Gas Source Isotope Ratio Mass Research Articles

Barite, Anhydrite and Gypsum Reference Materials for In Situ Oxygen and Sulfur Isotope Ratio Measurements

Secondary ion mass spectrometry was used to test the δ18O and δ34S nanogram‐scale homogeneity of a suite of candidate sulfate minerals, ultimately selecting three barite, two anhydrite, and two gypsum samples from the Royal Ontario Museum that have repeatabilities for their SIMS measurements of better than ±0.39‰ and ±0.37‰ (1s) for oxygen and sulfur isotope ratios, respectively. Metrological splits of each of the seven materials were sent to multiple gas source isotope ratio mass spectrometry laboratories in order to establish their absolute 18O/16O and 34S/32S ratios. The inter‐laboratory results of GS‐IRMS analyses yielded reasonably narrow ranges in δ18OVSMOW, whereas larger variations in δ34SVCDT values were found between the results from the gas source laboratories. All samples have good reproducibility within laboratories of GS‐IRMS 103δ18O values of between ±0.24‰ and ±0.44‰ (1s). The reproducibility within laboratories of GS‐IRMS 103δ34S values range from ±0.07‰ to ±0.99‰ (1s). Here we also discuss some of the current analytical limitations affecting these isotope‐mineral systems. A total of 256 metrological splits have been prepared from each of these seven materials; these aliquots will be made available to the global geochemical community.

Analysis of intramolecular carbon isotope distributions in alanine by electrospray ionization Orbitrap mass spectrometry

The ability to detect intramolecular isotopic differences within a single molecule can answer questions about molecule synthesis and alteration across numerous scientific fields. Until recently, intramolecular (i.e., position-specific isotope analysis, PSIA) isotope measurements were laborious, requiring large amounts of analyte or specialized instrumentation. Orbitrap™ mass spectrometers are capable of fragmenting molecules and have the high mass resolution needed to constrain position-specific isotopic differences among the resulting fragment ions. Orbitrap mass spectrometers with electrospray ionization accurately measured the molecular average isotope composition of acetate, nitrate, sulfate, phosphate, and the amino acid methionine, as well as the position-specific isotopic structure of methionine. Here, we document the ability of this method to measure the position-specific carbon isotope structure of the amino acid alanine. Data include measurements of 13C-enriched materials to assign specific atoms in fragments to the original molecular structure and detect any recombination of atoms in resultant fragments. We further demonstrate high-precision intramolecular isotope analyses for standards with independently determined position-specific carbon isotope compositions. Isotope data from ESI-Orbitrap-MS agrees with values obtained using gas source isotope ratio mass spectrometry, giving further confidence to this novel approach to PSIA. The carbon isotope analyses by Orbitrap-MS were rapid and required ∼5 μg of analyte to obtain both molecular average and position-specific values in triplicate with precision ≤1‰.

New Potential Sphalerite, Chalcopyrite, Galena and Pyrite Reference Materials for Sulfur Isotope Determination by Laser Ablation‐MC‐ICP‐MS

The sulfur isotope system is important in tracing a variety of geochemical processes. However, the availability of matrix‐matched reference materials for in situ sulfur isotope ratio measurement is limited. This study analysed four sulfide specimens to investigate their potential as reference materials for in situ sulfur isotope determination, including three natural sulfide minerals (sphalerite SPH‐1, chalcopyrite GC‐1 and galena NWU‐GN), and one resin preserved pyrite powder (RPPY). These four sulfides were analysed independently in four laboratories using both bulk and microbeam analysis techniques, all of which gave equivalent results within uncertainty with comparable accuracy and precision. The sulfur isotopes of these four sulfides are homogeneous based on a large number of isotope determinations by solution nebulisation multi‐collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS) and laser ablation (LA) MC‐ICP‐MS. The mean δ34S values for SPH‐1, RPPY, GC‐1 and NWU‐GN determined by gas source isotope ratio mass spectrometry (GS‐IRMS) and solution nebulisation MC‐ICP‐MS are −7.13 ± 0.41‰ (2s), 3.66 ± 0.24‰ (2s), −0.65 ± 0.28‰ (2s), 28.21 ± 0.17‰ (2s) relative to the Vienna‐Canyon Diablo troilite (V‐CDT) reference value, respectively, and which are proposed as potential preferred values. Over 400 spot analyses performed on randomly selected fragments of each sulfide by LA‐MC‐ICP‐MS yielded highly consistent δ34S values with the preferred values. Thus, SPH‐1, GC‐1, NWU‐GN and RPPY are considered a set of candidate matrix‐matched sulfide reference materials for in situ sulfur isotope determination.

Subnanomolar Sensitive Stable Isotopic Determination in CO2 by Tunable Infrared Laser Absorption Spectroscopy.

Oxygen and carbon stable isotope ratios (18O/16O, 13C/12C, and 17O/16O) of CO2 have been crucial in helping us understand Earth and planetary systems. These ratios have also been used in medicine for the noninvasive diagnosis of diseases from exhaled breath and for quantifying biochemical or metabolic reactions and in determining the production area of agricultural products. The current method for measuring the stable isotope ratios of CO2 is primarily gas-source isotope ratio mass spectroscopy (IRMS). Due to the recent demand for isotopic microanalysis of carbonates and organic compounds, the sample size required for isotopic measurements has been reduced to approximately 2 nmol CO2 (equivalent to 0.2 μg CaCO3 and 24 ng carbon) by using high-precision IRMS. We report a novel method using tunable mid-infrared laser direct absorption spectroscopy (TILDAS) for sensitive measurements of 18O/16O and 13C/12C in subnanomolar CO2. This method can accurately measure 18O/16O and 13C/12C in CO2 with a repeatability of less than 0.03‰ (n = 28) in a range of 0.3 nmol (equivalent to 0.03 μg CaCO3 and 3.8 ng carbon) to 30 nmol. This is a sample size 1 order of magnitude smaller than currently available sensitive analytical techniques. In addition, the TILDAS system measures 17O/16O simultaneously with a repeatability of less than 0.06‰ (n = 28). Our method is a major advance in supersensitive CO2 stable isotopic analyses for various fields.

Inter‐laboratory Characterisation of Apatite Reference Materials for Oxygen Isotope Analysis and Associated Methodological Considerations

Here we report on the oxygen isotope compositions of four proposed apatite reference materials (chlorapatite MGMH#133648 and fluorapatite specimens MGMH#128441A, MZ‐TH and ES‐MM). The samples were initially screened for 18O/16O homogeneity using secondary ion mass spectrometry (SIMS) followed by δ18O determinations in six gas source isotope ratio mass spectrometry laboratories (GS‐IRMS) using a variety of analytical protocols for determining either phosphate‐bonded or “bulk” oxygen compositions. We also report preliminary δ17O and Δ’17O data, major and trace element compositions collected using EPMA, as well as CO32− and OH− contents in the apatite structure assessed using thermogravimetric analysis and infrared spectroscopy. The repeatability of our SIMS measurements was better than ± 0.25 ‰ (1s) for all four materials that cover a wide range of 103δ18O values between +5.8 and +21.7. The GS‐IRMS results show, however, a significant offset of 103δ18O values between the “phosphate” and “bulk” analyses that could not be correlated with chemical characteristics of the studied samples. Therefore, we provide two sets of working values specific to these two classes of analytical methodologies as well as current working values for SIMS data calibration.

Cassiterite oxygen isotopes in magmatic-hydrothermal systems: in situ microanalysis, fractionation factor, and applications

Tin and tungsten are important metals for the industrializing society. Deciphering the origin and evolution of hydrothermal fluids responsible for their formation is critical to underpin genetic models of ore formation. Traditional approaches obtain isotopic information mainly from bulk analysis of both ore and gangue minerals, or less frequently from in situ analysis of gangue minerals, which either bear inherited complexities and uncertainties or are indirect constraints. Hence, directly obtaining isotopic information from ore minerals such as cassiterite by in situ techniques is warranted. However, this has been hampered by challenges from both analytical and applicational aspects. In this study, we first demonstrate a lack of crystallographic orientation effects during cassiterite ion microprobe oxygen isotope analysis. Along with our newly developed matrix-matched reference material, the Yongde-Cst, which has a recommended δ18O value of 1.36 ± 0.16‰ (VSMOW) as defined by gas source isotope ratio mass spectrometry, in situ oxygen isotope analysis of cassiterite now is possible. We further refine the oxygen isotope fractionation (1000 ln α) for quartz-cassiterite by first-principles calculations, which is given by the equation of 1.259 × 106/T2 + 8.15 × 103/T − 4.72 (T is temperature in Kelvin). The 1000 ln α for quartz-cassiterite has a sensitive response to temperature, and makes cassiterite-quartz an excellent mineral pair in oxygen isotope thermometry, as described by the equation of T (℃) = 2427 × (δ18Oqtz − δ18Ocst)−0.4326 − 492.4. Using the well-established 1000 ln α of quartz-water, 1000 ln α of cassiterite-water is derived as 2.941 × 106/T2 − 11.45 × 103/T + 4.72 (T in Kelvin), which shows a weak response to temperature. This makes cassiterite an ideal mineral from which to derive δ18O of fluids as robust temperature estimates are no longer a prerequisite. We have applied oxygen isotope analysis to cassiterite samples from six Sn(-W) deposits in China. The results show considerable variability in δ18O values both within a single deposit and among studied deposits. Combining the δ18O of cassiterite samples and the equilibrium oxygen isotope fractionation, we find that the δ18O values of ore-forming fluids show a strong magmatic affinity with variable but mostly no to low degree involvements (~0-10%) of meteoric water, hence our results invite a reassessment on the extent and role of meteoric water in Sn-W mineralization. This study demonstrates that in situ oxygen isotope analysis of cassiterite is a promising tool to refine sources of ore-forming fluids, and to decode hydrothermal dynamics controlling tin and tungsten mineralization.

NJUCal‐1: A New Calcite Oxygen Isotope Reference Material for Microbeam Analysis

Calcite is a common mineral in various types of rocks and mineral deposits, and its oxygen isotopic compositions are widely used to reconstruct palaeotemperature and to trace rock genesis and ore‐forming processes. However, the availability of high‐quality matrix‐matched calcite reference materials for oxygen isotope microanalysis is limited. A new natural calcite specimen (NJUCal‐1) was analysed in this study to investigate its potential as a reference material for microbeam oxygen isotope measurement. Detailed textural and major elemental examinations reveal no internal zoning of ground grains. During six measurement sessions, a total of 463 spot analyses of oxygen isotope ratios were applied on ~ 200 randomly selected grains using secondary ion mass spectrometry, Nanjing. Taking all six sessions together, the spot‐to‐spot repeatability is 0.28‰ (2s), and the repeatability is better than 0.31‰ (2s) for individual sessions, indicating that the NJUCal‐1 calcite is quite homogeneous in its oxygen isotopic compositions and can be used as a new reference material for microbeam oxygen isotope measurement. The mean δ18OVPDB value determined by gas source isotope ratio mass spectrometry is −22.58 ± 0.16‰ (2s, n = 22), which is proposed as the recommended value for this potential reference material.

Revisiting apatite SIMS oxygen isotope analysis and Qinghu-AP reference material

Oxygen is one of the most abundant elements in the universe and ubiquitously participates in geological and biological processes which play a critical role in shaping and maintaining habitability of the Earth. Its isotope composition is a key parameter to decode these geological and biological processes. For example, oxygen isotope composition of biogenic and abiogenic apatite are ideal proxies to reconstruct sea surface temperature and to tackle elemental cycling respectively, in which accurate and precise apatite oxygen isotope analysis is a basic requirement. Our robust evaluation demonstrates a lack of crystallographic orientation effects for Secondary Ion Mass Spectrometry (SIMS) apatite oxygen isotope analysis (expressed as δ18O). Apatite grains separated from the Qinghu monzonite were extensively analyzed by SIMS in the past decade in our laboratory, and 2041 individual δ18O measurements yield a two-standard-deviation of 0.47‰ (per mil, V-SMOW), or 0.16‰ when these measurements were integrated into 195 sessions. Homogenous oxygen isotope composition of Qinghu apatite (Qinghu-AP) at the micrometer level makes it a suitable reference material with a recommended δ18O value of 5.59 ± 0.19‰, as measured by Gas Source Isotope Ratio Mass Spectrometry with further validation from oxygen isotope thermometry. For seven randomly selected IGG (Institute of Geology and Geophysics)-Durango apatite crystals, our SIMS measurements suggest that they also have homogenous δ18O values (9.70 ± 0.22‰, 2 SD, n = 241, using Qinghu-AP as standard) without the presence of inter-grain and intra-grain heterogeneity. We documented a Y-coordinate related artificial effect, with measured δ18O values gradually increasing by ~1‰ from bottom to top of the mount over a distance of ~1.2 cm. This artifact is most likely caused by the presence of a slope (imperfect sample preparation and loading) or a gradient of accelerating voltage (conductivity of holder and sample) on the sample surface along the Y direction, which leads to an imperfect centering of the trajectory of the secondary ions. This effect can be reduced through applying a DTFA (dynamic transfer field aperture)-X correction. The presence of position-specific oxygen isotope fractionation could cause an artificial bias up to −6.7‰ when analyzing carbonated hydroxyapatite, and can reconcile the offset (e.g., −0.9 to 3.3‰) between δ18O values of conodonts measured by SIMS and IRMS. When working on hydrous systems (e.g., ore deposits) and low temperature environment (e.g., conodonts), where apatite is known to be water-rich and could be carbonated, a rigorous evaluation on this artificial bias is warranted. Since position-specific oxygen isotope fractionation is a common phenomenon observed in a variety of minerals (e.g., goethite, tourmaline and muscovite), attention should be paid in future studies. Our study highlights that SIMS is capable of providing accurate and precise δ18O measurements at the ~0.2‰ level, but high-quality reference materials and rigorous evaluations of potential artificial effects (e.g., topography, position, matrix, orientation, sample holder and position-specific oxygen isotope fractionation) degrading analytical accuracy are prerequisites.

Influences of oxidation ability on precision in nitrogen isotope measurements of organic reference materials using elemental analysis-isotope ratio mass spectrometry.

The precision of the standard measurements of reference materials (RMs) is critical to normalize the raw δ15 N values of samples to the international scale. Because the measurement configuration is required to moderate the properties of both RMs and samples for obtaining an ideal measurement precision, it is important to determine the appropriate measurement configurations of RMs to achieve accurate nitrogen isotope compositions of samples. Therefore, it is critical to systematically examine RMs for optimizing the configurations and further improve the measurement precision. Gas source isotope ratio mass spectrometry (IRMS) coupled with an online elemental analyzer (EA) equipped with a single-quartz reactor was performed to analyze the nitrogen isotope compositions. Some adjustments were made as follows: (a) as the in-house working standard, urea was used to investigate the influences of combustion through moderating the different oxygen injections (0-20 mL) and sample delay times (10-12 s) and optimize the combustion conditions to enhance oxidation ability; (b) CO2 from the sample gas stream was removed to prevent interferences between CO2 and N2 ; (c) international RMs, including USGS40 (l-glutamic), IAEA600 (caffeine), and soil standard (B2153) with a low organic content, were systematically analyzed under the optimized configurations, and the precisions of the δ15 N values were further examined. Our results showed that sufficient oxygen should be injected to improve the sample combustion when analyzing δ15 N in natural samples such as soil or marine sediment with low organic content. In addition, the measurement precision of δ15 N was affected by the tailing of the CO2 peak from the gas chromatography column into the subsequent sample measurement if the EA is equipped with a single-quartz reactor column. Our adjustments can produce an optimized repeatability and accuracy of the δ15 N value, especially for RMs with low organic content, and the uncertainty of the measurements is improved to be better than 0.1‰ under optimized configurations. The analytical conditions such as the oxygen flow rate and injection time or sample introduction time into the EA need to be adjusted depending on the combustion conditions of the RMs and samples to obtain a reliable accuracy of measurement. We recommend that when analyzing δ15 N of natural samples such as soil or marine sediment samples with low organic content, more oxygen should be injected to improve the combustion of the samples. In addition, CO2 should be removed from the sample gas stream before being introduced into the isotope ratio mass spectrometer when the EA is equipped with a single-quartz reactor.

Instrumental mass fractionation during sulfur isotope analysis by secondary ion mass spectrometry in natural and synthetic glasses

Sulfur isotope ratios are among the most commonly studied isotope systems in geochemistry. While sulfur isotope ratio analyses of materials such as bulk rock samples, gases, and sulfide grains are routinely carried out, in-situ analyses of silicate glasses such as those formed in magmatic systems are relatively scarce in the literature. Despite a number of attempts in recent years to analyse sulfur isotope ratios in volcanic and experimental glasses by secondary ion mass spectrometry (SIMS), the effects of instrumental mass fractionation (IMF) during analysis remain poorly understood. In this study we use more than 600 sulfur isotope analyses of nine different glasses to characterise the matrix effects that arise during sulfur isotope analysis of glasses by SIMS. Samples were characterised for major element composition, sulfur content, and sulfur isotope ratios by independent methods. Our glasses contain between 500 and 3400 ppm sulfur and cover a wide compositional range, including low-silica basanite, rhyolite, and phonolite, allowing us to investigate composition-dependent IMF. We use SIMS in multi-collection mode with a Faraday cup/electron multiplier detector configuration to achieve uncertainty of 0.3‰ to 2‰ (2σ) on measured δ34S. At high sulfur content, the analytical error of our SIMS analyses is similar to that of bulk analytical methods, such as gas-source isotope ratio mass spectrometry. We find IMF causes an offset of −12‰ to +1‰ between bulk sulfur isotope ratios and those measured by SIMS. Instrumental mass fractionation correlates non-linearly with glass sulfur contents and with a multivariate regression model combining glass Al, Na, and K contents. Both ln(S) and Al-Na-K models are capable of predicting IMF with good accuracy: 84% (ln(S)) and 87% (Al-Na-K) of our analyses can be reproduced within 2σ combined analytical uncertainty after a correction for composition-dependent IMF is applied. The process driving IMF is challenging to identify. The non-linear correlation between glass S content and IMF in our dataset resembles previously documented correlation between glass H2O abundance and IMF during D/H ratio analyses by SIMS, and could be attributed to changes in 32S− and 34S− ion yields with changing S content and glass composition. However, a clear correlation between S ion yields and S content cannot be identified in our dataset. We speculate that accumulation of alkalis at the SIMS crater floor may be the principal driving force of composition-dependent IMF. Nonetheless, other currently unknown factors could also influence IMF observed during S isotope ratio analyses of glasses by SIMS. Our results demonstrate that the use of multiple, well-characterised standards with a wide compositional range is required to calibrate SIMS instruments prior to sulfur isotope analyses of unknown silicate glasses. Matrix effects related to glass Al-Na-K contents are of particular importance for felsic systems, where alkali and aluminium contents can vary considerably more than in mafic magmas.

Fractionation of stable oxygen and clumped isotopes during acid digestion of calcite in the presence of an external direct current electric field.

Phosphoric acid digestion of carbonate generates CO2 for stable oxygen and clumped isotope analysis using a gas source isotope ratio mass spectrometer. The initial step of the digestion reaction is protonation of calcite while the product CO2 equilibrates with the system allowing further exchange of isotopes to various extents depending on the nature of the acid digestion methods. An external electric field is introduced in the break seal method to demonstrate the role of the protonation reaction and the post-digestion isotopic exchanges in the final isotopic composition of product CO2 . An acid digestion experiment following the break seal method was conducted at a constant temperature of 25 ± 0.5°C in the presence of a uniform external electric field of 0.5 kV/cm within a specially fabricated corona chamber. Replicate samples of a calcite powder of a reference standard (MAR J1) were reacted for 24 h in the presence and absence of an external electric field for varying exposure times (6 to 24 h) and the evolved CO2 was analyzed using a dual-inlet MAT 253 isotope ratio mass spectrometer. The CO2 yield from the phosphoric acid digestion of MAR J1 calcite was 20% lower during the reaction in presence of an electric field for an exposure time of 24 h, while the corresponding δ18 O value and Δ47 composition were 0.3‰ and 0.1‰ less, respectively, than without any electric field. We documented the systematic control of oxygen and clumped isotope ratios in CO2 evolved from the carbonate acid digestion reaction with varying exposure time to the external electric field. We provide a new method involving use of an external electric field to manipulate the isotopic fractionation during the acid digestion reaction of calcite. The experimental observation enabled theoretical understanding of the reaction mechanism of carbonate with phosphoric acid which will be useful for stable and clumped isotope studies.

Sulfur isotope behavior during metamorphism and anatexis of Archean sedimentary rocks: A case study from the Ghost Lake batholith, Ontario, Canada

Recycling of surface-derived sulfur into the deep earth can impart distinct sulfur isotope signatures to magmas. The details of sulfur transfer from sedimentary rocks to magmas (and ultimately igneous rocks) through metamorphism and devolatilization and/or partial melting, however, is difficult to trace. To understand this process in detail we studied multiple-sulfur isotope compositions of sulfides in the Archean (c. 2685 Ma) Ghost Lake batholith (GLB) and its surrounding host metasedimentary rocks of the Superior Craton (Ontario, Canada) by high spatial resolution secondary ion mass spectrometry, complemented by high-precision gas source isotope ratio mass spectrometry measurements. The GLB comprises strongly peraluminous biotite+cordierite, biotite+muscovite, and muscovite+garnet+tourmaline granites to leucogranites, which are thought to represent the partial melts of surrounding metagreywackes and metapelites. The metasedimentary rocks display a range of metamorphic grades increasing from biotite-chlorite (280-380°C) at ∼5 km away from the GLB to sillimanite-K-feldspar grade (∼660°C) immediately adjacent to the batholith, thus providing a natural experiment to understand sulfur isotope variations from low- to high-grade Archean sedimentary rocks, as well as granites representative of their partial melts.We find that metasedimentary sulfide δ34S values increase with progressive metamorphism at most 2-3‰ (from −1‰ up to +1 to +2‰). An increase in δ34S values in pyrrhotite during prograde metamorphism can be explained through Rayleigh fractionation during pyrite desulfidation reactions. Pyrite from all but one of the granite samples preserve δ34S values similar to that of the high-grade metasedimentary rocks, indicating that partial melting did not result in significant fractionation of δ34S. The exception to this is one granite sample from a part of the batholith characterized by abundant metasedimentary inclusions. This sample contains pyrite with heterogeneous and low δ34S values (down to −16‰) which likely resulted from incomplete homogenization of sulfur between the granitic melt and metasedimentary inclusions. Small (several tenths of a permil), mostly positive Δ33S are observed in both the metasedimentary rocks and granites.Our results suggest that Archean strongly peraluminous granites could be a high-fidelity archive to quantify the bulk sulfur isotope composition of the Archean siliciclastic sediments. Further, our findings indicate that subduction of reduced sulfur-bearing sediments in the Archean with δ34S at or near 0‰ should result in release of sulfur-bearing fluids in the mantle wedge with similar values (within a few permil). S-MIF (if initially present in Archean surface material) may be preserved during this process. However, the absence of S-MIF in igneous rocks does not preclude assimilation of Archean sedimentary material as either S-MIF may not be originally present in the Archean sedimentary sulfur and/or homogenization or dilution could obscure any S-MIF originally present in assimilated Archean sediments.

A Potential New Chalcopyrite Reference Material for Secondary Ion Mass Spectrometry Sulfur Isotope Ratio Analysis

Chalcopyrite is an important sulfide mineral in many types of ore deposits, but matrix‐matched chalcopyrite reference materials for microanalysis are lacking. A new natural chalcopyrite‐bearing specimen (HTS4‐6) was analysed in this study to investigate its potential as a reference material for microbeam sulfur isotope ratio measurement. Detailed textural examination and major element determination showed that the HTS4‐6 chalcopyrite grains have no growth rim or zoning. A total of 607 sulfur isotope ratio spot measurements with secondary ion mass spectrometry (SIMS) conducted on the cruciform sections, and over 120 randomly selected grains yielded highly consistent sulfur isotope ratio. The intermediate measurement precision for four measurement sessions of the 34S/32S measurement results was better than 0.39‰ (2s). Randomly selected chalcopyrite grains of HTS4‐6 were further analysed by LA‐MC‐ICP‐MS, which gave a mean δ34S value of +0.58 ± 0.38‰ (2s, n = 95). The maximum variance (expressed as intermediate precision from SIMS and LA‐MC‐ICP‐MS measurements) is not worse than 0.39‰ (the SIMS value), indicating that HTS4‐6 chalcopyrite is a potential reference material for in situ microbeam sulfur isotope measurements. The mean δ34S value determined by gas source isotope ratio mass spectrometry (GS‐IRMS) is +0.63 ± 0.16‰ (2s, n = 23), consistent with that derived by LA‐MC‐ICP‐MS, and can represent the recommended value for this potential reference material.

Isotopic fractionation during acid digestion of calcite: A combined ab initio quantum chemical simulation and experimental study.

Carbonate clumped isotope analysis involves the reaction of carbonate minerals with phosphoric acid to release CO2 for measurement in a gas-source isotope ratio mass spectrometer. Although the clumped isotope proxy is based on the temperature dependence of 13 C-18 O bonding preference in the mineral lattice, which is captured in the product CO2 , there is limited information on the phosphoric acid reaction mechanism and the magnitude of clumped isotopic fractionation (mass 63 in CO3 2- to mass 47 in CO2 ) during the acid digestion. We studied the reaction mechanism for the phosphoric acid digestion of calcite using first-principles density functional theory. We identified the transition state structures for each reaction involving different isotopologues and used the corresponding vibrational frequencies in reduced partition function theory to estimate the Δ47 acid fractionation. Experimental Δ47 data were acquired by processing the sample CO2 gas through the dual-inlet peripheral of a ThermoFinnigan MAT253 isotope ratio mass spectrometer. We showed that the acid digestion reaction, which results in the formation of CO2 enriched with 13 C-18 O bonds, began with the protonation of calcium carbonate in the presence of water. Our simulations yielded a relationship between the Δ47 acid fractionation and reaction temperature as Δ47 = -0.30175 + 0.57700 × (105 /T2 ) - 0.10791 × (105 /T2 )2 , with T varying between 298.15 and 383.15 K. We propose a reaction mechanism that shows a higher slope (Δ47 acid fractionation vs. 1/T2 curve) for the phosphoric acid digestion of calcite than in previous studies. The theoretical estimates from the present and earlier studies encapsulate experimental observations from both "sealed vessel" and "common acid bath" acid digestion methods.

Determination of the triple oxygen and carbon isotopic composition of CO2 from atomic ion fragments formed in the ion source of the 253 Ultra high-resolution isotope ratio mass spectrometer.

RationaleDetermination of δ17O values directly from CO2 with traditional gas source isotope ratio mass spectrometry is not possible due to isobaric interference of 13C16O16O on 12C17O16O. The methods developed so far use either chemical conversion or isotope equilibration to determine the δ17O value of CO2. In addition, δ13C measurements require correction for the interference from 12C17O16O on 13C16O16O since it is not possible to resolve the two isotopologues.MethodsWe present a technique to determine the δ17O, δ18O and δ13C values of CO2 from the fragment ions that are formed upon electron ionization in the ion source of the Thermo Scientific 253 Ultra high‐resolution isotope ratio mass spectrometer (hereafter 253 Ultra). The new technique is compared with the CO2‐O2 exchange method and the 17O‐correction algorithm for δ17O and δ13C values, respectively.ResultsThe scale contractions for δ13C and δ18O values are slightly larger for fragment ion measurements than for molecular ion measurements. The δ17O and Δ17O values of CO2 can be measured on the 17O+ fragment with an internal error that is a factor 1–2 above the counting statistics limit. The ultimate precision depends on the signal intensity and on the total time that the 17O+ beam is monitored; a precision of 14 ppm (parts per million) (standard error of the mean) was achieved in 20 hours at the University of Göttingen. The Δ17O measurements with the O‐fragment method agree with the CO2‐O2 exchange method over a range of Δ17O values of −0.3 to +0.7‰.ConclusionsIsotope measurements on atom fragment ions of CO2 can be used as an alternative method to determine the carbon and oxygen isotopic composition of CO2 without chemical processing or corrections for mass interferences.

High-Precision Simultaneous 18O/16O, 13C/12C, and 17O/16O Analyses for Microgram Quantities of CaCO3 by Tunable Infrared Laser Absorption Spectroscopy.

Stable isotope ratios (18O/16O, 13C/12C, and 17O/16O) in carbonates have contributed greatly to the understanding of Earth and planetary systems, climates, and history. The current method for measuring isotopologues of CO2 derived from CaCO3 is primarily gas-source isotope ratio mass spectroscopy (IRMS). However, IRMS has drawbacks, such as mass overlap by multiple CO2 isotopologues and contaminants, the requirement of careful sample purification, and the use of major instrumentation needing permanent installation and a high power electrical supply. Here, we report simultaneous 18O/16O, 13C/12C, and 17O/16O analyses for microgram quantities of CaCO3 using a tunable mid-infrared laser absorption spectroscopy (TILDAS) system, which has no mass overlap problem and yields high sensitivity/precision measurements on small samples, as small as 0.02 μmol of CO2 (equivalent to 2 μg of CaCO3) with standard errors of less than 0.08 ‰ for 18O/16O and 13C/12C (±0.136 ‰ and ±0.387 ‰ repeatability; n = 10). In larger samples of CO2, 0.68 μmol (or 68 μg of CaCO3), standard error is less than 0.04 ‰ for 18O/16O and 13C/12C (< ±0.1 ‰ repeatability; n = 10) and 0.03 ‰ for 17O/16O (±0.069 ‰ repeatability; n = 10). We also show, for the first time, the relationship between 17O/16O ratios measured using the TILDAS system and published δ17O values of international standard materials (NBS-18 and -19) measured by IRMS. The benchtop TILDAS system, with cryogen-free sample preparation vacuum lines for microgram quantities of carbonates, is therefore a significant advance in carbonate stable isotope ratio geochemistry and is a new alternative to conventional IRMS.

Sulfur isotope signatures in the lower crust: A SIMS study on S-rich scapolite of granulites

Scapolite is an important reservoir for volatiles in the deep crust and provides unique insights into the S isotope signatures at the mantle/crust interface. Here we document the first scapolite reference material (herein referred to as CB1) for in situ S isotope analysis. The chemical and isotopic composition of this euhedral, S-rich scapolite megacryst was characterized via LA-ICP-MS, EPMA, SIMS, and bulk fluorination gas source isotope ratio mass spectrometry. The CB1 scapolite is isotopically homogeneous and our results show that crystal orientation does not affect in situ S isotope SIMS analysis. This makes CB1 an ideal primary calibration standard for in situ analysis of S isotope ratios (36S/32S, 34S/32S and 33S/32S) in scapolite. With this reference material in hand, we then applied in situ SIMS analysis of S isotopes for the first time on scapolite in granulite samples from the lower crust/upper mantle. The analysed sample suite comprises rocks from classic granulite xenolith locations in southeastern Australia, as well as a sample from the high-grade suture zone of the Dahomeyides in south-eastern Ghana. The results show that scapolites in the lower crust have δ34S values between ~−0.5 and +4 (‰ VCDT). These values fall within the range of S isotope signatures present in mantle rocks and provide no evidence for the recycling of seawater-derived S into the lower crust. We propose that scapolite formed during granulite facies metamorphism of igneous cumulates, where S was sourced from precursor igneous sulfides. Sulfur isotope heterogeneities between individual scapolite grains in some of the studied samples may reflect non-uniform S-isotope compositions of igneous S-phases, which precipitated from mantle-derived melt.

Measurement of rare isotopologues of nitrous oxide by high-resolution multi-collector mass spectrometry.

Bulk and position-specific stable isotope characterization of nitrous oxide represents one of the most powerful tools for identifying its environmental sources and sinks. Constraining (14) N(15) N(18) O and (15) N(14) N(18) O will add two new dimensions to our ability to uniquely fingerprint N2 O sources. We describe a technique to measure six singly and doubly substituted isotopic variants of N2 O, constraining the values of δ(15) N, δ(18) O, ∆(17) O, (15) N site preference, and the clumped isotopomers (14) N(15) N(18) O and (15) N(14) N(18) O. The technique uses a Thermo MAT 253 Ultra, a high-resolution multi-collector gas source isotope ratio mass spectrometer. It requires 8-10 hours per sample and ~10 micromoles or more of pure N2 O. We demonstrate the precision and accuracy of these measurements by analyzing N2 O brought to equilibrium in its position-specific and clumped isotopic composition by heating in the presence of a catalyst. Finally, an illustrative analysis of biogenic N2 O from a denitrifying bacterium suggests that its clumped isotopic composition is controlled by kinetic isotope effects in N2 O production. We developed a method for measuring six isotopic variants of N2 O and tested it with analyses of biogenic N2 O. The added isotopic constraints provided by these measurements will enhance our ability to apportion N2 O sources.

Chemical and oxygen isotope composition of gem-quality apatites: Implications for oxygen isotope reference materials for secondary ion mass spectrometry (SIMS)

Oxygen isotope ratios (δ18O) of biogenic apatite have become a widely used tool for reconstructing palaeoenvironmental conditions in the past. Ongoing improvements in secondary ion mass spectrometry (SIMS) technology have made in situ δ18O analyses on sub-nanogram domains within single microfossil samples possible; however this method requires calibration with reference materials (RMs) with a matrix “similar” to that of the samples. Here we evaluated five sources of gem-quality, abiogenic apatites to assess their potentials as SIMS RMs. Our results show that all these gem-quality apatites are low-REEs calcium fluorapatites with δ18OVSMOW values between 6.6 and 11.4‰. Large variations in δ18O have been found for between crystals from a single deposit as well as within individual crystals. Durango apatite has an inter-crystal δ18O range of 4.4‰ (6.6–11.0‰, N=9 crystals). Madagascar Green apatite, Madagascar 1st Mine apatite and Ipira apatite have inter-crystal variations in δ18O of 4.1‰ (7.3–11.4‰, N=15 crystals), 3.5‰ (7.6–11.1‰, N=9 crystals) and 3.1‰ (7.1–10.2‰, N=11 crystals), respectively. South Africa Blue apatite has a smaller inter-crystal δ18O range of only 0.9‰ (8.7–9.6‰, N=6 crystals), though this might be an artefact due to the restricted number of samples studied. Intra-crystal δ18O variations of studied crystals generally range from 0.8 to 1.8‰. However, several gem apatite crystals from Madagascar have minor δ18O variation within 0.5‰ and represent most promising candidate RM.Gem-quality abiogenic apatites can be used as RMs for calibrating SIMS δ18O analyses, however these require homogeneity testing at sub-millimetre scale in advance. Durango crystals, commonly used as RMs in earlier SIMS-based studies, display a conspicuous heterogeneity in δ18O, with intra-crystal variations ranging from 0.7 to 2‰ as detected by both gas source isotope ratio mass spectrometry and large geometry SIMS. Thus, Durango apatite cannot be considered as suitable for SIMS calibration and alternative reference materials need to be sought.

Analysis of the site-specific carbon isotope composition of propane by gas source isotope ratio mass spectrometer

Site-specific isotope ratio measurements potentially provide valuable information about the formation and degradation of complex molecules—information that is lost in conventional bulk isotopic measurements. Here we discuss the background and possible applications of such measurements, and present a technique for studying the site-specific carbon isotope composition of propane at natural abundance based on mass spectrometric analysis of the intact propane molecule and its fragment ions. We demonstrate the feasibility of this approach through measurements of mixtures of natural propane and propane synthesized with site-specific 13C enrichment, and we document the limits of precision of our technique. We show that mass balance calculations of the bulk δ13C of propane based on our site-specific measurements is generally consistent with independent constraints on bulk δ13C. We further demonstrate the accuracy of the technique, and illustrate one of its simpler applications by documenting the site-specific carbon isotope signature associated with gas phase diffusion of propane, confirming that our measurements conform to the predictions of the kinetic theory of gases. This method can be applied to propane samples of moderate size (tens of micromoles) isolated from natural gases. Thus, it provides a means of studying the site-specific stable isotope systematics of propane at natural isotope abundances on sample sizes that are readily recovered from many natural environments. This method may also serve as a model for future techniques that apply high-resolution mass spectrometry to study the site-specific isotopic distributions of larger organic molecules, with potential applications to biosynthesis, forensics and other geochemical subjects.