Calibration and applications of the dolomite clumped isotope thermometer to high temperatures

Carbonate clumped isotope paleothermometry is based on the temperature-dependent formation of ^(13)C^(18)O^(16)O_2 ^(2-) ion groups within solid carbonate minerals. This thermometer has now been calibrated for various synthetic and natural biogenic and abiogenic minerals (calcite, aragonite and carbonateapatites [e.g., 1, 2]) at temperatures below ~ 50°C. Here we extend the use of the carbonate clumped isotope thermometer to shallow crustal environments by determining the Δ_(47) values of CO_2 extracted from natural and synthetic dolomites grown at know temperatures from 25 to 350ºC. The experimental temperature dependance is not linear in the Δ_(47) vs T^(-2) plot and resembles the predicted theoretical temperature dependence, both in shape and absolute value [3]. These data for synthetic dolomites overlap the previous calibrations for inorganic calcite and some forms of biogenic carbonates between 25 and 50˚C, and are consistent with a single trend that also intersects data for synthetic calcite equilibrated at 1200˚C. These observations suggest that a single temperature dependant relationship reasonably approximates the calibration for both phases. Data from a variety of slowly-cooled (i.e., over geological timescales) natural marbles and rapid (i.e., laboratory timescales) heating experiments provide insights into the kinetics of solid-state ^(13)C-^(18)O bond reordering in carbonates and its closure temperature. More generally, our new calibration and constraints on high-temperature kinetics have implications for the application of this technique to burial and metamorphic processes. These issues will be illustrated through estimates of the thermal history and oxygen isotopic compositions and abundances of pore fluids for several suites of late Neoproterozoic carbonates [e.g., 4]

Calibration of the dolomite clumped isotope thermometer from 25 to 350°C, and implications for a universal calibration for all (Ca, Mg, Fe)CO_3 carbonates

Carbonate clumped isotope thermometry is based on the temperature-dependent formation of ^(13)C^(18)O^(16)O_2^(2-) ion groups within the lattice of solid carbonate minerals. At low temperatures the bonds between rare, heavy ^(13)C and ^(18)O isotopes are thermodynamically favored, and thus at equilibrium they are present in higher than random abundances. Here we calibrate the use of this temperature proxy in a previously uncalibrated carbonate phase — dolomite [CaMg(CO_3)_2] — over a temperature range that extends to conditions typical of shallow crustal environments, by determining the Δ_(47) values of CO_2 extracted from synthetic or natural (proto)dolomites grown at known temperatures from 25 to 350°C and analyzed in two different laboratories using different procedures for sample analysis, purification and post-measurement data treatment. We found that the Δ_(47) – 1/T^2 dependence for (proto)dolomite is linear between 25 and 350°C, independent of their Mg/Ca compositions or cation order (or the laboratory in which they were analyzed), and offset from, but parallel to, the theoretical predictions of the Δ_(63) dependence to temperature of the abundance of the ^(13)C^(18)O^(16)O_2 isotopologue inside the dolomite and calcite mineral lattices as expected from ab-initio calculations (Schauble et al., 2006). This suggests that neither the equilibrium constant for ^(13)C–^(18)O clumping in (proto)dolomite lattice, nor the experimental fractionation associated with acid digestion to produce CO_2, depend on their formation mechanism, degree of cation order and/or stoichiometry (ie., Mg/Ca ratio) and/or δ^(18)O and δ^(13)C compositions (at least over the range we explored). Thus, we suggest the following single Δ_(47) – 1/T^2 linear regression for describing all dolomite minerals: with T in kelvin and Δ_(47) in the Carbon Dioxide Equilibrium Scale (CDES) of Dennis et al. (2011) and referring to CO_2 extracted by phosphoric acid digestion at 90°C. The listed uncertainties on slope and intercept are 95% confidence intervals. The temperature sensitivity (slope) of this relation is lower than those based on low temperature acid digestion of carbonates, but comparable to most of those based on high temperature acid digestion (with however significantly better constraints on the slope and intercept values, notably due to the large range in temperature investigated and the large number of Δ_(47) measurements performed here, n = 67). We also use this dataset to empirically determine that the acid fractionation factor associated with phosphoric acid digestion of dolomite at 90°C (Δ∗_(dolomite90)) is about + 0.176‰. This is comparable to the Δ∗_(calcite90) experimentally obtained for calcite (Guo et al., 2009), suggesting that the acid fractionation Δ∗ for acid digestion of dolomite and calcite are the same within error of measurement, with apparently no influence of the cation identity. This hypothesis is also supported by the fact that our dolomite calibration data are indistinguishable from published calibration data for calcite, aragonite and siderite generated in similar analytical conditions (ie., carbonate digested at T ⩾ 70°C and directly referenced into CDES), demonstrating excellent consistency among the four (Ca,Mg,Fe)CO_3 mineral phases analyzed in seven different laboratories (this represents a total of 103 mean Δ_(47) values resulting from more than 331 Δ_(47) measurements). These data are used to calculate a composite Δ_(47)–T universal relation for those carbonate minerals of geological interest, for temperatures between -1 and 300°C, that is found to be statistically indistinguishable from the one described by dolomite only: Thus, in order to standardize the temperature estimates out of different laboratories running high temperature digestion of (Ca,Mg,Fe)CO_3 carbonate minerals, we recommend the use of this single Δ_(47)-T calibration to convert Δ_(47CDES) data into accurate and precise temperature estimates. More widely, this study extends the use of the Δ_(47) thermometry to studies of diagenesis and low-grade metamorphism of carbonates with unprecedented precision on temperature estimates based on Δ_(47) measurements — environments where many other thermometers are generally empirical or semi-quantitative

The use of chlorate, nitrate, and perchlorate to promote crude oil mineralization in salt marsh sediments

Due to the high volume of crude oil released by the Deepwater Horizon oil spill, the salt marshes along the gulf coast were contaminated with crude oil. Biodegradation of crude oil in salt marshes is primarily limited by oxygen availability due to the high organic carbon content of the soil, high flux rate of S(2-), and saturated conditions. Chlorate, nitrate, and perchlorate were evaluated for use as electron acceptors in comparison to oxygen by comparing oil transformation and mineralization in mesocosms consisting of oiled salt marsh sediment from an area impacted by the BP Horizon oil spill. Mineralization rates were determined by measuring CO2 production and δ (13)C of the produced CO2 and compared to transformation evaluated by measuring the alkane/hopane ratios over a 4-month period. Total alkane/hopane ratios decreased (~55-70 %) for all treatments in the following relative order: aerated ≈ chlorate \u3e nitrate \u3e perchlorate. Total CO2 produced was similar between treatments ranging from 550-700 mg CO2-C. The δ (13)C-CO2 values generally ranged between the indigenous carbon and oil values (-17 and -27‰, respectively). Oil mineralization was greatest for the aerated treatments and least for the perchlorate amended. Our results indicate that chlorate has a similar potential as oxygen to support oil mineralization in contaminated salt marshes, but nitrate and perchlorate were less effective. The use of chlorate as a means to promote oil mineralization in situ may be a promising means to remediate contaminated salt marshes while preventing unwanted secondary impacts related to nutrient management as in the case of nitrate amendments

Salt effects on stable isotope partitioning and their geochemical implications for geothermal brines

The effects of dissolved salts (NaCI, KCI, MgCl{sub 2}, CaCI{sub 2}, Na{sub 2}SO{sub 4}, MgSO{sub 4}, and their mixtures) on oxygen and hydrogen isotope partitioning between brines and coexisting phases (vapor and calcite) were experimentally determined at 50-350{degree} C and 300{degree} C, respectively. In liquid-vapor equilibration experiments, for all of the salts studied, the hydrogen isotope fractionation factors between the salt solutions and vapor decreased appreciably (up to 20{permille}) compared to pure water-vapor. Except for KCI solutions at 500 C, the oxygen isotope fractionation factors between salt solutions and vapor were higher (up to 4{permille}) than, or very close to, that of pure water. The observed isotope salt effects are all linear with the molalities of the solutions. Mixed salt solutions mimicking natural geothermal brines exhibit salt effects additive of those of individual salts. The isotope exchange experiments of calcite-water at 300{degree}C and 1 kbar yielded a fractionation factor of 5.9+0.3{permille} for pure water and effects of NaCl consistent with those obtained from the liquid-vapor equilibration experiments. The isotope salt effects observed in this study are too large to be ignored, and must be taken into account for isotopic studies of geothermal systems (i.e., estimation of isotope ratios and temperatures of deep-seated geothermal brines)

Dawsonite synthesis and reevaluation of its thermodynamic properties from solubility measurements: Implications for mineral trapping of CO2

International audienceOver the last decade, a significant research effort has focused on determining the feasibility of sequestering large amounts of CO2 in deep, permeable geologic formations to reduce carbon dioxide emissions to the atmosphere. Most models indicate that injection of CO2 into deep sedimentary formations will lead to the formation of various carbonate minerals, including the common phases calcite (CaCO3), dolomite (CaMg(CO3)2), magnesite (MgCO3), siderite (FeCO3), as well as the far less common mineral, dawsonite (NaAlCO3(OH)2). Nevertheless, the equilibrium and kinetics that control the precipitation of stable carbonate minerals are poorly understood and few experiments have been performed to validate computer codes that model CO2 sequestration. In order to reduce this uncertainty we measured the solubility of synthetic dawsonite according to the equilibrium: NaAlCO(OH)+2HO?Al(OH)4-+HCO3-+Na+H, from under- and oversaturated solutions at 50 200 °C in basic media at 1.0 mol · kg?1 NaCl. The solubility products (Qs) obtained were extrapolated to infinite dilution to obtain the solubility constants (Kso). Combining the fit of these logKso values and fixing ?Cp,roat?185.5J·mol·K at 25 °C, which was derived from the calorimetric data of Ferrante et al. [Ferrante, M.J., Stuve, J.M., and Richardson, D.W., 1976. Thermodynamic data for synthetic dawsonite. U.S. Bureau of Mines Report Investigation, 8129, Washington, D.C., 13p.], the following thermodynamic parameters for the dissolution of dawsonite were calculated at 25 °C: ?Gro=102.1kJ·mol, ?Hro=97.0kJ·mol and ?Sro=-17.1J·mol·K. Subsequently, we were able to derive values for the Gibbs energy of formation (?fG298.15o=-1782±2kJ·mol), enthalpy of formation (?fH298.15o=-1960±7kJ·mol) and entropy (S298.15o=131±2J·mol·K) of dawsonite. These results are within the combined experimental uncertainties of the values reported by Ferrante et al. (1976). Predominance diagrams are presented for the dawsonite/boehmite and dawsonite/bayerite equilibria at 100 °C in the presence of a saline solution with and without silica-containing minerals

Measurements of water vapor adsorption on the Geysers rocks

The ORNL high temperature isopiestic apparatus was adapted for adsorption measurements. The quantity of water retained by rock samples taken from three different wells of The Geysers was measured at 150 °C and at 200 °C as a function of pressure in the range 0.00 ≤ p/p0 ≤ 0.98, where p0 is the saturated water vapor pressure. The rocks were crushed and sieved into three fractions of different grain sizes (with different specific surface areas). Both adsorption (increasing pressure) and desorption (decreasing pressure) runs were made in order to investigate the nature and extent of the hysteresis. Additionally, BET surface area analyses were performed by Porous Materials Inc. on the same rock samples using nitrogen or krypton adsorption measurements at 77 K. Specific surface areas and pore volumes were determined. These parameters are important in estimating water retention capability of a porous material. The same laboratory also determined the densities of the samples by helium pycnometry. Their results were then compared with our own density values obtained by measuring the effect of buoyancy in compressed argon. One of the goals of this project is to determine the dependence of the water retention capacity of the rocks as a function of temperature. The results show a significant dependence of the adsorption and desorption isotherms on the grain size of the sample. The increase in the amount of water retained with temperature observed previously (Shang et al., 1994a, 1994b, 1995) between 90 and 130°C for various reservoir rocks from The Geysers may be due to the contribution of slow chemical adsorption and may be dependent on the time allowed for equilibration. In contrast with the results of Shang et al. (1994a, 1994b, 1995), some closed and nearly closed hysteresis loops on the water adsorption/desorption isotherms (with closing points at p/p0 ≈ 0.6) were obtained in this study. In these cases the effects of activated processes were not present, and no increase in water adsorption with temperature was observe

The use of chlorate, nitrate, and perchlorate to promote crude oil mineralization in salt marsh sediments

Due to the high volume of crude oil released by the Deepwater Horizon oil spill, the salt marshes along the gulf coast were contaminated with crude oil. Biodegradation of crude oil in salt marshes is primarily limited by oxygen availability due to the high organic carbon content of the soil, high flux rate of S(2-), and saturated conditions. Chlorate, nitrate, and perchlorate were evaluated for use as electron acceptors in comparison to oxygen by comparing oil transformation and mineralization in mesocosms consisting of oiled salt marsh sediment from an area impacted by the BP Horizon oil spill. Mineralization rates were determined by measuring CO2 production and δ (13)C of the produced CO2 and compared to transformation evaluated by measuring the alkane/hopane ratios over a 4-month period. Total alkane/hopane ratios decreased (~55-70 %) for all treatments in the following relative order: aerated ≈ chlorate \u3e nitrate \u3e perchlorate. Total CO2 produced was similar between treatments ranging from 550-700 mg CO2-C. The δ (13)C-CO2 values generally ranged between the indigenous carbon and oil values (-17 and -27‰, respectively). Oil mineralization was greatest for the aerated treatments and least for the perchlorate amended. Our results indicate that chlorate has a similar potential as oxygen to support oil mineralization in contaminated salt marshes, but nitrate and perchlorate were less effective. The use of chlorate as a means to promote oil mineralization in situ may be a promising means to remediate contaminated salt marshes while preventing unwanted secondary impacts related to nutrient management as in the case of nitrate amendments

Temperature-Dependent Oxygen and Carbon Isotopes Fractionations of Biogenic Siderite

Isotopic compositions of biogenic iron minerals may be used to infer environmental conditions under which bacterial iron reduction occurs. The major goal of this study is to examine temperature-dependent isotope fractionations associated with biogenic siderite (FeCO3). Experiments were performed by using both mesophilic (\u3c35°C) and thermophilic (\u3e45°C) iron-reducing bacteria. In addition, control experiments were performed to examine fractionations under nonbiologic conditions. Temperature-dependent oxygen isotope fractionation occurred between biogenic siderite and water from which the mineral was precipitated. Samples in thermophilic cultures (45–75°C) gave the best linear correlation, which can be described as 103 lnαsid-wt = 2.56 × 106 T−2 (K) + 1.69. This empirical equation agrees with that derived from inorganically precipitated siderite by Carothers et al. (1988) and may be used to approximate equilibrium fractionation. Carbon isotope fractionation between biogenic siderite and CO2, based on limited data, also varied with temperature and was consistent with the inorganically precipitated siderite of Carothers et al. (1988). These results indicate that temperature is a controlling factor for isotopic variations in biogenic minerals examined in this study. The temperature-dependent fractionations under laboratory conditions, however, could be complicated by other factors including incubation time and concentration of bicarbonate. Early precipitated siderite at 120-mM initial bicarbonate tended to be enriched in 18O. Siderite formed at \u3c30 mM of bicarbonate tended to be depleted in 18O. Other\u3evariables, such as isotopic compositions of water, types of bacterial species, or bacterial growth rates, had little effect on the fractionation. In addition, siderite formed in abiotic controls had similar oxygen isotopic compositions as those of biogenic siderite at the same temperature, suggesting that microbial fractionations cannot be distinguished from abiotic fractionations under conditions examined here