【作者】 龚冰；

【导师】 郑永飞；

【作者基本信息】 中国科学技术大学 ， 同位素地球化学， 2010， 博士

【摘要】 大陆板块俯冲和折返过程中流体活动的研究已成为地球科学研究的前沿领域,而矿物中(尤其是名义上无水矿物中)水含量和氢同位素组成的研究能为大陆俯冲和折返过程不同阶段的流体演化提供新的视角。本文将热分解元素分析仪(TC/EA)与同位素气体质谱仪(MS)连接在一起,成功建立了TC/EA-MS在线连续流方法分析矿物水含量和氢同位素组成。应用这个新方法,对大别-苏鲁超高压变质带几个著名产地如碧溪岭、青龙山、桃行、仰口和荣成等五个地区的超高压变质岩进行了系统的矿物水含量和氢同位素分析。结合激光氟化法矿物氧同位素分析结果,为大陆板块俯冲和折返过程中的流体活动提供了新的化学地球动力学制约。应用TC/EA-MS联线的分析方法,不但可以分析含水矿物和名义上无水矿物中的水含量,同时也可以分析其氢同位素组成。我们的TC/EA-MS联线的方法分析样品中水含量和氢同位素组成的分析下限是样品中的水含量在0.01微升(μl)以上。对于已知氢含量为5.0 wt%的标准物质苯甲酸(C7H6O2),其水含量分析的误差在±0.05%以内。对于δD值为?65.7‰的标准物质黑云母NBS30,其氢同位素组成分析的全误差在±1.0‰以内。对于含水矿物,氢同位素组成和水含量的全分析误差分别为±0.5‰和±1%;而对于名义上无水矿物,由于其低的水含量,氢同位素组成和水含量的全分析误差分别高达±3‰和±5%。TC/EA-MS联线的分析方法的分析精度和准确性与传统分析方法具有可比性,因此这是用来测定名义上无水矿物中水含量和氢同位素组成的有效方法。直接应用TC/EA-MS方法分析得到的是矿物中全水的含量和氢同位素组成。为了能够区分名义上无水矿物中的结构OH和分子H2O,我们在进行TC/EA-MS联线分析之前,对样品进行了分步加热,在不同温度下,样品可以释放出不同形式的水。研究发现,超高压变质岩中粒径为50μm的石榴石单矿物颗粒,采用350°C加热4小时的预处理流程,分析得到的水含量和氢同位素组成分别为281±13 ppm (wt.)和?86±6‰。对同一个样品,利用FTIR分析的水含量结果为271±58 ppm (wt.),与TC/EA-MS分析的结果是一致的。因此,经过350°C,4小时加热预处理的石榴石样品中的分子H2O几乎完全除去,而结构OH几乎没有发生丢失,因此所测定的全水氢同位素组成代表了石榴石样品中结构OH的氢同位素组成。石榴石中分子水相对于结构羟基亏损D,并具有更大的活性。利用氢的扩散系数和扩散方程进行计算,可以排除在分步加热和TC/EA-MS分析中氢的扩散对分析结果的影响。应用分步加热和TC/EA-MS分析方法,本文研究了碧溪岭榴辉岩中名义上无水矿物中不同形式的水的含量和氢同位素组成。结果发现,这些名义上无水矿物中结构羟基含量明显大于分子水含量,而且分子水含量变化范围较小。分子水相比结构羟基显示出D的亏损,且具有较大的变化范围,指示了分子水来源的多样性。碧溪岭榴辉岩中石榴石和金红石结构羟基含量与薄片FTIR分析的水含量结果相似,绿辉石、蓝晶石和石英则显示出较高的水含量。碧溪岭榴辉岩中石榴石和绿辉石矿物对之间结构羟基氢同位素氢既呈现平衡分馏,也呈现不平衡分馏,指示在超高压岩片折返过程中,名义上无水矿物与具有内部缓冲性质的退变质流体之间发生了不同程度的氢同位素交换,与矿物氧同位素研究结果一致。碧溪岭榴辉岩中石榴石与绿辉石之间δD值差异与石榴石全水δD值呈正相关,而与绿辉石呈负相关。这种变化特征可能说明,石榴石与绿辉石之间全水氢同位素交换是在相对封闭的体系中进行的,也与矿物氧同位素结果一致。石榴石和绿辉石中结构羟基之间的氢同位素分馏表现出与全水相似的趋势,而分子水则不明显。这可能说明,石榴石与绿辉石之间的氢同位素分馏主要受结构羟基控制。对于超高压榴辉岩中的名义上无水矿物,其中分子水相对于结构羟基亏损D,指示名义上无水矿物在超高压岩石折返过程中,矿物降压脱水过程是首先结构羟基转化分子水,然后是亏损D的分子水优先释放。在深俯冲板片折返的降压过程中,由于结构羟基转化为分子水的过程中伴随着氢同位素动力学分馏,因此在超高压岩片折返过程中,名义上无水矿物中这种不同形式水之间的转化可能导致其氢同位素组成变化的原因之一。通过系统比较大别-苏鲁超高压变质带内碧溪岭、青龙山、桃行、仰口和荣成等五个地区超高压变质岩中单矿物的全水含量和氢同位素组成,结果发现:(1)大部分地区超高压榴辉岩样品中石榴石水含量与氢同位素组成之间表现出负相关性,由于氢同位素分馏的动力学效应,矿物在降压脱水过程中,一般是亏损D同位素的水优先释放,因此这种负相关性说明石榴石水含量的巨大变化主要是由于折返过程中的降压脱水造成的;(2)仰口榴辉岩中后成合晶具有比绿辉石明显较高的水含量和较低的δD值,说明退变质流体主要是亏损D的分子水;(3)在所有产地榴辉岩中,仰口榴辉岩具有相对较高的水含量和δD值,指示在超高压岩片折返过程中,仰口榴辉岩中矿物降压释放出的流体量最小,因此其中矿物的最高水含量可以作为峰期超高压(陆壳俯冲到最大深度)条件下矿物中水的最大溶解度。因此,仰口榴辉岩样品中的最高石榴石水含量近似于石榴石在峰期超高压条件下所能溶解的最大水含量,约为2500 ppm。类似地,与最高石榴石水含量达到平衡配分的绿辉石水含量可以作为绿辉石在峰期超高压条件下所能溶解水含量的最小估计,约为3500 ppm。更多还原

【Abstract】 The study of fluid activity during continental deep-subduction and exhumation has been one of forefront subject in earth science, and the study of the water content and hydrogen isotope composition of water in nominally anhydrous minerals can provide a new insight into fluid regime during continental deep-subduction and exhumation. In this thesis, I have developed the TC/EA-MS online method for extraction of water from natural minerals for water content and isotope analyses on the basis of integrating the thermal conversion elemental analyzer (TC/EA) with continuous-flow mass spectrometry (MS). By using the TC/EA-MS online technique, I have made a systematic analysis of the hydrogen isotopes and water contents (structural hydroxyl content and total water content) in nominally anhydrous minerals (NAMs) from ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu orogenic belt, China. In combination with the structural OH measurement by Fourier transform infrared spectroscopy (FTIR) and the oxygen isotope analysis by the laser fluorination (LF) technique, the results provide insights into fluid regime with reference to the chemical geodynamics of continental subduction-zone metamorphism.This on-line TC/EA/MS continuous flow method is not only capable of determining both total H2O concentration and H isotope composition of hydrous minerals, but also suitable for analyses of NAMs. Calibration curves for the H concentration analysis were obtained by variably weighing a standard material of benzoic acid (C7H6O2) that has an H concentration of 5.0 wt%, with analytical uncertainties better than±0.05% in our runs. Our protocols show that the routine analysis of sample sizes as small as 0.01μl H2O is suitable for both H isotope composition and H2O concentration in hydrous and nominally anhydrous minerals. AδD value for biotite NBS-30 is fixed at ?65.7‰for correction of instrumental fractionations during the routinely H isotopic analysis. The average reproducibility is better than±1.0‰when combining the bulk procedures of water extraction and mass spectrometry analysis. Bulk analytical errors appear to depend on mineral water contents, with absolute reproducibility of±0.5 to±2‰(1σ) forδD values and relative uncertainties of±1% to±3% (1σ) for H2O concentrations. In practice, the analytical errors for theδD value and the H2O content can respectively be as small as±0.5‰and±1% for hydrous minerals, but as large as±3‰and±5% for nominally anhydrous minerals of low water contents. Both precision and accuracy of the TC/EA-MS method are comparable to the conventional manometric methods. Therefore, the TC/EA-MS technique is a powerful tool to quantitatively determine both H2O concentration and H isotope composition of hydrous and nominally anhydrous minerals.Nominally anhydrous minerals contain small amounts of water in the forms of molecular H2O and structural OH. We have developed a stepwise-heating approach to extract the different forms of water for the TC/EA-MS analysis. Grained garnet which was preheated at 350°C for 4 hours also gave constantδD values of ?86±6‰and H2O contents of 281±13 ppm (wt.). The result for the H2O contents agrees with H2O contents of 271±58 ppm (wt.) measured by FTIR for quantitative analysis of structural hydroxyl in the same garnet. Stepwise-heating TC/EA-MS analyses for the garnet show that the molecular H2O are depleted in D relative to structural OH and has higher mobility than the structural OH. By using H diffusion coefficient and diffusion equation, the influence of H diffusion in the stepwise-heating approach and TC/EA-MS analyses was quantitatively evaluated. Therefore, the TC/EA-MS method can be used not only for quantitative determination of both H isotope composition and H2O concentration of hydrous and anhydrous minerals, but also for the concentration of structural hydroxyl in NAMs after stepwise-heating.Stepwise-heating TC/EA-MS analyses for NAMs from UHP eclogite at Bixiling in the Dabie orogen show that the molecular H2O is depleted in D relative to structural OH and has lower concentration than the structural OH. The results for the structural OH contents in garnet and rutile agree with those measured by FTIR. In contrast, the concentration of the structural OH in omphacite, kyanite and quartz is higher than those measured by FTIR. Both equilibrium and disequilibrium H isotope fractionations between garnet and omphacite occur in the eclogite at Bixiling. This indicates the contrasting fractionation behavior of H isotopes in the two series of minerals due to differential exchange of H isotopes with retrograde fluid during exhumation. This is concordant with the result from the mineral O isotope study of the eclogite. Garnet-omphacite H isotope fractionations are positively correlated with theδD values of total water in garnet, but negatively correlated with those in omphacite. This suggests that H isotope exchange between garnet and omphacite occurs in a relatively closed system, also concordant with the result from mineral O isotope study of the eclogite. For the structural OH and total water in garnet and omphacite, H isotope fractionations between garnet and omphacite show a similar tendency. This suggests that H isotope fractionations between garnet and omphacite are controlled by structural OH in NAMs, and hence structural OH is the dominant species in NAMs. Molecular H2O is preferentially liberated during the stepwise-heating analyses and depleted in D relative to structural OH in NAMs, indicating that the decompressional exsolution of molecular H2O proceeded prior to the exsolution of structural OH with decreasing pressure during exhumation. In this regard, structural OH may be released from NAMs by transforming into molecular H2O. H isotope kinetic fractionation may occur between structural OH and molecular H2O during the transformation of structural OH into molecular H2O, resulting in variations in the H isotope composition of NAMs.In view of the results for total water content and H isotopes in NAMs from UHP eclogites in the Dabie-Sulu orogenic belt, the following conclusions can be reached. (1) There is a negative correlation between the total water concentration andδD value of garnet, indicating that the decompressional exsolution of molecular water is a major cause for the large variation in the H isotope composition of NAMs. (2) Symplectites show higher concentration of water and lowerδD values than omphacites in eclogite samples from Yangkou, indicating that retrograde fluid is depleted in D relative to NAMs. (3) Among the all analyses eclogites, the Yangkou eclogite show both the highest water content andδD value of NAMs. This suggests that the Yangkou eclotite would experience least dehydration during exhumation. In this regard, the highest concentration of water in NAMs can be used to provide a proxy for the maximum solubility of water in NAMs at the peak UHP metamorphic conditions. As such, the measured maximum water contents of garnet and omphacite yield the the maximum water solubility of 2500 ppm and 3500 ppm, respectively.更多还原