Mass-independent fractionation of oxygen isotopes during thermal decomposition of divalent metal carbonates: Crystallographic influence, potential mechanism and cosmochemical significance

Few physical or chemical processes defy well-established laws of mass-dependent isotopic fractionation. A surprising example, discovered two decades ago, is that thermal decomposition of calcium and magnesium carbonate minerals (conducted in vacuo, to minimise back-reaction and isotopic exchange) causes the oxygen triple-isotope compositions of the resulting solid oxide and CO2 to fit on parallel mass-dependent fractionation lines in ln(1 + δ17O) versus ln(1 + δ18O) space, with anomalous depletion of 17O in the solid and equivalent enrichment of 17O in the CO2. By investigating the thermal decomposition of other natural divalent metal carbonates and one synthetic example, under similar conditions, we find that the unusual isotope effect occurs in all cases and that the magnitude of the anomaly (Δ′17O) seems to depend on the room temperature crystallographic structure of the carbonate. A lower cation coordination number (as associated with smaller cation radius) correlates with a Δ′17O value closer to zero. Local symmetry considerations may therefore be influential. Relative to a reference fractionation line of slope 0.524 and passing through VSMOW, solid oxides produced by thermal decomposition of orthorhombic carbonates were characterised by Δ′17O = −0.367 ± 0.004‰ (standard error). The comparable figure from rhombohedral examples was −0.317 ± 0.010‰, whereas from the sole monoclinic (synthesised) specimen it was −0.219 ± 0.011‰. The numerical values are, to some extent, dependent on details of the experimental procedure. We discuss potential origins of the isotopic anomaly, including the possibility of hyperfine coupling between 17O nuclei and unpaired electrons of transient radicals (the ‘magnetic isotope effect’). A new mechanism based on the latter process is proposed. The associated transition state is compatible with that suggested by recent quantum chemical and kinetic studies of the thermal decompositions of calcite and magnesite. An earlier suggestion based on the magnetic isotope effect is shown to be incompatible with the generation of a 17O anomaly, regardless of the identity of the carbonate. We cannot exclude the possibility that a Fermi resonance between states leading to dissociation may additionally affect the magnitude of Δ′17O in some cases. Our findings have cosmochemical implications, with thermal processing of carbonates providing a potential mechanism for the mass-independent fractionation of oxygen isotopes in protoplanetary systems

Carbon and oxygen isotopic fractionation in the products of low-temperature VUV photodissociation of carbon monoxide

The carbon and oxygen isotope fractionations occurring during photodissociation of carbon monoxide (CO) by vacuum ultraviolet photons (91–107 nm) at 80 K were measured and the isotopic fractionations due to direct and indirect predissociation processes individually quantified. The isotopic fractionations depend on the photodissociation wavelength. Slope values (δ'17O/δ'18O) in oxygen three-isotope space range from 0.75 to 1.1. The isotopic composition of the products depends on the dissociation dynamics at the upper electronic state (perturbation and coupling associated with that state), which in turn modulates the isotope effect (in this case an enrichment of minor isotopes) inside the gas column due to the saturation of major isotopologue (isotope self-shielding). An explanation in terms of isotope self-shielding would require a quantum yield of one for photodissociation of all isotopologues which is not consistent with the data

Nitrogen isotopic fractionations in the low temperature (80 K) vacuum ultraviolet photodissociation of N2

N2 is a diatomic molecule with complex electronic structure. Interstate crossings are prominent in the high energy domain, introducing significant perturbations to the system. Nitrogen mainly photodissociates in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum through both direct and indirect predissociation. Due to the complexity introduced by these perturbations, the nitrogen isotopic fractionation in N2 photodissociation is extremely hard to calculate, and an experimental approach is required. Here we present new data of N-isotopic fractionation in N2 photodissociation at low temperature (80 K), which shows a distinctly different 15N enrichment profile compared to that at relatively higher temperatures (200 and 300 K). The new data, important to understanding the N-isotopic compositions measured in meteorites and other planetary bodies, are discussed in light of the knowledge of N2 photochemistry and calculated photoabsorption cross sections in the VUV

Model for quantitative tip-enhanced spectroscopy and the extraction of nanoscale-resolved optical constants

Near-field infrared spectroscopy by elastic scattering of light from a probe tip resolves optical contrasts in materials at dramatically subwavelength scales across a broad energy range, with the demonstrated capacity for chemical identification at the nanoscale. However, current models of probe-sample near-field interactions still cannot provide a sufficiently quantitatively interpretation of measured near-field contrasts, especially in the case of materials supporting strong surface phonons. We present a model of near-field spectroscopy derived from basic principles and verified by finite-element simulations, demonstrating superb predictive agreement both with tunable quantum cascade laser near-field spectroscopy of SiO2 thin films and with newly presented nanoscale Fourier transform infrared (nanoFTIR) spectroscopy of crystalline SiC. We discuss the role of probe geometry, field retardation, and surface mode dispersion in shaping the measured near-field response. This treatment enables a route to quantitatively determine nanoresolved optical constants, as we demonstrate by inverting newly presented nanoFTIR spectra of an SiO2 thin film into the frequency dependent dielectric function of its mid-infrared optical phonon. Our formalism further enables tip-enhanced spectroscopy as a potent diagnostic tool for quantitative nanoscale spectroscopy. © 2014 American Physical Society

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

Rationale: Determination 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. Methods: We 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. Results: The 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‰. Conclusions: Isotope 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.</p