in the low-temperature metamorphic unit. Garnet, omphacite and rutile in UHP eclogites were analyzed by Micro-FTIR （Fourier Transform Infrared Spectroscopy） and EMP （Electron Microprobe） analysis, aimed to investigating the occurrence and content of structural water on different scales. The results of ～2000 IR analyzed points demonstrate that these NAMs contain trace/minor amount of structural water occurring as OH in point defects. The trace amount of water would drastically change the physical and chemical properties of the whole metamorphic system.In the typical OH vibration region (3000-3800 cm^-1), three group of absorbance bands at 3610-3630 cm^-1, 3560-3580 cm^-1 and 3400-3440 cm^-1 appear in the IR spectra of garnet. Group Ⅰ and Ⅱ bands arose from structural OH, and group Ⅲ was ascribed to H₂O in submicroscopic fluid inclusions. Water contents （wt. H₂O） range from 14 to 1915 ppm, with most being between 200 and 800 ppm, and independent on chemical compositions. The range of water contents in South Dabie garnets is identical with that in North Dabie garnets. Comparison with the IR spectra of hydrogrossular and hydroandradite suggests that group I and II bands in the Dabie garnets are caused by structural OH, and that the SiO₄^4-<->（O₄H₄）^4- substitution is possibly the main substitution mechanism. Three bands at ～3450 cm^-1, ～3520 cm^-1, ～3620 cm^-1 in IR spectra of omphacite were all ascribed to structural OH absorbance with contents varying from 115 to 695 ppm, and water contents in Huangzhen omphacites are slightly higher than that in South Dabie omphacites. A rough positive correlation between water contents and vacancies at M2 sites indicate that the possible substitution in omphacite includes OH occupying the 02 position and points to the M2 vacancy. The main OH absorbance bands of rutile appear at 3280 cm^-1 and 3295 cm^-1, sometimes with a weak and wide band at ～3360 cm^-1 which were all caused by structural OH with content varying from 161 to 854 ppm. Comparison with the experimental results suggests that substitutions of Ti³⁺+H⁺<->Ti⁴ and Fe³⁺+H⁺<->Ti⁴⁺ are important. The estimated whole-rock water content based on mineral water contents and their proportions is between 260 and 750 ppm, thereby implying that eclogitic rocks formed during continental subduction have the potential to recycle （at least） several hundreds ppm water into mantle depths, even the continental slab subducted to mantle depth beyond the stability fields of hydrous minerals.Heterogeneous distribution of water content of garnet, omphacite and rutile occurs at hand specimen scale with various water content of garnet within individual samples. The preserved chemical differences likely indicate that very limited fluid mobility occurred during UHP metamorphism and the eclogitic rocks resided at mantle conditions for a limited time span, and imply that they were exhumed shortly after更多还原