Hydrogen Isotope Fractionation Research Articles

Equilibrium Oxygen, Hydrogen and Carbon Isotope Fractionation Factors Applicable to Geologic Systems

As demonstrated by the chapters in this short course, stable isotope techniques are an important tool in almost every branch of the earth sciences. Central to many of these applications is a quantitative understanding of equilibrium isotope partitioning between substances. Indeed, it was Harold Urey’s (1947) thermodynamically based estimate of the temperature-dependence of 18O/16O fractionation between calcium carbonate and water, and a recognition of how this information might be used to determine the temperatures of ancient oceans, that launched the science of stable isotope geochemistry. The approach pioneered by Urey has since been used to estimate temperatures for a wide range of geological processes (e.g. Emiliani 1955; Anderson et al. 1971; Clayton 1986; Valley, this volume). In addition to their geothermometric applications, equilibrium fractionation data are also important in the study of fluid-rock interactions, including those associated with diagenetic, hydrothermal, and metamorphic processes (Baumgartner and Valley, this volume; Shanks, this volume). Finally, a knowledge of equilibrium fractionation is a necessary first step in evaluating isotopic disequilibrium, a widespread phenomenon that is increasingly being used to study temporal relationships in geological systems (Cole and Chakraborty, this volume). In the fifty-four years since the publication of Urey’s paper, equilibrium fractionation data have been reported for many minerals and fluids of geological interest. These data were derived from: (1) theoretical calculations following the methods developed by Urey (1947) and Bigeleisen and Mayer (1947); (2) direct laboratory experiments; (3) semi-empirical bond-strength models; and (4) measurement of fractionations in natural samples. Each of these methods has its advantages and disadvantages. However, the availability of a variety of methods for calibrating fractionation factors has led to a plethora of calibrations, not all of which are in agreement. In this chapter, we evaluate the major methods for determining fractionation factors. …

There is no Temperature Dependence of Net Biochemical Fractionation of Hydrogen and Oxygen Isotopes in Tree-Ring Cellulose

The isotopic composition of tree-ring cellulose was obtained over a two-year period from small diameter, riparian zone trees along an elevational transect in Big Cottonwood Canyon, Utah, USA to test for a possible temperature dependence of net biological fractionation during cellulose synthesis. The isotope ratios of stream water varied by only 3.6‰ and 0.2‰ in δD and δ18O, respectively, over an elevation change of 810m. The similarity in stream water and macroenvironment over the short (13km) transect produced nearly constant stem and leaf water δD and δ18O values. In addition, what few seasonal variations observed in the isotopic composition of source water and atmospheric water vapor or in leaf water evaporative enrichment were experienced equally by all sites along the elevational transect. The temperature at each site along the transect spanned a range of ≥ 5°C as calculated using the adiabatic lapse rate. Since the δD and δ18O values of stem and leaf water varied little for these trees over this elevation/ temperature transect, any differences in tree-ring cellulose δD and δ18O values should have been associated with temperature effects on net biological fractionation. However, the slopes of the regressions of elevation versus the δD and δ18O values of tree-ring cellulose were not significantly different from zero indicating little or no temperature dependence of net biological fractionation. Therefore, cross-site climatic reconstruction studies using the isotope ratios of cellulose need not be concerned that temperatures during the growing season have influenced results.

On the Deuterium Abundance on Mars and Some Related Problems

Strong fractionation of deuterium in photolysis of H 2O and above the hygropause reduces the production of HD relative to H 2 on Mars by a factor of 3.7 total. The model by Y. L. Yung et al. (1988, Icarus 76, 146–159) for deuterium fractionation in chemical reactions on Mars corrected for this factor results in (HD/H 2)/(HDO/H 2O)=0.43. This value may fit the deuterium abundance observed by V. A. Krasnopolsky et al. (1998, Science 280, 1576–1580) if the eddy diffusion coefficient does not depend on solar activity: K=1.4×10 13 n −1/2 cm 2 s −1 (model 2). The Mariner 9 observations show very low variability of atomic oxygen at the 1.2 n bar pressure level (h∼125 km) with solar activity. This requires eddy diffusion to be proportional to the solar activity index F 10.7: K=( F 10.7 cm/30)×10 13 n −1/2 cm 2 s −1 (model 1). The fractionation factor for escape of hydrogen isotopes is equal to 0.016 and 0.135 for models 1 and 2. These values have been averaged over the solar cycle. The three-reservoir model for hydrogen isotope fractionation suggested by Krasnopolsky et al. (1998) involves a reservoir composed primarily of water ice in the polar caps that isotopically interacts with the atmosphere. Assuming that water ice is half of the total volume of the polar caps and the polar-layered deposits, the total loss of water from Mars is equal to 65 and 120 m for models 1 and 2, respectively. Along with thermal and nonthermal escape, these values may include the loss of water by oxidation of regolith, if the released hydrogen escaped with isotopic fractionation. Although the solar-wind α particles are the main source of He on Mars, capture of the solar-wind H + and D + ions by Mars has a negligible effect on the thermospheric abundances of H and D. Improved observations of minor components in Mars' thermosphere may resolve the problem of eddy diffusion at various solar activity and choosing between the models.

Constraints on cooling and degassing of pumice during Plinian volcanic eruptions based on model calculations

During explosive volcanic eruptions, pumice clasts are transported into the atmosphere, and their thermal and degassing histories determine how much volatiles are lost syneruptively. This process is studied by combining a steady state eruption column model with a model for the cooling and degassing of pumice. In the model we investigate the impact of various parameters (e.g., mass eruption rate, Biot number, eruption temperature, and geometry) on the cooling and degassing of pumice and find that the Biot number and the eruption temperature are the most influential. During typical risetimes of pumices inside eruption columns (200–300 s), those smaller than 0.5 cm in diameter are found to lose little of their volatiles syneruptively and remain in thermal equilibrium with the plume. In the case of larger pumices the ratio of the cooling timescale (∝ rp2/k, where rp is the pumice radius and k is the thermal diffusivity) to the degassing timescale (∝ ξw2/D0′ where ξω is the vesicle wall thickness and D0′ is the species diffusivity) controls degassing. If this ratio is larger than 50, degassing of the pumice will be nearly complete with a few percent of the original volatiles left in the outer rind of the pumice. For a ratio of less than 0.1 no volatiles escape the pumice. Typical values for thermal and species diffusivity as well as vesicle wall thickness indicate that in the case of degassing Cl and H2O the transition from 0.1 to 50 will be in the pumice size range of 1–10 cm. Here volatile concentration is a strong function of position inside the pumice, suggesting that values for volatile contents measured on picked matrix glass from crushed samples may not be the best way to estimate the volatile content of matrix glass, a number which is frequently used to estimate the total volatile input of volcanic eruptions. This effect is even more pronounced when looking at hydrogen isotopic fractionation with larger pumices tending toward δD < −120‰ and small samples giving δD > −80‰.

Hydrogen Isotope Fractionation during Methanogenic Degradation of Toluene: Potential for Direct Verification of Bioremediation

Equilibrium headspace analysis of toluene for δ2H isotopic composition by continuous flow compound specific isotope mass spectrometry was determined to have an accuracy and reproducibility of ±5‰. Using this analytical approach, the hydrogen isotope fractionation produced by anaerobic biodegradation of toluene was evaluated in laboratory experiments using a mixed methanogenic consortium. A large, reproducible 2H-enrichment in the residual toluene of greater than 60‰ was observed at greater than 95% degradation, reflecting the preferential biodegradation of molecules containing the light (1H) isotope. Recent studies evaluating the magnitude of carbon isotope fractionation produced during biodegradation of aromatic hydrocarbons have documented heavy isotope (13C) enrichment in the residual contaminant approximately an order of magnitude smaller than those reported here for 2H. The very large isotopic enrichment in 2H suggests that under anaerobic conditions compound specific hydrogen isotope analysis may provide a more reliable means of validating intrinsic bioremediation of aromatic hydrocarbons than stable carbon isotope analysis. Combined application of stable carbon and hydrogen isotope analysis in an anaerobic groundwater has the potential to provide two important diagnostic tools. Relatively insensitive to biodegradation by mixed consortia, stable carbon isotope values may provide information about different sources of contaminant, while hydrogen isotope values provide an assessment of the degree of attenuation due to biodegradation.

Oxygen and hydrogen isotope fractionation by hydration complexes of Li +, Na +, K +, Mg 2+, F −, Cl −, and Br −: a theoretical study

Hydration complexes of the ions Li +, Na +, Mg 2+, F −, Cl −, and Br − that are expected to occur in natural brines, hydrothermal solutions, or volcanic vapors were studied theoretically using quantum mechanical methods, in particular density functional theory. The normal modes of vibration were calculated for minimum energy configurations obtained with the BLYP and B3LYP functionals in combination with the 6-31G∗∗ and 6-311G∗∗ basis sets. Enthalpies of dissociation for Li +, Na +, and Cl − hydrates calculated from these vibrational spectra are in good agreement with experimental values. From the theoretical normal modes of vibration, the reduced partition function ratios for D/H and 18O/ 16O substitution in the hydration complexes as well as the isotope fractionation between the complexes and monomeric water vapor were calculated. Following a scheme introduced by Bopp et al. (1974) we used the gas-phase results to model isotope salt effects in aqueous solutions. Good agreement with measured results was obtained for both oxygen and hydrogen isotope effects caused by cations, whereas anion results are less satisfactory, probably due to first shell–second shell interactions, which are not properly accounted for by the calculations. However, the calculations for the gas-phase cation hydration complexes can be utilized to gain insight into the nature of isotope “salt effects” in aqueous electrolyte solutions. Hydration-related “salt effects” are quantitatively dominated by the changes of O-H stretching frequencies due to the presence of the ions. Lower frequencies such as those related to the ion–water bonds are of minor importance. The distance up to which measurable effects on the O-H frequencies (and hence on the reduced partition function ratios) are induced is shown to be limited to the first hydration shell. These results may also be relevant for a better theoretical understanding of isotope fractionation involving clay minerals, adsorption of water on mineral surfaces, and isotope exchange at mineral–water interfaces.

The D/H content of methane emitted from biomass burning

The D/H content of methane emitted from biomass burning was measured using samples of smoke collected from large‐scale laboratory combustion experiments and from fires typical of slash burning of primary forest and of pasture burning in the Brazilian Amazon. In laboratory experiments, the mean δD of methane emitted from Pinus ponderosa needle combustion (δDCH4(bb)) was −233 ± 2‰. In Brazilian fires, δDCH4(bb) was −210 ± 16‰. The measured D/H of the fuel biomass ranged from −59 to −125‰ and indicated a significant hydrogen isotope fractionation (ϵDBURN) during combustion. For the laboratory experiments, ϵDBURN (=(D/H)CH4/(D/H)Biomass‐l)1000 = −124‰ and in the field studies, ϵDBURN ranged from −130 to −180‰. These results contrast those for δ13C which showed no fractionation during combustion, consistent with earlier studies. The measurements presented here suggest that a relatively narrow range exists for the δD of CH4 emitted from biomass burning on a global scale and that δD is an independent tracer for atmospheric CH4. Combining δDCH4(bb) ≈‐210‰ with literature values for the strength and δD of the major CH4 sources, we estimate a δD for the global CH4 source of ‐283 ± 13‰. This result provides a constraint for the D/H kinetic isotope effect associated with removal of atmospheric CH4 by atmospheric hydroxyl radicals.

Site-Specific Hydrogen Isotope Fractionation in the Biosynthesis of Glycerol

The nuclear magnetic resonance study of site-specific natural isotope fractionation (SNIF-NMR) produced in the glycolytic conversion of glucose into ethanol and glycerol provides isotopic transfer coefficients, aij, which relate sites in the products to sites j in the reactants. The isotopic connection between the carbon-bound hydrogens of glycerol and those of glucose and water in fermentation reactions carried out with Saccharomyces cerevisiae has been investigated. The aij coefficients provide mechanistic information on the genealogy of the glycerol hydrogens, on the relative rates of triose phosphate isomerization and reduction of dihydroxyacetone phosphate into glycerol 3-phosphate, on the stereospecificity of the reduction, on the percentages of intra- and intermolecular hydrogen transfer occurring in the course of the reaction, and on the order of magnitude of the active overall kinetic and thermodynamic isotope effects. Thus a close connection is determined between sites 3 pro-R of glycerol (stereospecific numbering) and H6 pro-R and to a lesser extent H2 of glucose and between site 3 pro-S of glycerol and H6 pro-S and H1 of glucose. Moreover site 2 of glycerol, which exhibits a strong correlation with water is also partly connected with glucose. Possible changes in the values of the isotopic transfer coefficients, as a function of the composition of the medium or of the environmental conditions, enable mechanistic perturbations such as variations in the percentage of intermolecular transfers to be detected. Analyzed in terms of stereospecificity of the reduction step the isotopic results provide a direct chemical shift assignment of the enantiomeric pairs (1S, 3R) and (1R, 3S) of glycerol triacetate. The influence of added bisulfite, which strongly increases the yield in glycerol is estimated. The isotopic characterization of the bioconversion producing both ethanol and glycerol has been extended to the determination of the carbon isotopic parameters by isotope ratio mass spectrometry. Although it usually occurs as a by-product of the fermentation, glycerol can be considered as a useful complementary probe for characterizing the glycolytic pathway and for inferring various properties of the carbohydrate precursors.

Hydrogen and Oxygen Isotope Fractionation in Water during Gas Hydrate Formation

Abstract: Isotope fractionation of hydrogen and oxygen in water caused by methane hydrate formation in saline water was investigated experimentally. The isotope fractionation factors were calculated by measuring the concentration of salts and hydrogen and oxygen isotopic ratios of water in both initial solution and residual solution after formation of methane hydrate. The amount of methane hydrates was estimated using two assumptions: complete desalination and gas consumption. As methane hydrate forms in saline water, the concentration of salt in solution increases and gas pressure decreases. We estimated the amount of methane hydrate not only from the increase of concentration of salts in the solution, but also from the decrease in gas pressure. The isotope fractionation factors of hydrogen and oxygen in the water between methane hydrate and saline water were obtained as 1.016 to 1.020 and 1.0028 to 1.0032, respectively. These are similar to the factors between ice and water.

Pressure Effect on Hydrogen Isotope Fractionation Between Brucite and Water at Elevated Temperatures.

Experimental evidence for a pressure effect on isotopic partitioning at elevated temperatures demonstrates that equilibrium deuterium-protium fractionation between the mineral brucite [Mg(OH)(2)] and pure water systematically increases by 12.4 per mil as pressure increases from 15 to 800 megapascals at 380 degrees C. A linear relation is observed between the measured fractionation factor and the density of water (0.070 to 1.035 grams per cubic centimeter). The trend of the isotope pressure effect is the same as that of recent theoretical studies, but the magnitude is smaller. The pressure effect must be accounted for in the interpretation of isotopic data of geologic systems involving water (paleotemperature, source of fluids).

Stable Isotope (C, Cl, and H) Fractionation during Vaporization of Trichloroethylene

Stable isotope fractionation during vaporization of trichloroethylene has been measured, with possible application as a technique to investigate subsurface behavior. The equilibrium value of Δ13Cvapor-liquid has been measured between 5 and 35 °C, and Δ13Cvapor-liquid, Δ37Clvapor-liquid, and ΔDvapor-liquid have been measured during progressive evaporation of liquid trichloroethylene at 22 ± 2 °C. Equilibrium values of Δ13Cvapor-liquid show a total range of 0.07−0.82‰, with a trend of decreasing Δ13Cvapor-liquid with increasing temperature, from approximately +0.7‰ at 5−15 °C to approximately +0.1‰ at 35 °C. Progressive evaporation experiments yield values of Δ13Cvapor-liquid = +0.35‰ and +0.24‰, Δ37Clvapor-liquid = −1.64‰, and ΔDvapor-liquid = +8.9‰. The positive values for carbon and hydrogen isotope fractionation, while unexpected, are consistent with available quantitative and qualititative data for trichloroethylene and other contaminant hydrocarbons, but a satisfactory explanation for these observations, particularly in combination with the negative value for chlorine, remains elusive. Vapor−liquid fractionation factors have application to the investigation of the behavior of trichloroethylene at contaminated sites, particularly sites undergoing remediation by techniques such as soil vapor extraction and soil bioventing.

Fractionation of hydrogen isotopes in lipid biosynthesis

Isotopic compositions of carbon-bound hydrogen in individual compounds from eight different organisms were measured using isotope-ratio-monitoring gas chromatography–mass spectrometry. This technique is capable of measuring D/H ratios at natural abundance in individual lipids yielding as little as 20 nmol of H 2, and is applicable to a wide range of compounds including hydrocarbons, sterols, and fatty acids. The hydrogen isotopic compositions of lipids are controlled by three factors: isotopic compositions of biosynthetic precursors, fractionation and exchange accompanying biosynthesis, and hydrogenation during biosynthesis. δD values of lipids from the eight organisms examined here suggest that all three processes are important for controlling natural variations in isotopic abundance. n-Alkyl lipids are depleted in D relative to growth water by 113–262‰, while polyisoprenoid lipids are depleted in D relative to growth water by 142–376‰. Isotopic variations within compound classes (e.g., n-alkanes) are usually less than ∼50‰, but variations as large as 150‰ are observed among isoprenoid lipids from a single organism. Phytol is consistently depleted in D by up to 50‰ relative to other isoprenoid lipids. Inferred isotopic fractionations between cellular water and lipids are greater than those indicated by previous studies.

Experimental studies of oxygen and hydrogen isotope fractionations between precipitated brucite and water at low temperatures

Oxygen and hydrogen isotope fractionation factors between brucite and water were experimentally determined by chemical synthesis techniques at low temperatures of 15° to 120°C. MgCl 2, Mg3N 2, and MgO were used as reactants, respectively, to produce brucite in aqueous solutions. All of the synthesis products were identified by x-ray diffraction (XRD) for crystal structure and by scanning electron microscope (SEM) for morphology. It is observed that oxygen isotope fractionations between brucite and water are temperature dependent regardless of variations in aging time, the chemical composition, and pH value of solutions. Brucites derived from three different starting materials yielded consistent fractionations with water at the same temperatures. These suggest that oxygen isotope equilibrium has been achieved between the synthesized brucite and water, resulting in the fractionation equation of 10 3 lnα=1.56×10 6/ T 2−14.1. When the present results for the brucite–water system are compared with those for systems of gibbsite–water and goethite–water, it suggests the following sequence of 18O-enrichment in the M−OH bonds of hydroxides: Al 3+ − OH > Fe 3+ − OH > Mg 2+ − OH. Hydrogen isotope fractionations between brucite and water obtained by the different synthesis methods have also achieved equilibrium, resulting in the fractionation equation of 10 3 lnα=−4.88×10 6/ T 2−22.5. Because of the pressure effect on hydrogen isotope fractionations between minerals and water, the present calibrations at atmospheric pressure are systematically lower than fractionations extrapolated from hydrothermal exchange experiments at high temperatures of 510° to 100°C and high pressures of 1060 to 1000 bar. Comparison of the present results with existing calibrations involving other low-temperature minerals suggests the following sequence of D-enrichment in hydroxyl-bearing minerals: Al 3+ − OH > Mg 2+ − OH > Fe 3+ − OH.

Stable isotope fractionation between liquid and vapour in water–salt systems up to 600°C

Hydrogen and oxygen isotope fractionation factors between liquid and vapour were measured between 350°C and 600°C in the systems H 2O–NaCl and H 2O–KCl. Fractionation factors are similar for the NaCl- and KCl-bearing systems. At all conditions, D preferentially fractionates into the vapour phase relative to H, 18 O into the liquid relative to 16 O . Fractionation factors are given in terms of 1000 ln αD/H (L–V) and 1000 ln α 18 O/ 16 O (L–V), respectively. At constant temperature, fractionations are approximated by linear correlations of the fractionation factor with the salt concentration in the liquid. Isotherms all have similar slopes and start at the respective critical composition where the fractionation is zero. D/H fractionation increases by 0.55‰ per wt.% NaCl (L), 18 O/ 16 O fractionation by about 0.05‰ per wt.% NaCl (L). If extrapolated up to the limit of salt-saturation in the system, maximum fractionations of 28‰ in D/H (V–L) and about 2‰ in 18 O/ 16 O (L–V) result along the liquid+vapour+halite curve over the temperature range from 350°C to 600°C. Maximum fractionations occur in boiling hydrothermal systems when the water–salt solvus opens up to salt-saturated conditions. This happens most likely in hydrothermal solutions at the roof of shallow intrusions. In this environment, salt-bearing solutions of magmatic, meteoric, or mixed origin intersect the solvus during isothermal or adiabatic decompression. Open-system behaviour may rapidly result in salt saturation of the liquid residue. Vapour leaving the hydrothermal system may have an isotopic signature similar to seawater whereas brine residues may become increasingly depleted in D and slightly enriched in 18 O . If the strong fractionation effect inherent in a boiling fluid system is disregarded, one may easily misinterpret the stable isotope ratios of hydrothermal minerals from such systems since boiling may strongly mask the source of the parent fluids.

Stable isotope characterization of olive oils: II‐deuterium distribution in fatty acids studied by nuclear magnetic resonance (SNIF‐NMR)

AbstractSite‐specific isotope fractionation of hydrogen was investigated, at natural abundance, by deuterium nuclear magnetic resonance (SNIF‐NMR) on nearly two hundred olive oil samples. Owing to the complexity of the 2H‐NMR spectra of the mixtures of fatty acids obtained after hydrolysis of the oils, the different signals were gathered into six clusters. Knowing the contribution to the clusters of each of the four fatty acids considered (C16:0, C18:0, C18:1, and C18:2) and the composition of the fatty acids in the mixture, it is possible to compute the site‐specific isotope ratios of the clusters from the molar fractions obtained from the 2H‐NMR‐spectra and from the total isotope ratio of the mixture, determined by isotope ratio mass spectrometry (IRMS). The results are discussed in terms of geographical (country, region and elevation) and temporal (year) parameters and they are tentatively explained on a climatic basis.

An empirical method for the determination of single ion hydrogen isotope salt effects in aqueous electrolyte solutions

An empirical method is presented that allows the determination of the individual contributions of anions and cations to the effect of dissolved salts on hydrogen isotope fractionation in aqueous systems (isotope salt effect). The method is solely based on experimental data and does not involve the choice of arbitrary reference values or theoretical assumptions. Plotting experimental liquid–vapor D/H fractionation factors for aqueous solutions of sodium salts vs. O–D stretching frequencies of water molecules in the hydration shells of the anions shows an excellent linear correlation. The distance between this line and the pure water liquid–vapor fractionation data point in the same plot gives the cation contribution to the isotope salt effects. The anion contribution can then simply be derived as the difference between the total salt effect and the cation salt effect. The validity of the concept is demonstrated using precise literature data for the O–D stretching frequencies in the hydration shells of individual ions at 20°C [Bergström, P.A., 1991. Single ion hydration properties in aqueous solution: a quantitative infrared spectroscopic study. PhD Thesis. Uppsala University] and for the liquid–vapor hydrogen isotope fractionation between aqueous solutions and water vapor at the same temperature [Stewart, M.K., Friedman, I., 1975. Deuterium fractionation between aqueous salt solutions and water vapor. Journal of Geophysical Research 80, 3812–3818]. Within the limits of experimental uncertainties, the data set shows internal consistency. Cation salt effects, 1000 ln Γ at 20°C, are (in per mil per mole per liter, using the convention of Horita et al. [Horita, J., Cole, D.R., Wesolowski, D.J., 1993a. The activity–composition relationship of oxygen and hydrogen isotopes in aqueous salt solutions: II. Vapor–liquid water equilibration of mixed salt solutions from 50–100°C. Geochimica et Cosmochimica Acta 57, 4703–4711]): Na ++0.7; K ++0.7; Mg 2++6.5; Ca 2++1.8; Al 3++12. The salt effect of H + cannot be determined unequivocally. The combined effect of the fractionation of H + itself plus its salt effect is +4.9. Anion effects are +1.4 for Cl −, +2.7 for Br −, +3.5 for I − and −1.4 for SO 4 2 −. Further single anion salt effects are being predicted as −1.8 for F −, +4.9 for NO 3 −, +6.9 for ClO 4 − and +5.4 for the triflate ion (CF 3SO 3 −).

A new technique for determining equilibrium hydrogen isotope fractionation factors using the ion microprobe: application to the epidote-water system

We have developed a new method for determining equilibrium hydrogen isotope fractionation factors between hydrous minerals and water. In contrast to traditional experimental methods that use finely powdered minerals as starting material and analyze the experimental run products with bulk analytical techniques, we use large single crystals as starting material and then analyze the outer 0.5–2.0 μm of the isotopically exchanged crystals by ion microprobe. Although providing somewhat less precise analytical data than can be obtained with conventional methods, the advantage of the present technique is that it allows fractionations to be determined in experiments in which isotopic exchange demonstrably occurs through a diffusional process and not by recrystallization. This, in turn, permits acquisition of unambiguously reversed experimental data on equilibrium fractionation factors. The technique was applied to the epidote-water system for which we report fractionation data between 300–600°C determined at a pressure of about 2.1 kbar. Despite initial epidote-water fractionations that were far from equilibrium in some experiments, diffusional exchange rates were sufficiently high in this system to bracket the equilibrium fractionation within analytical error to temperatures as low as 300°C. The measured fractionations vary systematically with temperature, decreasing from −36‰ at 300°C to −54‰ at 600°C. Within the experimental temperature range, these fractionations can be represented by the equation 1000 ln α (epidote-water) = 9.3 × 10 6/T 2 − 61.9. The present method should be applicable to other mineral-water systems and help to resolve some of the existing discrepancies with regard to the magnitude and temperature dependence of hydrogen isotope fractionation factors.

Recent Advances in Site-specific Natural Isotope Fractionation Studied by Nuclear Magnetic Resonance

Site-specific Natural Isotopic Fractionation of hydrogen and carbon in nicotine, glycerol and vanillin was studied by NMR spectroscopy (SNIF-NMR) in order to point out specific mechanistic and environmental effects. Several products (cellulose, sugars, acids, nicotine) isolated from tobacco leaves present related carbon and nitrogen fractionation profiles. In nicotine, it is possible to measure eleven site-specific isotope ratios (D/H)i which give useful information about the climate of the region of production. The origin of glycerol in fermented beverages can be determined by 2H and 13C-NMR, and the botanical origin of triglycerides can also be checked in the same way. Vanillin is a typical example of aroma which has undergone a series of sophisticated adulterations but which can be authentified by isotopic NMR techniques. The whole population of vanillin molecules can be split into more and more diversified sub-groups by 2H and 13C SNIF-NMR.

Experimental studies of oxygen and hydrogen isotope fractionations between brucite and water at low temperatures

Experimental studies of oxygen and hydrogen isotope fractionations between brucite and water at low temperatures

Water solubility and D/H fractionation in the system basaltic andesite–H 2O at 1250°C and between 0.5 and 3 kbars

Water solubilities in melt of basaltic andesite composition have been measured in the range 500 to 3000 bars, 1200–1250°C. The saturation curve obey an empirical relationship very close to the square root of pressure law of Hamilton et al. [Hamilton, D.L., Burnham, C.W., Osborn, E.F., 1964. The solubility of water and effects of oxygen fugacity and water content on crystallisation in mafic magmas. J. Petrol., 5, 21–39]. The best fit is obtained by a power law H 2O wt.%= aP n ( P in b), where a=0.05±0.01 and n=0.63; the departure from n=0.5 is mainly due to the increasing importance of dissolved molecular water at high pressures. Hydrogen isotopic fractionation between water vapor and total water dissolved in the melt (ΔD v–m) has been measured at 0.5, 2 and 3 kb. Δ D v–m decreases with pressure from 32 to 20±2‰. This variation is related to the increase in molecular water proportions relative to hydroxyl group when water solubility increases. The variation of the D/H ratios in water dissolved in such melts thus appears as a possible indirect measurement of the original OH/H 2O ratio of total dissolved water, not subject to variation during quenching as the later is. This correlation shows that the Δ D v–m isotopic fractionation at very high water vapor pressure tend toward a limit of 16±3‰ for basic magmas.