Principles of calibration of HCO3- analysis from Raman spectra at ambient temperature

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Renishaw Raman spectrometer (Renishaw, UK)

In State Key Lab for Mineral Deposit Research (Nanjing University, China), there is a Renishaw® RM2000 type of micro-Raman spectrometer (Figure I-9) from Renishaw plc., United Kingdom (http://www.renishaw.com/). This microprobe is equipped with a Notch® filter and with a CCD detector cooled by air and with a confocal research-grade microscope (Leica®) matching the high quality long working distance objectives of ~ 100×. A diffraction grating of 1800 grooves/mm is chosen to combine reasonable spectral resolution (down to 2 cm-1). Spectral window is around 1200 cm-1. Figure I-10 shows the optical path of this Raman microspectrometer starting from laser light, the exciting radiation at 514.532 nm provided by an air-cooled Ar+ laser (type 5490A, Aiao®) and ending to the collection by CCD detector. Raman data are obtained at different temperature using a heating-freezing stage fixed on the microscope and are present through WIRETM instrument control software by which cosmic ray strikes can be removed automatically from the spectrum.

Synthetic fluid inclusions

It is well-documented that synthetic fluid inclusions can be used to determine P-V-T-X properties and phase relations of fluids, to understand the behaviour of natural fluid inclusions, to calibrate the instruments applied to the analysis of fluid inclusion. Roedder & Kopp (1975) first used the inclusions in synthetic quartz to study interesting problem (errors in measurement of homogenization temperature, errors in calculating pressure corrections, etc.) on fluid inclusion. Shelton & Orville (1980) first documented that synthetic fluid inclusions trap representative samples of fluids at elevated temperatures and pressures. Sterner & Bodnar (1984) summarized the different compositional types of synthesized fluid inclusions in geologically applicable systems. Bodnar & Sterner (1987) introduced and developed the technique to synthesize fluid inclusions by healing preformed micro-fractures in quartz by the fluid at fixed P-T experimental conditions. Studies related to synthetic fluid inclusions include work with pure fluids, H2O and binary and ternary H2O-salt systems (e.g. Bodnar et al., 1985; Zhang & Frantz, 1987; Sterner et al., 1988), and work with volatile-bearing fluids, binary H2O-volatile systems and ternary H2O-volatile-salt volatile systems (e.g. Zhang & Frantz, 1992; Schmidt & Bodnar, 2000; Dubessy et al., 2001). The validity of application of synthetic fluid inclusions have been documented (e.g. Bodnar and Sterner, 1985; Zhang and Frantz, 1987; Sterner and Bodnar, 1991).

Autoclaves used for the synthesis of fluid inclusions

Large volume autoclaves and high P-T vessel are used to synthesize fluid inclusions in this work. These are introduced in the following.
Two types of large volume autoclaves are applied for the synthesis of aqueous inclusions and gas-bearing aqueous inclusions. For the synthesis of aqueous inclusions without gas, a simple autoclave is put in an oven to be heated. Pressure evolves along the saturation curves by the fluid system. Figure I-11a is an illustration of the autoclave, which is made of stainless steel. The fluid sample is isolated from the stainless steel by a golden cup with a gold cover (Diameter of underside of cup: 3 cm, height of cup: 4 cm), which is enclosed in the centre of the autoclave. The autoclave is closed by eight hexagonal bolts (figure I-11b). The maximum temperature is 400 °C for this autoclave.
The autoclave for the synthesis of gas-bearing aqueous inclusion is a little different from the previous one and is made in Hastalloy B2. The experimental fluid is loaded in a Teflon cup covered by a Teflon lid and connected to two tubes (figure I-12). One tube extends into the liquid phase and the second ends at the top of the vapour phase. A CO2 tank is connected to the tube extending into the liquid phase and air is removed from the system by CO2 bubbling into the liquid phase and evacuation is carried out through the second tube. The autoclave is loaded inside cylindrical heating element. The temperature is measured by a thermocouple inserted into the liquid phase. The pressure is measured using a gas pressure gauge in connection with the vapour phase.

Adaptation of the system of I-Ming Chou (by J. Dubessy and P. Robert) and general procedure of capillary loading

Figure I-16 illustrates the adaptation system of I-Ming Chou system, carried out at G2R with Pascal Robert, used for sample loaded in sealed capillaries. All the system is linked by stainless metal tubes (1/16 inch or 1/8 inch) and Swagelok® connections and valves. A vacuum pump producing a vacuum down to 0.03 mbar is permanently connected to the stainless steel line heated at 100 °C ~ 120 °C to evacuate any impurity. One-end-sealed silica capillary with or without samples is linked to the system by a special device. Then, air in the sample capillary is evacuated through the metal tubes by vacuum pump. Subsequently, gas samples with triple point temperature higher than the temperature of liquid N2 (-195.8 °C) are added cryogenically by immersing the sealed end of the capillary in liquid nitrogen, and the gas is condensed or frozen into the sealed end of capillary to a solid for several minutes. The pressure in the system lines is measured by a high precision of pressure detector (~ 0.1 mbar). Because of the limitation of pressure detector (~ 1 bar), the gas tank is linked to the system lines by adding a gas reservoir or a stainless metal tube as a pressure buffer and the large difference of volume between the whole system lines and gas reservoir. Valves which separate the system into several parts, also control the gas pressure in the reference volume. As the capillary is loaded, the capillary is evacuated again and the open end is sealed by melting the silica glass using a hydrogen flame while the loaded material at the other extremity of the capillary is frozen, see figure I-17. The typical length of the capillary ranges is between 8 mm for the shortest ones to 15 mm.
Three devices to link the silica glass capillary to the stainless steel line have been designed and checked. The three following constraints are required for these devices: 1) the lack of leakage at the junction between the silica glass capillary (360 μm external diameter) and the stainless steel line (1/16 inch external diameter); 2) the mobile character of the link between the stainless metal and capillary tube; 3) differences in material of two types of capillary. In this work, three methods have been tried.
The first method designed by I-Ming Chou is illustrated in figure I-18a. A hard plastic tube with two ends closed by amber rubber cap. The cap is elastic and has a hole in the centre, which permits to hold the tube when it is inserted into the hole. Because of the micro-dimension of the capillary tube, the open end of capillary tube is previously inserted inside a glass tube for increasing the quality of imperviousness between the amber rubber and the tube by a higher diameter dimension. Glue is used to fix the silica glass capillary tube inside the glass tube. This method has two shortcomings, the imperviousness of this junction is not perfect and the use of rubber cap is limited by its fatigue.

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Principles of calibration of HCO3- analysis from Raman spectra at ambient temperature

Referring to the Raman active bands of relative species, the spectra of the aqueous phase in inclusion are collected in two spectrum windows. The first is covering the wavenumber range 550 – 2150 cm-1, including bicarbonate bands 1017 cm-1 (ν5 C-OH stretching vibration) and 1360 cm-1 (ν3 symmetric CO stretching vibration) and the bending band of liquid water around 1640 cm-1. The second spectral window between 2800 cm-1 and 4000 cm-1, contains the symmetric stretching band of liquid water. The ratio of area bands of bicarbonate with the bending band of liquid water is calculated to calibrate the analysis of concentration of bicarbonate in aqueous solution (Eq. II-4 and II-5).

Method for quantifying CO2 condensed in capillary and the precision of the method

It is possible to quantify the amount of CO2 trapped in sealed capillary. When the capillary sample with pure CO2 is done, it can be quantified by determining the density of CO2 in capillary and measuring the volume occupies within the capillary. This method has been developed for pure CO2 and thus, the volume occupied by CO2 is the total inner capillary volume. Considering that several substances are sealed in capillary at the same time, it is necessary to directly quantify the gas when it is loaded into capillary. This approach is developed to directly estimate the mole number of gas loaded in the capillary using the variation of gas pressure at known temperature and the equation of state of perfect gas. The following describes the approach in detail.
In this study, referring to figure II-18, volume V2 is filled with gas CO2 at first, and then gas enters into the lines marked Vcap. After the closed-end of the capillary is immerged into the liquid nitrogen, gas CO2 is condensed as solid phase in the cold part of the capillary. Before the capillary is sealed, the rest of remaining vapor CO2 in the volume Vcap is evacuated from Vcap at different steps which are described below. The pressure in each step is measured at each step of evacuation by pressure gauge connected with line V2.
The critical point of the procedure consists to know exactly the number of moles of CO2 contained in volume V2. This can be achieved only if pressure P, volume V2 and temperature are known according to the equation of state of perfect gas: TRnVPCO××=×22. As pressure and temperature are measured, the only remaining unknown is V2.

Table of contents :

INTRODUCTION
1. A survey of the main crustal fluids
2. Parameters controlling mass transport by fluids
3. PH of present day geological fluid
3.1. Definition of pH
3.2. Main acid-base equilibria
3.3. PH data and pH measurement of present-day geological fluids
3.4. PH of palaeo-fluids
3.4.1. Palaeo-fluid and fluid inclusions: a brief summary
3.4.2. Use of fluid inclusions for palaeo-pH estimation
CHAPTER I: EXPERIMENTAL METHODS
I.1. Microthermometric measurements
I.1.1. Linkam geology heating-freezing stage
I.1.2. USGS gas-flow heating-freezing stage
I.1.3. Calibration of Linkam and USGS stages
I.2. Raman measurements
I.2.1. Principles of Raman spectroscopy
I.2.2. Principles of Raman spectrometers
I.2.3. Micro-Raman spectrometers used in this work
I.2.3.1. Labram Raman spectrometer (Dilor/Jobin-Yvon/Horiba, USA)
I.2.3.2. Renishaw Raman spectrometer (Renishaw, UK)
I.3. Synthetic samples
I.3.1. Synthetic fluid inclusions
I.3.1.1. Autoclaves used for the synthesis of fluid inclusions
I.3.1.2. Methods for synthesizing fluid inclusions in cold sealed autoclaves
I.3.1.3. Samples of synthetic fluid inclusions
I.3.2. Pure silica capillary
I.3.2.1. Pure silica capillary tube
I.3.2.2. Adaptation of the system of I-Ming Chou (by J. Dubessy and P. Robert) and general procedure of capillary loading
I.3.2.3. Procedures for loading silica glass capillary with different gases, liquids and solids
I.3.2.4. Capillary samples
CHAPTER II: THE SYSTEM H2O-CO2-NaCl-H+-HCO3-: RESULTS OF EXPERIMENTS
II.1. Objectives of the experimental part
II.2. Calibration of analysis of bicarbonates by Raman spectroscopy
II.2.1. Vibrational modes and bands of Raman active of related species in HCO3–bearing aqueous solution
II.2.2. Principles of calibration of HCO3- analysis from Raman spectra at ambient temperature
II.2.2.1. Results obtained from synthetic fluid inclusion
II.2.2.2. Results obtained from silica glass capillaries
II.2.2.3. Comparison between the results from synthetic fluid inclusion and the results from silica glass capillary
II.2.3. Calibration as a function of temperature
II.2.3.1. Results obtained from synthetic fluid inclusions
II.2.3.2. Results obtained from capillary samples
II.3. Calibration of the analysis of CO2 by Raman spectroscopy
II.3.1. Synthesis of fluid from the H2O-CO2-(NaCl) system in capillary
II.3.1.1. Method for loading pure CO2 in silica glass capillary
II.3.1.2. Method for quantifying CO2 condensed in capillary and the precision of the method
II.3.1.3. Validation of the approach and determinations the density of CO2
II.3.1.4. Results and discussion
II.3.1.5. Method for loading CO2 with H2O liquid in silica glass capillary
II.3.2. Synthesis of CO2-bearing silica glass capillary by the thermal decomposition of organic acids: oxalic acid and formic acid
II.3.2.1. Oxalic acid as a source of CO2
II.3.2.2. Formic acid as a source of CO2
II.4. Conclusions and perspectives
CHAPTER III: METHODOLOGY OF DETERMINATION OF PH APPLICATED TO FLUID INCLUSIONS
III.1. Determination of pH in fluid inclusions
III.1.1. The thermodynamic models
III.1.1.1. Thermodynamic model of the aqueous phase: the Pitzer model
III.1.1.2. The equation of state: the Duan model
III.1.2. Theoretical principles
III.1.2.1. Calculation of the concentration of Na+ and Cl-
III.1.2.2. Calculation of bulk composition and density
III.1.2.3. Calculation of molar volume
III.1.2.4. Calculation of pH
III.1.3. The algorithm of program
III.2. Application to natural case
III.2.1. Geology background of Mokrsko gold deposit
III.2.2. Experimental measurements of fluid inclusions
III.2.3. Calculation of pH
CONCLUSIONS AND PERSPECTIVES
1.Conclusions
1.1. A new method for synthesizing fluid inclusions by sealing material into fused-silica glass capillary tubing
1.2. Calibration of the concentration analysis of aqueous species HCO3- and CO2 using micro Raman spectroscopy
1.3. Algorithm for the calculation of pH
1.4. The application on natural case
2. Perspectives
2.1. The calibration of CO2 amount by controlling the loading materials in system H2O-CO2 in silica glass capillary
2.2. The quantification of CO2 and H2O in system H2O-CO2-salts systems should be fully investigated
2.3. The liquid-vapor isopleths of bicarbonate-rich fluids
2.4. The use of temperature of relevant phase equilibria other than Tmice
2.5. Thermodynamic models above 250 °C
2.6. More applications to the natural cases are needed for verifying and improving the methodology
2.7. Other acido-base equilibria of interest

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