A TC/EA-MS Online Technique Of Determining Mineral Water Content And Hydrogen Isotope And Its Applications To UHP Metamorphic Rock In The Dabie-Sulu Orogenic Belt

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.