Acoustic velocity measurements on Na2O-TiO2-SiO2 liquids: Evidence for a highly compressible TiO2 component related to five-coordinated Ti

Longitudinal acoustic velocities were measured at 1 bar in 10 Na₂O-TiO₂-SiO₂ (NTS) liquids for which previous density and thermal expansion data are reported in the literature. Data were collected with a frequency-sweep acoustic interferometer at centered frequencies of 4.5, 5, and 6 MHz between 1233 and 1896 K; in all cases, the sound speeds decrease with increasing temperature. Six of the liquids have a similar TiO₂ concentration (∼25 mol %), so that the effect of varying Na/Si ratio on the partial molar compressibility of the TiO₂ component can be evaluated. Theoretically based models for β_T and (∂V/∂P)_T as a function of composition and temperature are presented. As found previously for the partial molar volume of TiO₂ in sodium silicate melts, values of (13.7-18.8 × 10⁻²/GPa) vary systematically with the Na/Si and Na/(Si + Ti) ratio in the liquid. In contrast values of for the SiO₂ and Na₂O components (6.6 and 8.0 × 10⁻²/GPa, respectively, at 1573 K) are independent of composition. Na₂O is the only component that contributes to the temperature dependence of the compressibility of NTS liquids (1.13 ± 0.04 × 10⁻⁴/GPa K). The results further indicate that the TiO₂ component is twice as compressible as the Na₂O and SiO₂ components. The enhanced compressibility of TiO₂ appears to be related to the abundance of five-coordinated Ti (^[5]Ti) in these liquids, but not with a change in Ti coordination. Instead, it is proposed that the asymmetric geometry of ^[5]Ti in a square pyramidal site promotes different topological rearrangements in alkali titanosilicate liquids, which lead to the enhanced compressibility of TiO₂.     

A laser fluorination method for oxygen isotope analysis of biogenic silica and a new oxygen isotope calibration of modern diatoms in freshwater environments

An empirical calibration for the oxygen isotope fractionation between biogenic silica and water was determined for diatom frustules sampled from living diatom communities in the Jemez Mountains of northern New Mexico, USA. Over a temperature range from 5.1 to 37.8 °C, the silica-water fractionation is defined by the equation 1000 ln α_{(silica-water)} = 2.39(±0.13) × 10⁶T⁻² + 4.23(±1.49). This relationship is in close agreement with other published silica-water fractionation factors for laboratory cultured diatom samples; however, it is as much as 8‰ lower than equilibrium quartz-water fractionations and 3-4‰ lower than observed silica-water fractionations in diatomaceous silica collected from sediment traps and sediment cores. There are three possible explanations for the disparate silica-water fractionation factors observed in diatom silica: (1) silica does not precipitate in equilibrium with ambient water, (2) silica does precipitate in equilibrium with ambient water, but the silica-water fractionation factor for diatom silica is considerably less than the equilibrium fractionation factor for quartz-water, or (3) silica precipitation is influenced by a ‘vital’ effect, where the δ¹⁸O value of the water inside the diatom cell walls is lower than the δ¹⁸O values of ambient water.Post-mortem loss of organic material results in an alteration or ‘maturation’ of diatom silica in which silica reequilibrates with a silica-water fractionation closer to the equilibrium quartz-water fractionation. Alteration is likely to occur rapidly after the diatom frustule loses its organic coating, either as it settles through the water column or at the sediment-water interface; δ¹⁸O values recorded by paleo-diatom silica therefore do not record growing conditions but more likely record conditions at the sediment-water interface. In the case of lacustrine environments, where the bottom water remains at a nearly constant 4 °C, the reequilibration of diatom silica with bottom conditions could reduce or remove the conflating effects of temperature on δ¹⁸O values recorded by paleo-diatom silica and provide direct information on the δ¹⁸O value of the lake water.     

Experimental study of the aragonite to calcite transition in aqueous solution

The experimental replacement of aragonite by calcite was studied under hydrothermal conditions at temperatures between 160 and 200 °C using single inorganic aragonite crystals as a starting material. The initial saturation state and the total [Ca²⁺]:[CO₃²⁻] ratio of the experimental solutions was found to have a determining effect on the amount and abundance of calcite overgrowths as well as the extent of replacement observed within the crystals. The replacement process was accompanied by progressive formation of cracks and pores within the calcite, which led to extended fracturing of the initial aragonite. The overall shape and morphology of the parent aragonite crystal were preserved. The replaced regions were identified with scanning electron microscopy and Raman spectroscopy.Experiments using carbonate solutions prepared with water enriched in ¹⁸O (97%) were also performed in order to trace the course of this replacement process. The incorporation of the heavier oxygen isotope in the carbonate molecule within the calcite replacements was monitored with Raman spectroscopy. The heterogeneous distribution of ¹⁸O in the reaction products required a separate study of the kinetics of isotopic equilibration within the fluid to obtain a better understanding of the ¹⁸O distribution in the calcite replacement. An activation energy of 109 kJ/mol was calculated for the exchange of oxygen isotopes between [C¹⁶O₃²⁻]_aq and [H₂¹⁸O] and the time for oxygen isotope exchange in the fluid at 200 °C was estimated at ∼0.9 s. Given the exchange rate, analyses of the run products imply that the oxygen isotope composition in the calcite product is partly inherited from the oxygen isotope composition of the aragonite parent during the replacement process and is dependent on access of the fluid to the reaction interface rather than equilibration time. The aragonite to calcite fluid-mediated transformation is described by a coupled dissolution-reprecipitation mechanism, where aragonite dissolution is coupled to the precipitation of calcite at an inwardly moving reaction interface.     

C-O bonds in carbonate minerals: A new kind of paleothermometer

The abundance of the doubly substituted CO₂ isotopologue, ¹³C¹⁸O¹⁶O, in CO₂ produced by phosphoric acid digestion of synthetic, inorganic calcite and natural, biogenic aragonite is proportional to the concentration of ¹³C-¹⁸O bonds in reactant carbonate, and the concentration of these bonds is a function of the temperature of carbonate growth. This proportionality can be described between 1 and 50 °C by the function: Δ₄₇ = 0.0592 · 10⁶ · T⁻² − 0.02, where Δ₄₇ is the enrichment, in per mil, of ¹³C¹⁸O¹⁶O in CO₂ relative to the amount expected for a stochastic (random) distribution of isotopes among all CO₂ isotopologues, and T is the temperature in Kelvin. This relationship can be used for a new kind of carbonate paleothermometry, where the temperature-dependent property of interest is the state of ordering of ¹³C and ¹⁸O in the carbonate lattice (i.e., bound together vs. separated into different CO₃²⁻ units), and not the bulk δ¹⁸O or δ¹³C values. Current analytical methods limit precision of this thermometer to ca. ± 2 °C, 1σ. A key feature of this thermometer is that it is thermodynamically based, like the traditional carbonate-water paleothermometer, and so is suitable for interpolation and even modest extrapolation, yet is rigorously independent of the δ¹⁸O of water and δ¹³C of DIC from which carbonate grew. Thus, this technique can be applied to parts of the geological record where the stable isotope compositions of waters are unknown. Moreover, simultaneous determinations of Δ₄₇ and δ¹⁸O for carbonates will constrain the δ¹⁸O of water from which they grew.     

Oxygen isotope exchange between H2O and CO2 at elevated CO2 pressures: Implications for monitoring of geological CO2 storage

Traditionally, the application of stable isotopes in Carbon Capture and Storage (CCS) projects has focused on δ¹³C values of CO₂ to trace the migration of injected CO₂ in the subsurface. More recently the use of δ¹⁸O values of both CO₂ and reservoir fluids has been proposed as a method for quantifying in situ CO₂ reservoir saturations due to O isotope exchange between CO₂ and H₂O and subsequent changes in δ¹⁸O_H2O values in the presence of high concentrations of CO₂. To verify that O isotope exchange between CO₂ and H₂O reaches equilibrium within days, and that δ¹⁸O_H2O values indeed change predictably due to the presence of CO₂, a laboratory study was conducted during which the isotope composition of H₂O, CO₂, and dissolved inorganic C (DIC) was determined at representative reservoir conditions (50 °C and up to 19 MPa) and varying CO₂ pressures. Conditions typical for the Pembina Cardium CO₂ Monitoring Pilot in Alberta (Canada) were chosen for the experiments. Results obtained showed that δ¹⁸O values of CO₂ were on average 36.4 ± 2.2‰ (1σ, n = 15) higher than those of water at all pressures up to and including reservoir pressure (19 MPa), in excellent agreement with the theoretically predicted isotope enrichment factor of 35.5‰ for the experimental temperatures of 50 °C. By using ¹⁸O enriched water for the experiments it was demonstrated that changes in the δ¹⁸O values of water were predictably related to the fraction of O in the system sourced from CO₂ in excellent agreement with theoretical predictions. Since the fraction of O sourced from CO₂ is related to the total volumetric saturation of CO₂ and water as a fraction of the total volume of the system, it is concluded that changes in δ¹⁸O values of reservoir fluids can be used to calculate reservoir saturations of CO₂ in CCS settings given that the δ¹⁸O values of CO₂ and water are sufficiently distinct.     

Oxygen and iron isotope constraints on near-surface fractionation effects and the composition of lunar mare basalt source regions

Oxygen and iron isotope analyses of low-Ti and high-Ti mare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources. An internally-consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low-Ti basalts from the Apollo 12 and 15 missions and eight types of high-Ti basalts from the Apollo 11 and 17 missions. High-precision whole-rock δ¹⁸O values (referenced to VSMOW) of low-Ti and high-Ti basalts correlate with major-element compositions (Mg#, TiO₂, Al₂O₃). The observed oxygen isotope variations within low-Ti and high-Ti basalts are consistent with crystal fractionation and match the results of mass-balance models assuming equilibrium crystallization. Whole-rock δ⁵⁶Fe values (referenced to IRMM-014) of high-Ti and low-Ti basalts range from 0.134‰ to 0.217‰ and 0.038‰ to 0.104‰, respectively. Iron isotope compositions of both low-Ti and high-Ti basalts do not correlate with indices of crystal fractionation, possibly owing to small mineral-melt iron fractionation factors anticipated under lunar reducing conditions.The δ¹⁸O and δ⁵⁶Fe values of low-Ti and the least differentiated high-Ti mare basalts are negatively correlated, which reflects their different mantle source characteristics (e.g., the presence or absence of ilmenite). The average δ⁵⁶Fe values of low-Ti basalts (0.073 ± 0.018‰, n = 8) and high-Ti basalts (0.191 ± 0.020‰, n = 7) may directly record that of their parent mantle sources. Oxygen isotope compositions of mantle sources of low-Ti and high-Ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing, the estimated equilibrium mantle olivine δ¹⁸O value, and equilibrium oxygen-fractionation between olivine and other mineral phases. The differences between the calculated whole-rock δ¹⁸O values for source regions, 5.57‰ for low-Ti and 5.30‰ for high-Ti mare basalt mantle source regions, are solely a function of the assumed source mineralogy. The oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values. The δ¹⁸O and δ⁵⁶Fe values of the lunar upper mantle are estimated to be 5.5 ± 0.2‰ (2σ) and 0.085 ± 0.040‰ (2σ), respectively. The oxygen isotope composition of lunar upper mantle is identical to the current estimate of Earth’s upper mantle (5.5 ± 0.2‰), and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle (0.044 ± 0.030‰).     

Microscale oxygen isotope variations in 1.9 Ga Gunflint cherts: Assessments of diagenesis effects and implications for oceanic paleotemperature reconstructions

Variations in the oxygen isotope composition (δ¹⁸O) of five cherts from the 1.9 Ga Gunflint iron formation (Canada) were studied at the micrometer scale by ion microprobe to try to better understand the processes that control δ¹⁸O values in cherts and to improve seawater paleotemperature reconstructions. Gunflint cherts show clearly different δ¹⁸O values for different types of silica with for instance a difference of ≈15‰ between detrital quartz and microquartz. Microquartz in the five samples is characterized by large intra sample variations in δ¹⁸O values, (δ¹⁸O of quartz varies from 4.6‰ to 6.6‰ at the 20 μm scale and from ≈12‰ to 14‰ at 2 μm scale). Isotopic profiles in microquartz adjacent to hydrothermal quartz veins demonstrate that microquartz more than ≈200 μm away from the veins has preserved its original δ¹⁸O value.At the micrometer spatial resolution of the ion probe, data reveal that microquartz has preserved a considerable δ¹⁸O heterogeneity that must be regarded as a signature inherited from its diagenetic history. Modelling of the δ¹⁸O variations produced during the diagenetic transformation of sedimentary amorphous silica precursors into microquartz allows us to calculate seawater temperature (T_sw at which the amorphous silica precipitated) and diagenesis temperature (T_diagenesis at which microquartz formed) that reproduce the δ¹⁸O distributions (mean, range and shape) measured at micrometer scale in microquartz. The two critical parameters in this modelling are the δ¹⁸O value and the mass fraction of the diagenetic fluid. Under these assumptions, the most likely ranges for T_sw and T_diagenesis are from 37 to 52 °C and from 130 to 170 °C, respectively.     

Interaction between sulphide and H2O in silicate melts

Reaction between dissolved water and sulphide was experimentally investigated in soda-lime-silicate (NCS) and sodium trisilicate (NS3) melts at temperatures from 1000 to 1200 °C and pressures of 100 or 200 MPa in internally heated gas pressure vessels. Diffusion couple experiments were conducted at water-undersaturated conditions with one half of the couple being doped with sulphide (added as FeS or Na₂S; 1500-2000 ppm S by weight) and the other with H₂O (∼3.0 wt.%). Additionally, two experiments were performed using a dry NCS glass cylinder and a free H₂O fluid. Here, the melt was water-saturated at least at the melt/fluid interface. Profiling by electron microprobe (sulphur) and infrared microscopy (H₂O) demonstrate that H₂O diffusion in the melts is faster by 1.5-2.3 orders of magnitude than sulphur diffusion and, hence, H₂O can be considered as a rapidly diffusing oxidant while sulphur is quasi immobile in these experiments.In Raman spectra a band at 2576 cm⁻¹ appears in the sulphide - H₂O transition zone which is attributed to fundamental S-H stretching vibrations. Formation of new IR absorption bands at 5025 cm⁻¹ (on expense of the combination band of molecular H₂O at 5225 cm⁻¹) and at 3400 cm⁻¹ was observed at the front of the in-diffusing water in the sulphide bearing melt. The appearance and intensity of these two IR bands is correlated with systematic changes in S K-edge XANES spectra. A pre-edge excitation at 2466.5 eV grows with increasing H₂O concentration while the sulphide peak at 2474.0 eV decreases in intensity relative to the peak at 2477.0 eV and the feature at 2472.3 eV becomes more pronounced (all energies are relative to the sulphate excitation, calibrated to 2482.5 eV). The observations by Raman, IR and XANES spectroscopy indicate a well coordinated S²⁻ - H₂O complex which was probably formed in the glasses during cooling at the glass transition. No oxidation of sulphide was observed in any of the diffusion couple experiments. On the contrary, XANES spectra from experiments conducted with a free H₂O fluid show complete transformation of sulphide to sulphate near the melt surface and coexistence of sulphate and sulphide in the center of the melt. This can be explained by a lower H₂O activity in the diffusion couple experiments or by the need of a sink for hydrogen (e.g., a fluid which can dissolve high concentration of hydrogen) to promote oxidation of sulphide by H₂O via the reaction S²⁻ + 4H₂O = SO₄²⁻ + 4H₂. Sulphite could not be detected in any of the XANES spectra implying that this species, if it exists in the melt, it is a subordinate or transient species only.     

O and H diffusion in uraninite: Implications for fluid-uraninite interactions, nuclear waste disposal, and nuclear forensics

Diffusion coefficients for oxygen and hydrogen were determined from a series of natural uraninite-H₂O experiments between 50 and 700 °C. Under hydrous conditions there are two diffusion mechanisms: (1) an initial extremely fast-path diffusion mechanism that overprinted the oxygen isotopic composition of the entire crystals regardless of temperature and (2) a slower volume-diffusive mechanism dominated by defect clusters that displace or eject nearest neighbor oxygen atoms to form two interstitial sites and two partial vacancies, and by vacancy migration. Using the volume diffusion coefficients in the temperature range of 400-600 °C, diffusion coefficients for oxygen can be represented by D = 1.90e⁻⁵ exp (−123,382 J/RT) cm²/s and for temperatures between 100 and 300 °C the diffusion coefficients can be represented by D = 1.95e⁻¹⁰ exp (−62484 J/RT) cm²/s, where the activation energies for uraninite are 123.4 and 62.5 kJ/mol, respectively. Hydrogen diffusion in uraninite appears to be controlled by similar mechanisms as oxygen. Using the volume diffusion coefficients for temperatures between 50 and 700 °C, diffusion coefficients for hydrogen can be represented by D = 9.28e⁻⁶ exp (−156,528 J/RT) cm²/s for temperatures between 450 and 700 °C and D = 1.39e⁻¹⁴ exp (−34518 J/RT) cm²/s for temperatures between 50 and 400 °C, where the activation energies for uraninite are 156.5 and 34.5 kJ/mol, respectively.Results from these new experiments have implications for isotopic exchange during natural UO₂-water interactions. The exceptionally low δ¹⁸O values of natural uraninites (i.e. −32‰ to −19.5‰) from unconformity-type uranium deposits in Saskatchewan, in conjunction with theoretical and experimental uraninite-water and UO₃-water fractionation factors, suggest that primary uranium mineralization is not in oxygen isotopic equilibrium with coeval clay and silicate minerals. The low δ¹⁸O values have been interpreted as resulting from the low temperature overprinting of primary uranium mineralization in the presence of relatively modern meteoric fluids having δ¹⁸O values of ca. −18‰, despite petrographic and U-Pb isotope data that indicate limited alteration. Our data show that the anomalously low oxygen isotopic composition of the uraninite from the Athabasca Basin can be due to meteoric water overprinting under reducing conditions, and meteoric water or groundwater can significantly affect the oxygen isotopic composition of spent nuclear fuel in a geologic repository, with minimal change to the chemical composition or texture. Moreover, the rather fast oxygen and hydrogen diffusion coefficients for uraninite, especially at low temperatures, suggest that oxygen and hydrogen diffusion may impart characteristic isotopic signals that can be used to track the route of fissile material.     

Calibration of the calcite-water oxygen-isotope geothermometer at Devils Hole, Nevada, a natural laboratory

The δ¹⁸O of ground water (−13.54 ± 0.05 ‰) and inorganically precipitated Holocene vein calcite (+14.56 ± 0.03 ‰) from Devils Hole cave #2 in southcentral Nevada yield an oxygen isotopic fractionation factor between calcite and water at 33.7 °C of 1.02849 ± 0.00013 (1000 ln α_{calcite-water} = 28.09 ± 0.13). Using the commonly accepted value of ∂(α_{calcite-water})/∂T of −0.00020 K⁻¹, this corresponds to a 1000 ln α_{calcite-water} value at 25 °C of 29.80, which differs substantially from the current accepted value of 28.3. Use of previously published oxygen isotopic fractionation factors would yield a calcite precipitation temperature in Devils Hole that is 8 °C lower than the measured ground water temperature. Alternatively, previously published fractionation factors would yield a δ¹⁸O of water, from which the calcite precipitated, that is too negative by 1.5 ‰ using a temperature of 33.7 °C. Several lines of evidence indicate that the geochemical environment of Devils Hole has been remarkably constant for at least 10 ka. Accordingly, a re-evaluation of calcite-water oxygen isotopic fractionation factor may be in order.Assuming the Devils Hole oxygen isotopic value of α_{calcite-water} represents thermodynamic equilibrium, many marine carbonates are precipitated with a δ¹⁸O value that is too low, apparently due to a kinetic isotopic fractionation that preferentially enriches ¹⁶O in the solid carbonate over ¹⁸O, feigning oxygen isotopic equilibrium.     

Abundance of mass 47 CO2 in urban air, car exhaust, and human breath

Atmospheric carbon dioxide is widely studied using records of CO₂ mixing ratio, δ¹³C and δ¹⁸O. However, the number and variability of sources and sinks prevents these alone from uniquely defining the budget. Carbon dioxide having a mass of 47 u (principally ¹³C¹⁸O¹⁶O) provides an additional constraint. In particular, the mass 47 anomaly (Δ₄₇) can distinguish between CO₂ produced by high temperature combustion processes vs. low temperature respiratory processes. Δ₄₇ is defined as the abundance of mass 47 isotopologues in excess of that expected for a random distribution of isotopes, where random distribution means that the abundance of an isotopologue is the product of abundances of the isotopes it is composed of and is calculated based on the measured ¹³C and ¹⁸O values. In this study, we estimate the δ¹³C (vs. VPDB), δ¹⁸O (vs. VSMOW), δ47, and Δ₄₇ values of CO₂ from car exhaust and from human breath, by constructing ‘Keeling plots’ using samples that are mixtures of ambient air and CO₂ from these sources. δ47 is defined as , where is the R⁴⁷ value for a hypothetical CO₂ whose δ¹³C_VPDB = 0, δ¹⁸O_VSMOW = 0, and Δ₄₇ = 0. Ambient air in Pasadena, CA, where this study was conducted, varied in [CO₂] from 383 to 404 μmol mol⁻¹, in δ¹³C and δ¹⁸O from −9.2 to −10.2‰ and from 40.6 to 41.9‰, respectively, in δ47 from 32.5 to 33.9‰, and in Δ₄₇ from 0.73 to 0.96‰. Air sampled at varying distances from a car exhaust pipe was enriched in a combustion source having a composition, as determined by a ‘Keeling plot’ intercept, of −24.4 ± 0.2‰ for δ¹³C (similar to the δ¹³C of local gasoline), δ¹⁸O of 29.9 ± 0.4‰, δ47 of 6.6 ± 0.6‰, and Δ₄₇ of 0.41 ± 0.03‰. Both δ¹⁸O and Δ₄₇ values of the car exhaust end-member are consistent with that expected for thermodynamic equilibrium at∼200 °C between CO₂ and water generated by combustion of gasoline-air mixtures. Samples of CO₂ from human breath were found to have δ¹³C and δ¹⁸O values broadly similar to those of car exhaust-air mixtures, −22.3 ± 0.2 and 34.3 ± 0.3‰, respectively, and δ47 of 13.4 ± 0.4‰. Δ₄₇ in human breath was 0.76  ± 0.03‰, similar to that of ambient Pasadena air and higher than that of the car exhaust signature.     

Oxygen isotopes in calcite grown under cave-analogue conditions

Speleothem oxygen isotopes and growth rates are valuable proxies for reconstructing climate history. There is debate, however, about the conditions that allow speleothems to grow in oxygen isotope equilibrium, and about the correct equilibrium fractionation factors. We report results from a series of carbonate growth experiments in karst-analogue conditions in the laboratory. The setup closely mimics natural processes (e.g. precipitation driven by CO₂-degassing, low ionic strength solution, thin solution film) but with a tight control on growth conditions (temperature, pCO₂, drip rate, calcite saturation index and the composition of the initial solution). Calcite is dissolved in water in a 20,000 ppmV pCO₂ environment. This solution is dripped onto glass plates (coated with seed-carbonate) in a lower pCO₂ environment (<2500 ppmV), where degassing leads to calcite growth. Experiments were performed at 7, 15, 25 and 35 °C. At each temperature, calcite was grown at three drip rates (2, 6 and 10 drips per minute) on separate plates. The mass of calcite grown in these experiments varies with temperature (T in K) and drip rate (d_r in drips min⁻¹) according to the relationship daily growth mass = 1.254 + 1.478 ∗ 10⁻⁹ ∗ e^0.0679T + (e^0.00248T − 2) ∗ (−0.779d_r² + 10.05d_r + 11.69). This relationship indicates a substantial increase of growth mass with temperature, a smaller influence of drip rate on growth mass at low temperature and a non-linear relationship between drip rate and growth mass at higher temperatures. Low temperature, fast dripping conditions are found to be the most favourable for reducing effects associated with evaporation and rapid depletion of the dissolved inorganic carbon reservoir (rapid DIC-depletion). The impact of evaporation can be large so caves with high relative humidity are also preferable for palaeoclimate reconstruction. Even allowing for the maximum offsets that may have been induced by evaporation and rapid DIC-depletion, δ¹⁸O measured in some of our experiments remain higher than those predicted by Kim and O’Neil (1997). Our new results are well explained by equilibrium at a significantly higher α_{calcite-water}, with a kinetic-isotope effect that favours ¹⁶O incorporation as growth rate increases. This scenario agrees with recent studies by [Coplen, 2007] and [Dietzel et al., 2009]. Overall, our results suggest that three separate processes cause δ¹⁸O to deviate from true isotope equilibrium in the cave environment. Two of these drive δ¹⁸O to higher values (evaporation and rapid DIC-depletion) while one drives δ¹⁸O to lower values (preferential incorporation of ¹⁶O in the solid carbonate at faster growth rates). While evaporation and DIC-depletion can be avoided in some settings, the third may be inescapable in the cave environment and means that any temperature to δ¹⁸O relationship is an approximation. The controlled conditions of the present experiments also display limitations in the use of the Hendy test to identifying equilibrium growth.     

Stable isotopic compositions of waters in the karst environments of China: Climatic implications

A total of 117 water samples, including cave water, ground water, spring water and river water, collected from the monsoonal area of China have been analyzed for their H- and O-isotope composition. Overall, a δ¹⁸O–δD correlation is observed of δD = −4.45 + 6.6δ¹⁸O (R² = 0.90) and a significant evaporation effect observed for the southern sites. Average δ¹⁸O and δD site values generally correspond to those of precipitation in nearby cities, with correlations of δD = 2.18 + 7.23δ¹⁸O (R² = 0.95) for the sample sites and δD = 11.05 + 7.95δ¹⁸O (R² = 0.95) for the cities. The effects of rainfall amount and temperature on precipitation δ¹⁸O were calculated using a simplified theoretical model derived from the Rayleigh distillation equation, which demonstrated that the sign of δ¹⁸O_pvs. T correlation is dependent on precipitation intensity. The mean δ¹⁸O value of cave waters exhibit decreasing trends with increasing latitude and reveal a spatial pattern of positive correlation with annual mean temperature and precipitation, mainly reflecting isotopic fractionations in the moisture source traveling from the ocean side to the inland continent. This spatial pattern implies that the δ¹⁸O values recorded in the proxy climate records derived from speleothems might be influenced by shifts in monsoon boundary during the past, especially between glacial and interglacial intervals.     

Polycrystalline apatite synthesized by hydrothermal replacement of calcium carbonates

Aragonite and calcite single crystals can be readily transformed into polycrystalline hydroxyapatite pseudomorphs by hydrothermal treatment in a (NH₄)₂HPO₄ solution. Scanning electron microscopy of the reaction products showed that the transformation of aragonite to apatite is characterised by the formation of a sharp interface between the two phases and by the development of intracrystalline porosity in the hydroxyapatite phase. In addition, electron backscattered diffraction (EBSD) imaging showed that the c-axis of apatite is predominantly oriented perpendicular to the reaction front with no crystallographic relationship to the aragonite lattice. However, the Ca isotopic composition of the parent aragonite, measured by thermal ionization mass spectrometry was inherited by the apatite product.Hydrothermal experiments conducted with use of phosphate solutions prepared with water enriched in ¹⁸O (97%) further revealed that the ¹⁸O from the solution is incorporated in the product apatite, as measured by micro-Raman spectroscopy. Monitoring the distribution of ¹⁸O with Raman spectroscopy was possible because the incorporation of ¹⁸O in the PO₄ group of apatite generates four new Raman bands at 945.8, 932, 919.7 and 908.8 cm⁻¹, in addition to the ν₁(PO₄) symmetric stretching band of apatite located at 962 cm⁻¹, which can be assigned to four ¹⁸O-bearing PO₄ species. The relative intensities of these bands reflect the ¹⁸O content in the PO₄ group of the apatite product. By using equilibrated and non-equilibrated solutions, with respect to the ¹⁸O distribution between aqueous phosphate and water, we could show that the concentration of ¹⁸O in the apatite product is linked to the degree of ¹⁸O equilibration in the solution. The textural and chemical observations are indicative of a coupled mechanism of aragonite dissolution and apatite precipitation taking place at a moving reaction interface.     

The triple isotopic composition of oxygen in leaf water

The isotopic composition of atmospheric O₂ depends on the rates of oxygen cycling in photosynthesis, respiration, photochemical reactions in the stratosphere and on δ¹⁷O and δ¹⁸O of ocean and leaf water. While most of the factors affecting δ¹⁷O and δ¹⁸O of air O₂ have been studied extensively in recent years, δ¹⁷O of leaf water—the substrate for all terrestrial photosynthesis—remained unknown. In order to understand the isotopic composition of atmospheric O₂ at present and in fossil air in ice cores, we studied leaf water in field experiments in Israel and in a European survey. We measured the difference in δ¹⁷O and δ¹⁸O between stem and leaf water, which is the result of isotope enrichment during transpiration. We calculated the slopes of the lines linking the isotopic compositions of stem and leaf water. The obtained slopes in ln(δ¹⁷O + 1) vs. ln(δ¹⁸O + 1) plots are characterized by very high precision (∼0.001) despite of relatively large differences between duplicates in both δ¹⁷O and δ¹⁸O (0.02-0.05‰). This is so because the errors in δ¹⁸O and δ¹⁷O are mass-dependent. The slope of the leaf transpiration process varied between 0.5111 ± 0.0013 and 0.5204 ± 0.0005, which is considerably smaller than the slope linking liquid water and vapor at equilibrium (0.529). We further found that the slope of the transpiration process decreases with atmospheric relative humidity (h) as 0.522-0.008 × h, for h in the range 0.3-1. This slope is neither influenced by the plant species, nor by the environmental conditions where plants grow nor does it show strong variations along long leaves.     

Intra-shell oxygen isotope ratios in the benthic foraminifera genus Amphistegina and the influence of seawater carbonate chemistry and temperature on this ratio

Using secondary ion mass spectrometry (SIMS) we looked at the natural variability in the oxygen isotope ratio of the shallow water, symbionts-bearing foraminiferan Amphistegina lobifera. Live foraminifera were collected in February 2005 in the Gulf of Eilat, Israel. Vertical section exposing the knob area of this species represents the growth history of this species from August 2004 to February 2005. SIMS profile at a resolution of ∼15 μm (representing about 2 weeks considering the size of the knob area and the life span of ≈6 months of this foraminifera species) yielded δ¹⁸O changes of ∼1.5‰ that are compatible with the known temperature changes for the Gulf of Eilat for this period (21-27 °C). Natural variability between primary and secondary calcite at the knob area were obtained on horizontal section of the upper knob area. This section is semi-tangential to the growth lines and exposes relatively wide belts of the primary calcite which could be analysed using the SIMS (beam size of 10 × 20 μm). The primary calcite δ¹⁸O value is on average more than 3‰ lower than the secondary calcite that represents the bulk of the skeleton (more than 95% by weight). A vertical profile at the knob was obtained by rastering an area of 50 × 50 μm at vertical steps of roughly 1 μm. The profile revealed a narrow zone of lower δ¹⁸O compared to the higher values above and below it. The difference between the lowest δ¹⁸O and the highest one was also close to 2‰. The δ¹⁸O in the margin - keel area of A. lobifera is also lower compared to the bulk secondary calcite. Specimens that were cultured in the laboratory at a constant temperature and inorganic carbon but at different pH have increased their CaCO₃ weight by roughly a factor of 8. Single specimen from each pH (ranging between 7.90 and 8.45) were investigated with the SIMS at the knob area. While there is some variability within each specimen (perhaps related to the primary calcite), the general trend was a decrease in δ¹⁸O with increasing pH (or CO₃²⁻ concentration), in agreement with previous studies on planktonic foraminifera. Some other specimens grown at different temperatures (between 21 and 33 °C) were also measured with the SIMS at the knob area. For each temperature, we observed also some variability, nevertheless the trend of −0.2‰/°C in δ¹⁸O is observed.     

Zircons from kimberlite: New insights from oxygen isotopes, trace elements, and Ti in zircon thermometry

Zircons found in mantle-sourced kimberlite provide probes into the isotopic chemistry of the asthenosphere and subcontinental lithospheric mantle. However, little is known about the conditions of formation of these zircons. A suite of 88 zircons found in kimberlites from Africa, Siberia, Brazil, and the United States have been analyzed for their Ti concentration and selected zircons were analyzed for their Rare Earth Element (REE) concentrations by ion microprobe. In addition, precise and accurate laser-fluorination oxygen isotope data were obtained for zircons from Brazil (5.1 ± 0.3‰, 1SD) and the Midwest United States (5.3 ± 0.3‰), yielding mantle-like δ¹⁸O values similar to published data for Africa (5.2 ± 0.3‰) and Siberia (5.3 ± 0.2‰). Most megacrysts in this study preserve fine-scale, oscillatory zoning in CL and are generally homogenous in oxygen isotopic composition, consistent with preservation of primary compositions. A few zircons from Brazil show some evidence of chemical zoning due to recrystallization. The Ti content of mantle zircon is in general low with average compositions from each locality of 13 ± 8.4 ppm (1SD, Kaapvaal craton), 12 ± 8.7 ppm (Siberian platform), 18 ± 11 ppm (Brazil), and 4.8 ±4.3 ppm (United States). The recently calibrated Ti in zircon thermometer yields an average temperature of 744 ±62 °C (1SD) for the average of 13 ± 9 ppm Ti, with no correction for pressure, a_TiO2, or a_SiO2. The Ti content of zircons found within rutile nodules from the Orapa kimberlite (Kaapvaal craton) is almost indistinguishable from those with no constraint on a_TiO2, suggesting that reduced a_TiO2 is not responsible for lower than expected mantle temperatures. The average temperature in this study corresponds to ∼3 GPa on a 40 mW/m² cratonic geotherm. If correct, this would suggest that zircon megacrysts from all four cratons formed in the shallow lithospheric mantle. However, there are several possibly confounding effects to this thermometer, including: a pressure correction and disequilibrium zircon growth. Zircons from rutile nodules have REE contents that span the range of mantle zircon REE and are similar to both zircon megacrysts and zircons from metasomatic assemblages.     

The potential origins and palaeoenvironmental implications of high temporal resolution δO heterogeneity in coral skeletons

δ¹⁸O was determined at high spatial resolution (beam diameter ∼30 μm) by secondary ion mass spectrometry (SIMS) across 1-2 year sections of 2 modern Porites lobata coral skeletons from Hawaii. We observe large (>2‰) cyclical δ¹⁸O variations that typically cover skeletal distances equivalent to periods of ∼20-30 days. These variations do not reflect seawater temperature or composition and we conclude that skeletal δ¹⁸O is principally controlled by other processes. Calcification site pH in one coral record was estimated from previous SIMS measurements of skeletal δ¹¹B. We model predicted skeletal δ¹⁸O as a function of calcification site pH, DIC residence time at the site and DIC source (reflecting the inputs of seawater and molecular CO₂ to the site). We assume that oxygen isotopic equilibration proceeds at the rates observed in seawater and that only the aqueous carbonate ion is incorporated into the precipitating aragonite. We reproduce successfully the observed skeletal δ¹⁸O range by assuming that DIC is rapidly utilised at the calcification site (within 1 h) and that ∼80% of the skeletal carbonate is derived from seawater. If carbonic anhydrase catalyses the reversible hydration of CO₂ at the calcification site, then oxygen isotopic equilibration times may be substantially reduced and a larger proportion of the skeletal carbonate could be derived from molecular CO₂. Seasonal skeletal δ¹⁸O variations are most pronounced in the skeleton deposited from late autumn to winter (and coincide with the high density skeletal bands) and are dampened in skeleton deposited from spring to summer. We observed no annual pattern in sea surface temperature or photosynthetically active radiation variability which could potentially correlate with the coral δ¹⁸O. At present we are unable to resolve an environmental cue to drive seasonal patterns of short term skeletal δ¹⁸O heterogeneity.     

Fractionation of oxygen and hydrogen isotopes in evaporating water

Variations in oxygen and hydrogen isotope ratios of water and ice are powerful tools in hydrology and ice core studies. These variations are controlled by both equilibrium and kinetic isotope effects during evaporation and precipitation, and for quantitative interpretation it is necessary to understand how these processes affect the isotopic composition of water and ice. Whereas the equilibrium isotope effects are reasonably well understood, there is controversy on the magnitude of the kinetic isotope effects of both oxygen and hydrogen and the ratio between them. In order to resolve this disagreement, we performed evaporation experiments into air, argon and helium over the temperature range from 10 to 70 °C. From these measurements we derived the isotope effects for vapor diffusion in gas phase (ε_{diff(HD¹⁶O)} for D/H and ε_{diff(H2¹⁸O)} for ¹⁸O/¹⁶O). For air, the ratio ε_{diff(HD¹⁶O)}/ε_{diff(H2¹⁸O)} at 20 °C is 0.84, in very good agreement with Merlivat (1978) (0.88), but in considerable inconsistency with Cappa et al. (2003) (0.52). Our results support Merlivat’s conclusion that measured ε_{diff(HD¹⁶O)}/ε_{diff(H2¹⁸O)} ratios are significantly different than ratios calculated from simplified kinetic theory of gas diffusion. On the other hand, our experiments with helium and argon suggest that this discrepancy is not due to isotope effects of molecular collision diameters. We also found, for the first time, that the ε_{diff(HD¹⁶O)}/ε_{diff(H2¹⁸O)} ratio tends to increase with cooling. This new finding may have important implications to interpretations of deuterium excess (d-excess = δD − 8δ¹⁸O) in ice core records, because as we show, the effect of temperature on d-excess is of similar magnitude to glacial interglacial variations in the cores.     

Thermal history of the unsaturated zone at Yucca Mountain,Nevada, USA

Secondary calcite, silica and minor amounts of fluorite deposited in fractures and cavities record the chemistry, temperatures, and timing of past fluid movement in the unsaturated zone at Yucca Mountain, Nevada, the proposed site of a high-level radioactive waste repository. The distribution and geochemistry of these deposits are consistent with low-temperature precipitation from meteoric waters that infiltrated at the surface and percolated down through the unsaturated zone. However, the discovery of fluid inclusions in calcite with homogenization temperatures (T_h) up to ∼80 °C was construed by some scientists as strong evidence for hydrothermal deposition. This paper reports the results of investigations to test the hypothesis of hydrothermal deposition and to determine the temperature and timing of secondary mineral deposition. Mineral precipitation temperatures in the unsaturated zone are estimated from calcite- and fluorite-hosted fluid inclusions and calcite δ¹⁸O values, and depositional timing is constrained by the ²⁰⁷Pb/²³⁵U ages of chalcedony or opal in the deposits. Fluid inclusion T_h from 50 samples of calcite and four samples of fluorite range from ∼35 to ∼90 °C. Calcite δ¹⁸O values range from ∼0 to ∼22‰ (SMOW) but most fall between 12 and 20‰. The highest T_h and the lowest δ¹⁸O values are found in the older calcite. Calcite T_h and δ¹⁸O values indicate that most calcite precipitated from water with δ¹⁸O values between −13 and −7‰, similar to modern meteoric waters.