中国大陆科学钻探工程主孔超高压岩石中名义上无水矿物的结构水.doc

中国大陆科学钻探工程主孔超高压岩石中名义上无水矿物的结构水 【中文摘要】中国大陆科学钻探工程CCSD位于苏鲁造山带根部的超高压变质带上,主孔井深5158m,获得了很多重要的超高压变质岩样品,受到研究者的广泛关注。钻孔中的榴辉岩累积厚度可达1300米,主要组成矿物为石榴石、绿辉石、石英、金红石等名义上无水矿物NAMs,为研究大陆深俯冲过程中的流体含量、存在形成及演化过程提供了难得的样品。本文利用傅立叶变换红外光谱FTIR技术对CCSD主孔及地表超高压岩石中NAMs进行了原位分析,结果表明,所有的NAMs中都含有一定的结构水,主要以羟基OH的形式存在。其中残留在绿辉石中的结构水含量为23398ppm,表明绿辉石可能是榴辉岩中水的主要寄主矿物。根据红外光谱中吸收峰的外形和位置的不同可以将石榴石分为三类,其中A类石榴石的结构水为经历超高压过程后所保存下来的结构水,B类和C类可能受到后期流体的扩散作用的影响。金红石中残留的结构水可高达1494ppm,蓝晶石为2022ppm,石英中存在有OH形式的水,但含量很低。各种矿物中的结构水的分布在宏观标准和微观标准都表现为不均一,证实超高压变质后期缺乏大规模流体的活动。绿辉石中残余的结构水部分地受矿物成分、全岩成分及矿物组合控制,与峰期变质条件、氧同位素值及岩石退变质程度无关。超基性岩中橄榄石的结构水含量为6377ppm,单斜辉石53163ppm,斜方辉石46ppm。根据各种矿物在岩石中所占的体积含量计算出全岩的结构水含量,榴辉岩中残留的结构水14220ppm,均匀值为76ppm,超基性岩为55.5ppm。根据估算,石榴石和绿辉石各占一半的榴辉岩在峰期变质条件下的含水量为1225ppm,上地幔中的水接近1000ppm。在板块折返过程中将会有1142ppm左右的水发生出溶,参与早期退变质过程。深俯冲过程中发生拆沉的板块将携带着大约1225ppm的水进进上地幔,成为水在壳-幔循环中重要的一环。; 【Abstract】 The Chinese Continental Scientific Drilling Project CCSD is located in the Southern Sulu Ultrahigh-pressure UHP metamorphic belt. The CCSD main hole is 5158m deep. The depth of eclogite in the main hole reaches 1300m, and researchers pay close attention to it. UHP metamorphic rock such as eclogite is composed with garnet, omphacite, quartz and rutile. These minerals are usually called nominally anhydrous minerals NMAs for there is no H in their ulas. Researches in these decades show that almost all the NMAs could dissolve trace water.Analysis of Structure hydroxyl content in NAMs of UHP metamorphic rocks from CCSD and other area was carried out in situ, using the of Fourier trans infrared spectroscopy FTIR. The results show that all these NAMs contain trace water, mainly in the of hydroxyl OH-. FTIR analysis yield H2O concentrations of 23398ppm for omphacite, indicate that omphacite might be the major hydrous phase in eclogite. Garnet is sorted of 3 types depend on the absorption characters of infrared spectroscopy. Water in A type garnet should undergo the UHP metamorphism. Water in type B and C could be affected by diffusion of the later fluid. The hydroxyl remained in rutile reaches 1494ppm at maximum, and a range of 2022ppm for kyanite. Hydroxyl exists in quartz, but the concentrations are especially low. Heterogeneity is found in the same kind of minerals from different samples, in different grains from the same sample and different places in the same grain. The heterogeneity indicates that there are no fluid actives cosmically in the later process. Hydroxyl content in omphacite is correlated partly with its composition, the geochemistry of the whole rock and mineral assemblage, but it shows no correlation with the peak metamorphism conditions, isotope of hydrogen and the retrograde degree. Olivine in ultramafic rocks contains 6377ppm water, 53163ppm for clinopyroxene, and 46ppm for orthopyroxene.Water remains in the whole rock was calculated depend the bulk of minerals. The result shows that there is 14220ppm in eclogite with an average of 76ppm, and 55.5ppm in the ultramafic rocks. It is assumed that 1225ppm water in eclogite with 50 garnet and 50 omphacite during the peak metamorphism process, and the water in the upper mantle should near 1000 ppm. About 1142ppm water exsolves from NAMs when the continental crust exhumation and the fluid is the main source of the HP hydrations of retrogression during the early stage. Descend crust into the deep mantle carries about 1225ppm, and this process is an important part of the circle of water between the earth crust and mantle.