An experimental study of oxygen isotope fractionation between inorganically precipitated aragonite and water at low temperatures

To determine oxygen isotope fractionation between aragonite and water, aragonite was slowly precipitated from Ca(HCO₃)₂ solution at 0 to 50°C in the presence of Mg²⁺ or SO₄²⁻. The phase compositions and morphologies of synthetic minerals were detected by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The effects of aragonite precipitation rate and excess dissolved CO₂ gas in the initial Ca(HCO₃)₂ solution on oxygen isotope fractionation between aragonite and water were investigated. For the CaCO₃ minerals slowly precipitated by the CaCO₃ or NaHCO₃ dissolution method at 0 to 50°C, the XRD and SEM analyses show that the rate of aragonite precipitation increased with temperature. Correspondingly, oxygen isotope fractionations between aragonite and water deviated progressively farther from equilibrium. Additionally, an excess of dissolved CO₂ gas in the initial Ca(HCO₃)₂ solution results in an increase in apparent oxygen isotope fractionations. As a consequence, the experimentally determined oxygen isotope fractionations at 50°C indicate disequilibrium, whereas the relatively lower fractionation values obtained at 0 and 25°C from the solution with less dissolved CO₂ gas and low precipitation rates indicate a closer approach to equilibrium. Combining the lower values at 0 and 25°C with previous data derived from a two-step overgrowth technique at 50 and 70°C, a fractionation equation for the aragonite-water system at 0 to 70°C is obtained as follows:

     

Phosphoric acid fractionation factors for smithsonite and cerussite between 25 and 72°C

The intramolecular kinetic oxygen isotope fractionation between CO₂ and CO₃²⁻ during reaction of phosphoric acid with natural smithsonite (ZnCO₃) and cerussite (PbCO₃) has been determined between 25 and 72°C. While cerussite decomposes in phosphoric acid within a few hours at 25°C, smithsonite reacts very slowly with the acid at 25°C providing yields of CO₂ < 25% after 2 weeks. The low yields result in a low precision for oxygen isotope measurements of the acid-liberated CO₂ (±1.65‰, 1σ, n = 9). The yield and reproducibility of oxygen isotope values of the acid-liberated CO₂ from smithsonite can be improved, the latter to ∼±0.15‰, by increasing the reaction temperature to 50°C for 12 h or to 72°C for 1 h. Our new phosphoric acid fractionation factor for natural cerussite at 25°C deviates significantly from a previously published value on synthetic material. The temperature dependence of the oxygen isotope factionation factor, α between acid-liberated CO₂ and carbonate at 25 to 72°C is given by the following equations

     

Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights from first-principles theoretical modeling and clumped isotope measurements

Phosphoric acid digestion has been used for oxygen- and carbon-isotope analysis of carbonate minerals since 1950, and was recently established as a method for carbonate ‘clumped isotope’ analysis. The CO₂ recovered from this reaction has an oxygen isotope composition substantially different from reactant carbonate, by an amount that varies with temperature of reaction and carbonate chemistry. Here, we present a theoretical model of the kinetic isotope effects associated with phosphoric acid digestion of carbonates, based on structural arguments that the key step in the reaction is disproportionation of H₂CO₃ reaction intermediary. We test that model against previous experimental constraints on the magnitudes and temperature dependences of these oxygen isotope fractionations, and against new experimental determinations of the fractionation of ¹³C-¹⁸O-containing isotopologues (‘clumped’ isotopic species). Our model predicts that the isotope fractionations associated with phosphoric acid digestion of carbonates at 25 °C are 10.72‰, 0.220‰, 0.137‰, 0.593‰ for, respectively, ¹⁸O/¹⁶O ratios (1000 lnα^∗) and three indices that measure proportions of multiply-substituted isotopologues . We also predict that oxygen isotope fractionations follow the mass dependence exponent, λ of 0.5281 (where ). These predictions compare favorably to independent experimental constraints for phosphoric acid digestion of calcite, including our new data for fractionations of ¹³C-¹⁸O bonds (the measured change in Δ₄₇ = 0.23‰) during phosphoric acid digestion of calcite at 25 °C.We have also attempted to evaluate the effect of carbonate cation compositions on phosphoric acid digestion fractionations using cluster models in which disproportionating H₂CO₃ interacts with adjacent cations. These models underestimate the magnitude of isotope fractionations and so must be regarded as unsucsessful, but do reproduce the general trend of variations and temperature dependences of oxygen isotope acid digestion fractionations among different carbonate minerals. We suggest these results present a useful starting point for future, more sophisticated models of the reacting carbonate/acid interface. Examinations of these theoretical predictions and available experimental data suggest cation radius is the most important factor governing the variations of isotope fractionation among different carbonate minerals. We predict a negative correlation between acid digestion fractionation of oxygen isotopes and of ¹³C-¹⁸O doubly-substituted isotopologues, and use this relationship to estimate the acid digestion fractionation of for different carbonate minerals. Combined with previous theoretical evaluations of ¹³C-¹⁸O clumping effects in carbonate minerals, this enables us to predict the temperature calibration relationship for different carbonate clumped isotope thermometers (witherite, calcite, aragonite, dolomite and magnesite), and to compare these predictions with available experimental determinations. The success of our models in capturing several of the features of isotope fractionation during acid digestion supports our hypothesis that phosphoric acid digestion of carbonate minerals involves disproportionation of transition state structures containing H₂CO₃.     

Kinetic and equilibrium Fe isotope fractionation between aqueous Fe(II) and Fe(III)

Equilibrium and kinetic Fe isotope fractionation between aqueous ferrous and ferric species measured over a range of chloride concentrations (0, 11, 110 mM Cl⁻) and at two temperatures (0 and 22°C) indicate that Fe isotope fractionation is a function of temperature, but independent of chloride contents over the range studied. Using ⁵⁷Fe-enriched tracer experiments the kinetics of isotopic exchange can be fit by a second-order rate equation, or a first-order equation with respect to both ferrous and ferric iron. The exchange is rapid at 22°C, ∼60-80% complete within 5 seconds, whereas at 0°C, exchange rates are about an order of magnitude slower. Isotopic exchange rates vary with chloride contents, where ferrous-ferric isotope exchange rates were ∼25 to 40% slower in the 11 mM HCl solution compared to the 0 mM Cl⁻ (∼10 mM HNO₃) solutions; isotope exchange rates are comparable in the 0 and 110 mM Cl⁻ solutions.The average measured equilibrium isotope fractionations, Δ_{Fe(III)-Fe(II)}, in 0, 11, and 111 mM Cl⁻ solutions at 22°C are identical within experimental error at +2.76±0.09, +2.87±0.22, and +2.76±0.06 ‰, respectively. This is very similar to the value measured by Johnson et al. (2002a) in dilute HCl solutions. At 0°C, the average measured Δ_{Fe(III)-Fe(II)} fractionations are +3.25±0.38, +3.51±0.14 and +3.56±0.16 ‰ for 0, 11, and 111 mM Cl⁻ solutions. Assessment of the effects of partial re-equilibration on isotope fractionation during species separation suggests that the measured isotope fractionations are on average too low by ∼0.20 ‰ and ∼0.13 ‰ for the 22°C and 0°C experiments, respectively. Using corrected fractionation factors, we can define the temperature dependence of the isotope fractionation from 0°C to 22°C as: where the isotopic fractionation is independent of Cl⁻ contents over the range used in these experiments. These results confirm that the Fe(III)-Fe(II) fractionation is approximately half that predicted from spectroscopic data, and suggests that, at least in moderate Cl⁻ contents, the isotopic fractionation is relatively insensitive to Fe-Cl speciation.     

The origin of Zn isotope fractionation in sulfides

Isotope fractionation of Zn between aqueous sulfide, chloride, and carbonate species (Zn²⁺, Zn(HS)₂, , , ZnS(HS)⁻, ZnCl⁺, ZnCl₂, , and ZnCO₃) was investigated using ab initio methods. Only little fractionation is found between the sulfide species, whereas carbonates are up to 1‰ heavier than the parent solution. At pH > 3 and under atmospheric-like CO₂ pressures, isotope fractionation of Zn sulfides precipitated from sulfidic solutions is affected by aqueous sulfide species and the δ⁶⁶Zn of sulfides reflect these in the parent solutions. Under high P_CO2 conditions, carbonate species become abundant. In high P_CO2 conditions of hydrothermal solutions, Zn precipitated as sulfides is isotopically nearly unfractionated with respect to a low-pH parent fluid. In contrast, negative δ⁶⁶Zn down to at least −0.6‰ can be expected in sulfides precipitated from solutions with pH > 9. Zinc isotopes in sulfides and rocks therefore represent a potential indicator of mid to high pH in ancient hydrothermal fluids.     

Oxygen isotope fractionation between apatite-bound carbonate and water determined from controlled experiments with synthetic apatites precipitated at 10-37 °C

The oxygen isotope fractionation between the structural carbonate of inorganically precipitated hydroxyapatite (HAP) and water was determined in the range 10-37 °C. Values of 1000 ln α() are linearly correlated with inverse temperature (K) according to the following equation: 1000 ln α() = 25.19 (±0.53)·T⁻¹ − 56.47 (±1.81) (R² = 0.998). This fractionation equation has a slightly steeper slope than those already established between calcite and water ( [O’Neil et al., 1969] and [Kim and O’Neil, 1997]) even though measured fractionations are of comparable amplitude in the temperature range of these experimental studies. It is consequently observed that the oxygen isotope fractionation between apatite carbonate and phosphate increases from about 7.5‰ up to 9.1‰ with decreasing temperature from 37 °C to 10 °C. A compilation of δ¹⁸O values of both phosphate and carbonate from modern mammal teeth and bones confirms that both variables are linearly correlated, despite a significant scattering up to 3.5‰, with a slope close to 1 and an intercept corresponding to a 1000 ln α() value of 8.1‰. This apparent fractionation factor is slightly higher or close to the fractionation factor expected to be in the range 7-8‰ at the body temperature of mammals.     

Carbon isotope fractionation in synthetic magnesian calcite

Mg-calcite was precipitated at 25 °C in closed system, free-drift experiments, from solutions containing NaHCO₃, CaCl₂ and MgCl₂. The carbon stable isotope composition of bulk solid and solution were analyzed from subsamples collected during time course experiments of 24 h duration. Considering only the Mg-content and δ¹³C values for the bulk solid, the carbon isotope fractionation factor for the Mg-calcite-HCO_3(aq)⁻ system (as ) increased with average mol percentage of Mg (X_Mg) in the solid at a rate of (0.024 ± 0.011) per mol% MgCO₃. Extrapolation of this relationship to the pure calcite end member yields a value of 0.82 ± 0.09, which is similar to published values for the calcite-HCO_3(aq)⁻ system. Although did not vary for precipitation rates that ranged from 10^3.21 to 10^4.60 μmol m⁻² h⁻¹, it was not possible to hold Mg-content of the solid constant, so kinetic effect on 10³ ln α could not be evaluated from these experiments.     

Controls on ostracod valve geochemistry: Part 2. Carbon and oxygen isotope compositions

The stable carbon and oxygen isotope compositions of fossil ostracods are powerful tools to estimate past environmental and climatic conditions. The basis for such interpretations is that the calcite of the valves reflects the isotopic composition of water and its temperature of formation. However, calcite of ostracods is known not to form in isotopic equilibrium with water and different species may have different offsets from inorganic precipitates of calcite formed under the same conditions. To estimate the fractionation during ostracod valve calcification, the oxygen and carbon isotope compositions of 15 species living in Lake Geneva were related to their autoecology and the environmental parameters measured during their growth. The results indicate that: (1) Oxygen isotope fractionation is similar for all species of Candoninae with an enrichment in ¹⁸O of more than 3‰ relative to equilibrium values for inorganic calcite. Oxygen isotope fractionation for Cytheroidea is less discriminative relative to the heavy oxygen, with enrichments in ¹⁸O for these species of 1.7 to 2.3‰. Oxygen isotope fractionations for Cyprididae are in-between those of Candoninae and Cytheroidea. The difference in oxygen isotope fractionation between ostracods and inorganic calcite has been interpreted as resulting from a vital effect. (2) Comparison with previous work suggests that oxygen isotope fractionation may depend on the total and relative ion content of water. (3) Carbon isotope compositions of ostracod valves are generally in equilibrium with DIC. The specimens’ δ¹³C values are mainly controlled by seasonal variations in δ¹³C_DIC of bottom water or variation thereof in sediment pore water. (4) Incomplete valve calcification has an effect on carbon and oxygen isotope compositions of ostracod valves. Preferential incorporation of at the beginning of valve calcification may explain this effect. (5) Results presented here as well as results from synthetic carbonate growth indicate that different growth rates or low pH within the calcification site cannot be the cause of oxygen isotope ‘vital effects’ in ostracods. Two mechanisms that might enrich the ¹⁸O of ostracod valves are deprotonation of that may also contribute to valve calcification, and effects comparable to salt effects with high concentrations of Ca and/or Mg within the calcification site that may also cause a higher temperature dependency of oxygen isotope fractionation.     

Climatic dependence of stable carbon and oxygen isotope signals recorded in speleothems: From soil water to speleothem calcite

Understanding the relationship between stable isotope signals recorded in speleothems (δ¹³C and δ¹⁸O) and the isotopic composition of the carbonate species in the soil water is of great importance for their interpretation in terms of past climate variability. Here the evolution of the carbon isotope composition of soil water on its way down to the cave during dissolution of limestone is studied for both closed and open-closed conditions with respect to CO₂.The water entering the cave flows as a thin film towards the drip site. CO₂ degasses from this film within approx. 10 s by molecular diffusion. Subsequently, chemical and isotopic equilibrium is established on a time scale of several 10-100 s. The δ¹³C value of the drip water is mainly determined by the isotopic composition of soil CO₂. The evolution of the δ¹⁸O value of the carbonate species is determined by the long exchange time T_ex, between oxygen in carbonate and water of several 10,000 s. Even if the oxygen of the CO₂ in soil water is in isotopic equilibrium with that of the water, dissolution of limestone delivers oxygen with a different isotopic composition changing the δ¹⁸O value of the carbonate species. Consequently, the δ¹⁸O value of the rainwater will only be reflected in the drip water if it has stayed in the rock for a sufficiently long time.After the water has entered the cave, the carbon and oxygen isotope composition of the drip water may be altered by CO₂-exchange with the cave air. Exchange times, , of about 3000 s are derived. Thus, only drip water, which drips in less than 3000 s onto the stalagmite surface, is suitable to imprint climatic signals into speleothem calcite deposited from it.Precipitation of calcite proceeds with time constants, τ_p, of several 100 s. Different rate constants and equilibrium concentrations for the heavy and light isotopes, respectively, result in isotope fractionation during calcite precipitation. Since T_ex ? τ_p, exchange with the oxygen in the water can be neglected, and the isotopic evolution of carbon and oxygen proceed analogously. For drip intervals T_d < 0.1τ_p the isotopic compositions of both carbon and oxygen in the solution evolve linearly in time. The calcite precipitated at the apex of the stalagmite reflects the isotopic signal of the drip water.For long drip intervals, when calcite is deposited from a stagnant water film, long drip intervals may have a significant effect on the isotopic composition of the DIC. In this case, the isotopic composition of the calcite deposited at the apex must be determined by averaging over the drip interval. Such processes must be considered when speleothems are used as proxies of past climate variability.     

N/N and O/O stable isotope ratios of nitrous oxide produced during denitrification in temperate forest soils

Anaerobic incubations of upland and wetland temperate forest soils from the same watershed were conducted under different moisture and temperature conditions. Rates of nitrous oxide (N₂O) production by denitrification of nitrate () and the stable isotopic composition of the N₂O (δ¹⁵N, δ¹⁸O) were measured. In all soils, N₂O production increased with elevated temperature and soil moisture. At each temperature and moisture level, the rate of N₂O production in the wetland soil was greater than in the upland soil. The ¹⁵N isotope effect (ε) _{(product − substrate)} ranged from −20‰ to −29‰. These results are consistent with other published estimates of ¹⁵N fractionation from both single species culture experiments and soil incubation studies from different ecosystems.A series of incubations were conducted with ¹⁸O-enriched water (H₂O) to determine if significant oxygen exchange (O-exchange) occurred between H₂O and N₂O precursors during denitrification. The exchange of H₂O-O with nitrite () and/or nitric oxide (NO) oxygen has been documented in single organism culture studies but has not been demonstrated in soils prior to this study. The fraction of N₂O-O derived from H₂O-O was confined to a strikingly narrow range that differed between soil types. H₂O-O incorporation into N₂O produced from upland and wetland soils was 86% to 94% and 64% to 70%, respectively. Neither the temperature, soil moisture, nor the rate of N₂O production influenced the magnitude of O-exchange. With the exception of one treatment, the net ¹⁸O isotope effect (ε_net) _{(product-substrate)} ranged from +37‰ to +43‰.Most previous studies that have reported ¹⁸O isotope effects for denitrification of to N₂O have failed to account for the effect of oxygen exchange with H₂O. When high amounts of O-exchange occur after fractionation during reductive O-loss, the ¹⁸O-enrichment is effectively lost or diminished and δ¹⁸O-N₂O values will be largely dictated by δ¹⁸O-H₂O values and subsequent fractionation. The process and extent of O-exchange, combined with the magnitude of oxygen isotope fractionation at each reduction step, appear to be the dominant controls on the observed oxygen isotope effect. In these experiments, significant oxygen isotope fractionation was observed to occur after the majority of water O-exchange. Due to the importance of O-exchange, the net oxygen isotope effect for N₂O production in soils can only be determined using δ¹⁸O-H₂O addition experiments with δ¹⁸O-H₂O close to natural abundance.The results of this study support the continued use of δ¹⁵N-N₂O analysis to fingerprint N₂O produced from the denitrification of . The utilization of ¹⁸O/¹⁶O ratios of N₂O to study N₂O production pathways in soil environments is complicated by oxygen exchange with water, which is not usually quantified in field studies. The oxygen isotope fractionation observed in this study was confined to a narrow range, and there was a clear difference in water O-exchange between soil types regardless of temperature, soil moisture, and N₂O production rate. This suggests that ¹⁸O/¹⁶O ratios of N₂O may be useful in characterizing the actively denitrifying microbial community.     

Experimental studies of oxygen isotope fractionation between rhodochrosite (MnCO3) and water at low temperatures

Rhodochrosite crystals were precipitated from Na-Mn-Cl-HCO₃ parent solutions following passive, forced and combined passive-to-forced CO₂ degassing methods. Forced and combined passive-to-forced CO₂ degassing produced rhodochrosite crystals with a small non-equilibrium oxygen isotope effect whereas passive CO₂ degassing protocols yielded rhodochrosite in apparent isotopic equilibrium with water. On the basis of the apparent equilibrium isotopic data, a new temperature-dependent relation is proposed for the oxygen isotope fractionation between rhodochrosite and water between 10 and 40 °C:

1000lnα_{rhodochrosite-water}=17.84±0.18(10³/T)-30.24±0.62     

Modeling the effects of bond environment on equilibrium iron isotope fractionation in ferric aquo-chloro complexes

Four or five sets of ab initio models, including Unrestricted Hartree Fock (UHF) and hybrid Density Functional Theory (DFT) are calculated for each species in a series of aqueous ferric aquo-chloro complexes: , , , FeCl₃(H₂O)₃, FeCl₃(H₂O)₂, , FeCl₅H₂O²⁻, , ) in order to determine the relative isotopic fractionation among the complexes, to compare the results of different models for the same complexes, to examine factors that influence the magnitude of the isotopic fractionation, and to compare bond-partner-driven fractionation with redox-driven fractionation.Relative to , all models show a nearly linear decrease in ⁵⁶Fe/⁵⁴Fe as the number of Cl⁻ ions per Fe³⁺ ion increases, with slopes of −0.8‰ to −1.0‰ per Cl⁻ at 20 °C. At 20 °C, 1000 ln β (β = ⁵⁶Fe/⁵⁴Fe reduced partition function ratio relative to a dissociated Fe atom) values range from 8.93‰ to 9.73‰ for , 8.04-9.12‰ for , 7.61-8.73‰ for , 7.14-8.25‰ for , and 3.09-4.41‰ for . The fractionation between and ranges from 1.5‰ to 2.6‰, depending on the model; this is comparable in magnitude to fractionation effects due to Fe³⁺/Fe²⁺ redox reactions. β values from the UHF models are consistently higher than those from the hybrid DFT models.Isotopic fractionation is shown to be sensitive to differences in ligand bond stiffness (above), coordination number, bond length, and the frequency of the asymmetric Fe-X stretching vibrational mode, as predicted by previous theoretical studies. Complexes with smaller coordination numbers have higher 1000 ln β (7.46‰, 5.25‰, and 3.48‰ for , ,, respectively, from the B3LYP/6-31G(d) model). Species with the same number of chlorides but fewer waters also show the effect of coordination number on 1000 ln β: (7.46‰ vs. 7.05‰ for FeCl₃(H₂O)₂ vs. FeCl₃(H₂O)₃ and 5.25‰ vs. 4.94‰ for vs. FeCl₅H₂O²⁻ with the B3LYP/6-31G(d) model). As more Fe-Cl bonds substitute for Fe-OH₂ bonds (with a resulting decrease in β), the lengths of the Fe-Cl bonds and the Fe-O bonds increase.Preliminary modeling of shows an Fe³⁺/Fe²⁺ fractionation of 3.2‰ for the B3LYP/6-31G(d) model, in agreement with previous studies. The addition of an explicit outer hydration sphere of 12 H₂O molecules to models of improves agreement with measured vibrational frequencies and bond lengths; 1000 ln β increases by 0.8-1.0‰. An additional hydration sphere around increases 1000 ln β by only 0.1‰.Isotopic fractionations predicted for this simple system imply that ligands present in an aqueous iron environment are potentially important drivers of fractionation, and suggest that significant fractionation effects are likely in other aqueous systems containing sulfides or organic ligands. Fractionation effects due to both speciation and redox must be considered when interpreting iron isotope fractionations in the geological record.     

Oxygen and sulfur isotope systematics of sulfate produced by bacterial and abiotic oxidation of pyrite

To better understand reaction pathways of pyrite oxidation and biogeochemical controls on δ¹⁸O and δ³⁴S values of the generated sulfate in acid mine drainage (AMD) and other natural environments, we conducted a series of pyrite oxidation experiments in the laboratory. Our biological and abiotic experiments were conducted under aerobic conditions by using O₂ as an oxidizing agent and under anaerobic conditions by using dissolved Fe(III)_aq as an oxidant with varying δ¹⁸O_H2O values in the presence and absence of Acidithiobacillus ferrooxidans. In addition, aerobic biological experiments were designed as short- and long-term experiments where the final pH was controlled at ∼2.7 and 2.2, respectively. Due to the slower kinetics of abiotic sulfide oxidation, the aerobic abiotic experiments were only conducted as long term with a final pH of ∼2.7. The δ³⁴S_SO4 values from both the biological and abiotic anaerobic experiments indicated a small but significant sulfur isotope fractionation (∼−0.7‰) in contrast to no significant fractionation observed from any of the aerobic experiments. Relative percentages of the incorporation of water-derived oxygen and dissolved oxygen (O₂) to sulfate were estimated, in addition to the oxygen isotope fractionation between sulfate and water, and dissolved oxygen. As expected, during the biological and abiotic anaerobic experiments all of the sulfate oxygen was derived from water. The percentage incorporation of water-derived oxygen into sulfate during the oxidation experiments by O₂ varied with longer incubation and lower pH, but not due to the presence or absence of bacteria. These percentages were estimated as 85%, 92% and 87% from the short-term biological, long-term biological and abiotic control experiments, respectively. An oxygen isotope fractionation effect between sulfate and water (ε¹⁸O_SO4-H2O) of ∼3.5‰ was determined for the anaerobic (biological and abiotic) experiments. This measured value was then used to estimate the oxygen isotope fractionation effects between sulfate and dissolved oxygen in the aerobic experiments which were −10.0‰, −10.8‰, and −9.8‰ for the short-term biological, long-term biological and abiotic control experiments, respectively. Based on the similarity between δ¹⁸O_SO4 values in the biological and abiotic experiments, it is suggested that δ¹⁸O_SO4 values cannot be used to distinguish biological and abiotic mechanisms of pyrite oxidation. The results presented here suggest that Fe(III)_aq is the primary oxidant for pyrite at pH < 3, even in the presence of dissolved oxygen, and that the main oxygen source of sulfate is water-oxygen under both aerobic and anaerobic conditions.     

Oxygen and hydrogen isotope fractionation in serpentine-water and talc-water systems from 250 to 450 °C, 50 MPa

Oxygen and hydrogen isotope fractionation factors in the talc-water and serpentine-water systems have been determined by laboratory experiment from 250 to 450 °C at 50 MPa using the partial exchange technique. Talc was synthesized from brucite + quartz, resulting in nearly 100% exchange during reaction at 350 and 450 °C. For serpentine, D-H exchange was much more rapid than ¹⁸O-¹⁶O exchange when natural chrysotile fibers were employed in the initial charge. In experiments with lizardite as the starting charge, recrystallization to chrysotile enhanced the rate of ¹⁸O-¹⁶O exchange with the coexisting aqueous phase. Oxygen isotope fractionation factors in both the talc-water and serpentine-water systems decrease with increasing temperature and can be described from 250 to 450 °C by the relationships: 1000 ln  = 11.70 × 10⁶/T² − 25.49 × 10³/T + 12.48 and 1000 ln  = 3.49 × 10⁶/T² − 9.48 where T is temperature in Kelvin. Over the same temperature interval at 50 MPa, talc-water D-H fractionation is only weakly dependent on temperature, similar to brucite and chlorite, and can be described by the equation: 1000 ln = 10.88 × 10⁶/T² − 41.52 × 10³/T + 5.61 where T is temperature in Kelvin. Our D-H serpentine-water fractionation factors calibrated by experiment decrease with temperature and form a consistent trend with fractionation factors derived from lower temperature field calibrations. By regression of these data, we have refined and extended the D-H fractionation curve from 25 to 450 °C, 50 MPa as follows: 1000 ln  = 3.436 × 10⁶/T² − 34.736 × 10³/T + 21.67 where T is temperature in Kelvin. These new data should improve the application of D-H and ¹⁸O-¹⁶O isotopes to constrain the temperature and origin of hydrothermal fluids responsible for serpentine formation in a variety of geologic settings.     

Oxygen isotopic fractionation during inorganic calcite precipitation ― Effects of temperature, precipitation rate and pH

Stable oxygen isotopic fractionation during inorganic calcite precipitation was experimentally studied by spontaneous precipitation at various pH (8.3 < pH < 10.5), precipitation rates (1.8 < log R < 4.4 μmol m^− 2 h^− 1) and temperatures (5, 25, and 40 °C) using the CO₂ diffusion technique.The results show that the apparent stable oxygen isotopic fractionation factor between calcite and water (α_{calcite–water}) is affected by temperature, the pH of the solution, and the precipitation rate of calcite. Isotopic equilibrium is not maintained during spontaneous precipitation from the solution. Under isotopic non-equilibrium conditions, at a constant temperature and precipitation rate, apparent 1000lnα_{calcite–water} decreases with increasing pH of the solution. If the temperature and pH are held constant, apparent 1000lnα_{calcite–water} values decrease with elevated precipitation rates of calcite. At pH = 8.3, oxygen isotopic fractionation between inorganically precipitated calcite and water as a function of the precipitation rate (R) can be described by the expressions

at 5, 25, and 40 °C, respectively.The impact of precipitation rate on 1000lnα_{calcite–water} value in our experiments clearly indicates a kinetic effect on oxygen isotopic fractionation during calcite precipitation from aqueous solution, even if calcite precipitated slowly from aqueous solution at the given temperature range. Our results support Coplen's work [Coplen T. B. (2007) Calibration of the calcite–water oxygen isotope geothermometer at Devils Hole, Nevada, a natural laboratory. Geochim. Cosmochim. Acta 71, 3948–3957], which indicates that the equilibrium oxygen isotopic fractionation factor might be greater than the commonly accepted value.     

Coupled phase and aqueous species equilibrium of the H2O-CO2-NaCl-CaCO3 system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to saturation of halite

A model is developed for the calculation of coupled phase and aqueous species equilibrium in the H₂O-CO₂-NaCl-CaCO₃ system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to saturation of halite. The vapor-liquid-solid (calcite, halite) equilibrium together with the chemical equilibrium of H⁺, Na⁺, Ca²⁺, , Ca(OH)⁺, OH⁻, Cl⁻, , , CO_2(aq) and CaCO_3(aq) in the aqueous liquid phase as a function of temperature, pressure, NaCl concentrations, CO_2(aq) concentrations can be calculated, with accuracy close to those of experiments in the stated T-P-m range, hence calcite solubility, CO₂ gas solubility, alkalinity and pH values can be accurately calculated. The merit and advantage of this model is its predictability, the model was generally not constructed by fitting experimental data.One of the focuses of this study is to predict calcite solubility, with accuracy consistent with the works in previous experimental studies. The resulted model reproduces the following: (1) as temperature increases, the calcite solubility decreases. For example, when temperature increases from 273 to 373 K, calcite solubility decreases by about 50%; (2) with the increase of pressure, calcite solubility increases. For example, at 373 K changing pressure from 10 to 500 bar may increase calcite solubility by as much as 30%; (3) dissolved CO₂ can increase calcite solubility substantially; (4) increasing concentration of NaCl up to 2 m will increase calcite solubility, but further increasing NaCl solubility beyond 2 m will decrease its solubility.The functionality of pH value, alkalinity, CO₂ gas solubility, and the concentrations of many aqueous species with temperature, pressure and NaCl_(aq) concentrations can be found from the application of this model. Online calculation is made available on www.geochem-model.org/models/h2o_co2_nacl_caco3/calc.php.     

Characterization of calcium isotopes in natural and synthetic barite

The mineral barite (BaSO₄) accommodates calcium in its crystal lattice, providing an archive of Ca-isotopes in the highly stable sulfate mineral. Holocene marine (pelagic) barite samples from the major ocean basins are isotopically indistinguishable from each other (δ^44/40Ca = −2.01 ± 0.15‰) but are different from hydrothermal and cold seep barite samples (δ^44/40Ca = −4.13 to −2.72‰). Laboratory precipitated (synthetic) barite samples are more depleted in the heavy Ca-isotopes than pelagic marine barite and span a range of Ca-isotope compositions, Δ^44/40Ca = −3.42 to −2.40‰. Temperature, saturation state, , and aCa²⁺/aBa²⁺ each influence the fractionation of Ca-isotopes in synthetic barite; however, the fractionation in marine barite samples is not strongly related to any measured environmental parameter. First-principles lattice dynamical modeling predicts that at equilibrium Ca-substituted barite will have much lower ⁴⁴Ca/⁴⁰Ca than calcite, by −9‰ at 0 °C and −8‰ at 25 °C. Based on this model, none of the measured barite samples appear to be in isotopic equilibrium with their parent solutions, although as predicted they do record lower δ^44/40Ca values than seawater and calcite. Kinetic fractionation processes therefore most likely control the extent of isotopic fractionation exhibited in barite. Potential fractionation mechanisms include factors influencing Ca²⁺ substitution for Ba²⁺ in barite (e.g. ionic strength and trace element concentration of the solution, competing complexation reactions, precipitation or growth rate, temperature, pressure, and saturation state) as well as nucleation and crystal growth rates. These factors should be considered when investigating controls on isotopic fractionation of Ca²⁺ and other elements in inorganic and biogenic minerals.     

Isotopic fractionation of Mg(aq), Ca(aq), and Fe(aq) with carbonate minerals

Density-functional electronic structure calculations are used to compute the equilibrium constants for ²⁶Mg/²⁴Mg and ⁴⁴Ca/⁴⁰Ca isotope exchange between carbonate minerals and uncomplexed divalent aquo ions. The most reliable calculations at the B3LYP/6-311++G(2d,2p) level predict equilibrium constants K, reported as 10³ln (K) at 25 °C, of −5.3, −1.1, and +1.2 for ²⁶Mg/²⁴Mg exchange between calcite (CaCO₃), magnesite (MgCO₃), and dolomite (Ca_0.5Mg_0.5CO₃), respectively, and Mg²⁺(aq), with positive values indicating enrichment of the heavy isotope in the mineral phase. For ⁴⁴Ca/⁴⁰Ca exchange between calcite and Ca²⁺(aq) at 25 °C, the calculations predict values of +1.5 for Ca²⁺(aq) in 6-fold coordination and +4.1 for Ca²⁺(aq) in 7-fold coordination. We find that the reduced partition function ratios can be reliably computed from systems as small as and embedded in a set of fixed atoms representing the second-shell (and greater) coordination environment. We find that the aqueous cluster representing the aquo ion is much more sensitive to improvements in the basis set than the calculations on the mineral systems, and that fractionation factors should be computed using the best possible basis set for the aquo complex, even if the reduced partition function ratio calculated with the same basis set is not available for the mineral system. The new calculations show that the previous discrepancies between theory and experiment for Fe³⁺-hematite and Fe²⁺-siderite fractionations arise from an insufficiently accurate reduced partition function ratio for the Fe³⁺(aq) and Fe²⁺(aq) species.     

The partitioning of molybdenum(VI) between aqueous liquid and vapour at temperatures up to 370 °C

We have conducted experiments to evaluate the vapour-liquid fractionation of Mo(VI) in the system MoO₃-NH₃-H₂O at 300-370 °C and saturated vapour pressure, using a two-chamber autoclave that allows separate trapping of the vapour and liquid. The measured total Mo concentrations in each phase were used to calculate a distribution coefficient, , which increases as the density of the vapour approaches that of the liquid, and is greater than one for pH ? 4. Molybdenum speciation in the vapour is described by a single complex, MoO₃H₂O. By contrast, thermodynamic modeling of the distribution of Mo species in the liquid indicates that bimolybdate (HMoO₄⁻) is the dominant aqueous species at the conditions of our experiments, and that molybdate (MoO₄²⁻) and molybdic acid (H₂MoO₄⁰) are present in smaller quantities. As vapour-liquid fractionation occurs between neutral species, it is governed by the reaction H₂MoO₄⁰(aq) = MoO₃ · H₂O(g). Fractionation is therefore controlled by the concentration of H₂MoO₄⁰ in the liquid, which increases with increasing temperature and decreasing pH. Owing to the pH dependence of , it cannot be used to describe Mo fractionation in aqueous vapour-liquid systems with compositions different than those of this study. We have therefore calculated a composition-independent (Henry’s Law) constant, , for each experimental point, using the measured total Mo concentration in the vapour and the modeled concentration of H₂MoO₄⁰ in the liquid. This constant may be applied to aqueous vapour-liquid systems of known liquid composition to estimate the concentration of Mo in a vapour for which little chemical information is available, and thereby supplement the available fractionation data for natural porphyry-forming systems. The results of this study demonstrate that at conditions typical of natural porphyry ore-forming systems, a significant amount of molybdenum fractionates into the vapour over the liquid, and the vapour may transport quantities of Mo in excess of that in the liquid at pH conditions below those of the muscovite-microcline reaction boundary.     

Interdiffusion of H2O and CO2 in metamorphic fluids at ∼490 to 690°C and 1 GPa

Interdiffusion coefficients have been determined for H₂O-CO₂ mixtures by quantifying the flux of CO₂ between two fluid-filled chambers in a specially designed piston-cylinder cell. The two chambers, which are maintained at 1.0 GPa and at temperatures differing by ∼100°C, each contain the X_CO2-buffering assemblage calcite + quartz + wollastonite, in H₂O. The positive dependence of X_CO2 on temperature results in a down-temperature, steady-state flux of CO₂ through a capillary tube that connects the two chambers. This flux drives the wollastonite = calcite + quartz equilibrium to the right in the cooler chamber, producing a measurable amount of calcite that is directly related to CO₂-H₂O interdiffusion rates. Diffusivities calculated from seven experiments range from 1.0 × 10⁻⁸ to 6.1 × 10⁻⁸ m²/s for mean capillary temperatures between ∼490 and 690°C. The data set can be approximated by an Arrhenius-type relation: