Jean Dubessy

Fluid regimes during late stages of a continental collision: Physical, chemical, and stable isotope measurements of fluid inclusions in fissure quartz from a geotraverse through the Central Alps, Switzerland☆

Fluid evolution during neo-alpine metamorphism during late stages of the continental collision between Europe and Africa was studied by analyzing fluid inclusions in alpine fissure quartz collected in forty-nine localities along a geotraverse through the Central Alps, Switzerland. The methods employed include microthermometry, micro-Raman spectroscopy, K/Na thermometry, and stable isotope analysis. Early fluid inclusions provide evidence of close to peak metamorphic temperatures of the late Tertiary or neo-alpine metamorphic event. Fluid composition evolved along the geotraverse from north to south as follows: higher hydrocarbons were dominant in the low- and medium-grade diagenetic zones, methane was the main volatile in the high-grade diagenetic and low-grade anchizone, water dominated in the highgrade anchizone and low-grade epizone, with CO2 > 10 mol% in the high-grade epizone and in the mesozone. Higher hydrocarbons and CH4 were the products of kerogen maturation and cracking of preexisting petroleum. Large water supplies originated from the dehydration of cooler metasedimentary rocks that were overthrust by crystalline basements of the Lepontines, Aar, and Gotthard massifs. Carbon isotope analyses suggest that the CO2 component was derived from oxidation of graphitic matter, especially in the vicinity of sulfate-bearing metasediments and from decarbonation reactions. In the Aar and Gotthard massifs as well as in the Helvetic Axen nappe and its underlying North Helvetic flysch, high fluid pressures prevailed and favored nappe transport. By contrast, in the southern Lepontine area, very low early fluid pressures were probably related to dry rocks and scarce metasediments, and to high geothermal gradients that resulted from intense uplift and erosion between 26 and 18 Ma. Retrograde fluid evolution was recorded by a succession of fluid inclusion populations in each alpine fissure. It was controlled by uplift and cooling and characterized by decreasing contents of volatiles and an increase in δ18O of host quartz. Tectonic activity led to episodic pressure drops of at least 0.5 to 2 kbar and promoted fluid unmixing, channelized flow, and rapid growth of skeletal quartz. Channelized rather than pervasive fluid migration at temperatures < 450°C and under conditions of brittle deformation is documented by episodic increases in salinity and by fluid flushing through the massifs. There is stable isotope evidence for involvement of meteoric water only in late-crystallizing quartz. Formation of Alpine fissures and fissure minerals was the result of a unique coincidence of late continental collision (< 450°C), fluid expulsion from overthrust metasediments, uplift, and erosion.

An optical cell for Raman spectroscopic studies of supercritical fluids and its application to the study of water to 500°C and 2000 bar

A high-temperature, high-pressure optical cell has been developed for the study of aqueous solutions by Raman spectroscopy. The disk-shaped cell has a sample volume of < 1 ml and utilizes diamond or sapphire windows set at 90° to one-another. Temperatures to 700°C and pressures to 4000 bar have been attained as measured using an internal thermocouple and a strain gauge. The apparatus was employed in the study of water to 500°C and 2000 bar with spectra of the OH stretching mode being collected at intervals of 50°C and 250 bar. A low-frequency shoulder between 3250 and 3300 cm−1 was found to persist to the maximum temperatures to at least 450°C at pressures above those of the liquid vapor curve; its intensity decreased with increasing temperature and decreasing pressure. The frequency of the maximum intensity of the spectral envelope increased dramatically with temperature to above 250°C and was found to be linear with respect to density and independent of temperature at constant density above 250°C. Similar behavior is seen for the viscosity, dielectric constant and the limiting equivalent conductances of pure water. The data indicate the presence of intermolecular hydrogen bonding to temperatures to well above 300°C at densities above the critical density.

Radiolysis evidenced by H2-O2 and H2-bearing fluid inclusions in three uranium deposits

H2 with or without O2 has been identified by Raman spectroscopy in fluid inclusions in quartz from three Precambrian uranium deposits: Oklo (Gabon), Rabbit Lake and Cluff Lake D (Saskatchewan, Canada). In the Oklo uranium deposit, heterogeneous trapping of fluids in the system H2O-H2-CH4-salts is spatially associated with the natural nuclear reaction zones. At Rabbit Lake and Cluff Lake D, fluid inclusions show heterogeneous trapping of an aqueous liquid phase and a vapour phase with variable X(O2)X(H2) ratios always greater than one. H2 and O2 formation is discussed in terms of chemical reactions, chemical reactions indirectly induced by nuclear reactions and radiolytic processes. Numerical calculations of water radiolysis were carried out for two models: 1) water at the contact with an uranium oxide and 2) water inside a fluid inclusion containing either dissolved uranium or a small UO2 crystal. The concentrations of O2 and H2 inside fluid inclusions from Rabbit Lake and Cluff Lake D deposits are consistent with the first model. In the Oklo deposit, the H2-bearing fluid inclusions found in the quartz and the anomalous high OC atomic ratio of organic matter from nuclear reactor zones could result from radiolysis of water and organic matter due to the fission products of the chain nuclear reactions, but α radiolysis is not definitely ruled out.

Characterization of graphite alteration in an uranium deposit by micro-Raman spectroscopy, X-ray diffraction transmission, electron microscopy and scanning electron microscopy

Abstract A series of graphitic samples associated with a uranium deposit has been studied by micro-Raman spectrometry, transmission electron microscopy and X-Ray diffraction. The dependence of the Raman spectrum on the orientation with respect to the laser beam is explained both from the structure of the tensor components associated with the different vibrational modes ( E 2 g 2 and defect bands) and from the analysed volume by the spectrometer. Doubly polished thin rock sections, usually made for classical petrographic observations, are not suitable for Raman analysis because polishing damages the structural order of graphitic compounds. A progressive and continuous but still small loss of structural ordering along the c axis is shown from both second order Raman spectra, interpreted by the model of Lespade and TEM measurements. Comparison with XRD data leads to the conclusion that the degradation of graphite occurs only on the surface and is probably linked with the alteration of its host-rock. In addition to these defects, which were suggested by variations of the reflectance, graphite leaves close to uranium concentrations exhibit hollow points with diameters up to several micrometers. Their characterization by TEM and Raman spectrometry has shown an amorphous structure of the carbon. They probably originate from a higher degree of graphite alteration.

Determination of Chlorinity in Aqueous Fluids Using Raman Spectroscopy of the Stretching Band of Water at Room Temperature: Application to Fluid Inclusions

A new analytical method, based on the Raman spectroscopy of the ν(OH) stretching vibration of water, has been developed for the determination of the concentration of chloride in aqueous solutions with the goal of reconstructing the bulk ion content of fluid inclusions that are relics of paleo-fluid circulation in rocks. The method involves calibrating the area of one band of the spectrum difference between pure water and solutions of appropriate composition with respect to the chloride concentration. Calibration curves were constructed for the major geological chemical salts LiCl, NaCl, KCl, CaCl2, and MgCl2, and NaCl–CaCl2 systems. The application to fluid inclusions has been confirmed using synthetic fluid inclusions. For cubic minerals such as fluorite, the calibration curve for the NaCl system correctly estimates the chlorinity. For birefringent minerals, such as quartz, the Raman spectrum of the aqueous solution depends on the orientation of the host crystal. The crystal must be oriented in such a way that one axis of the ellipse of the indicatrix projects parallel to the spectrometer slit. This method complements micro-thermometric data and allows the determination of chlorinity when ice-melting temperature cannot be used.

Methane-bearing aqueous fluid inclusions: Raman analysis, thermodynamic modelling and application to petroleum basins

Abstract Calibration for the determination of the CH4/H2O ratio using Raman spectroscopy is carried out using synthetic fluid inclusions with 0 m NaCl. Spectra of the symmetric stretching band of methane (ν1,CH4), and the bending (ν2,H2O) and stretching (νS,H2O) bands of water were obtained in the aqueous phase co-existing with the vapour phase at variable temperatures from 25° to a few degrees above the homogenisation temperature. A software program, based on the model of Duan et al. (1992a) [Duan, Z., Moller, N., Greenberg, J.H., Weare, J.H. 1992a. The prediction of methane solubility in natural waters to high ionic strength from 0°C to 250°C and from 0 to 1600 bar. Geochim. Cosmochim. Acta, 56, 1451–1460.] has been developed to calculate the composition of the aqueous liquid phase co-existing with the vapour phase in the synthetic fluid inclusions. Application to natural samples from the Alwyn South area (North Sea) shows that Raman analyses permit discrimination between inclusion populations that are not recognised using only microthermometric measurements. These results provide important constraints on the P–T conditions of inclusion formation and suggest that oil migration occurred at fluid pressures of approximately 240 bars.

Improvements in clathrate modelling: I. The H2O-CO2 system with various salts

Abstract The formation of clathrates in fluid inclusions during microthermometric measurements is typical for most natural fluid systems which include a mixture of H 2 O, gases, and electrolytes. A general model is proposed which gives a complete description of the CO 2 clathrate stability field between 253–293 K and 0–200 MPa, and which can be applied to NaCl, KCl, and CaCl 2 bearing systems. The basic concept of the model is the equality of the chemical potential of H 2 O in coexisting phases, after classical clathrate modelling. None of the original clathrate models had used a complete set of the most accurate values for the many parameters involved. The lack of well-defined standard conditions and of a thorough error analysis resulted in inaccurate estimation of clathrate stability conditions. According to our modifications which include the use of the most accurate parameters available, the semi-empirical model for the binary H 2 O-CO 2 system is improved by the estimation of numerically optimised Kihara parameters σ = 365.9 pm and ɛ/k = 174.44 K at low pressures, and σ = 363.92 pm and e/k = 174.46 K at high pressures. Including the error indications of individual parameters involved in clathrate modelling, a range of 365.08–366.52 pm and 171.3–177.8 K allows a 2% accuracy in the modelled CO 2 clathrate formation pressure at selected temperatures below Q 2 conditions. A combination of the osmotic coefficient for binary salt-H 2 O systems and Henrys constant for gas-H 2 O systems is sufficiently accurate to estimate the activity of H 2 O in aqueous solutions and the stability conditions of clathrate in electrolyte-bearing systems. The available data on salt-bearing systems is inconsistent, but our improved clathrate stability model is able to reproduce average values. The proposed modifications in clathrate modelling can be used to perform more accurate estimations of bulk density and composition of individual fluid inclusions from clathrate melting temperatures. Our model is included in several computer programs which can be applied to fluid inclusion studies.

CO2-CO fluid inclusions in a composite peridotite xenolith: implications for upper mantle oxygen fugacity

AbstractFluid inclusions occur in a composite xenolith from the Lunar Crater Volcanic Field, Nevada, U.S.A. The xenolith is an amphibole-bearing wehrlite that is cut by an andesine-amphibole vein. The compositions of individual fluid inclusions in both portions of the xenolith have been determined using microthermometry and micro Laser-Raman spectroscopy. Fluids in the host wehrlite are nearly pure CO2 (>99 mol%) whereas those in the vein contain from 8.5 to 12.0 mol % CO in CO2. Chemical modelling shows that the composition of the vein fluids at Troom is representative of the composition at the high P, T conditions of trapping. Graphite has not been observed by optical microscopy in any of the fluid inclusions. Graphite is probably absent (although stable at T<800° C) most probably because of the kinetically unfavorable CO decomposition reaction and rapid quenching. By combining the measured fluid compositions with fluid P-V-T data and the chemical equilibrium CO2⇋CO +1/2 O2, we have calculated the oxygen fugacity of the fluid inclusions at 1200° C: log

Linked fluid and tectonic evolution in the High Himalaya mountains (Nepal)

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