Article

Mass-independent fractionation of oxygen isotopes during thermal decomposition of carbonates.

Planetary and Space Sciences Research Institute, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom.
Proceedings of the National Academy of Sciences (Impact Factor: 9.81). 09/2002; 99(17):10988-93. DOI: 10.1073/pnas.172378499
Source: PubMed

ABSTRACT Nearly all chemical processes fractionate 17O and 18O in a mass-dependent way relative to 16O, a major exception being the formation of ozone from diatomic oxygen in the presence of UV radiation or electrical discharge. Investigation of oxygen three-isotope behavior during thermal decomposition of naturally occurring carbonates of calcium and magnesium in vacuo has revealed that, surprisingly, anomalous isotopic compositions are also generated during this process. High-precision measurements of the attendant three-isotope fractionation line, and consequently the magnitude of the isotopic anomaly (delta17O), demonstrate that the slope of the line is independent of the nature of the carbonate but is controlled by empirical factors relating to the decomposition procedure. For a slope identical to that describing terrestrial silicates and waters (0.5247 +/- 0.0007 at the 95% confidence level), solid oxides formed during carbonate pyrolysis fit a parallel line offset by -0.241 +/- 0.042 per thousand. The corresponding CO2 is characterized by a positive offset of half this magnitude, confirming the mass-independent nature of the fractionation. Slow, protracted thermolysis produces a fractionation line of shallower slope (0.5198 +/- 0.0007). These findings of a 17O anomaly being generated from a solid, and solely by thermal means, provide a further challenge to current understanding of the nature of mass-independent isotopic fractionation.

0 Bookmarks
 · 
106 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present petrologic and Secondary Ion Mass Spectrometry (SIMS) oxygen isotope analyses of Ca-carbonate within a group of paired Antarctic CM2 chondrites. The carbonates can be grouped into two isotopically and morphologically distinct populations. Type 1 grains (small matrix grains) possess average δ18O of 33.7±2.3‰ (1σ) and average Δ17O of −0.81‰±0.90‰ (1σ). Type 2 grains (calcite aggregates) possess distinct oxygen isotopic compositions, average δ18O of 19.4‰±1.5‰ (1σ) and average Δ17O of −1.98±0.9‰ (1σ). These differences are interpreted to indicate that the two populations of calcite formed under different conditions at different times. The carbonates have textural features that suggest an extraterrestrial origin. The data presented here fall within error of a previously measured array for carbonates from CM falls (Benedix et al., 2003). The presence of two generations of carbonate suggests carbonate formation in two discrete events on the parent body of these meteorites. The oxygen isotopic data presented here deviate from prior bulk carbonate measurements undertaken for these meteorites. Most likely, this deviation is because bulk carbonate analyses included vein carbonate which formed during terrestrial weathering.
    Geochimica et Cosmochimica Acta 01/2012; · 4.25 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Oxygen triple isotope measurements can be used to calculate aquatic gross oxygen production rates. Past studies have emphasised the appropriate definition of the 17O excess and often used an approximation to derive production rates from the 17O excess. Here, I show that the calculation can be phrased more consistently and without any approximations using the relative 17O/16O and 18O/16O isotope ratio differences directly. The 17O excess is merely a mathematical construct and the derived production rate is independent of its definition, provided all calculations are performed with a consistent definition. I focus on the mixed layer, but also show how time series of triple oxygen measurements below the mixed layer can be used to derive gross production. In the calculation of mixed layer productivity, I explicitly include isotopic fractionation during gas invasion and evasion, which requires the oxygen supersaturation s to be measured as well. I also suggest how bubble injection could be considered in the same mathematical framework. I distinguish between concentration steady state and isotopic steady state and show that only the latter needs to be assumed in the calculation. It is even possible to derive an estimate of the net production rate in the mixed layer that is independent of the assumption of concentration steady state. I review measurements of the parameters required for the calculation of gross production rates and show how their systematic uncertainties as well as the use of different published calculation methods can cause large variations in the production rates for the same underlying isotope ratios. In particular, the 17O excess of dissolved O2 in equilibrium with atmospheric O2 and the 17O excess of photosynthetic O2 need to be re-measured. Because of these uncertainties, all calculation parameters should always be fully documented and the measured isotope ratio differences as well as the oxygen supersaturation should be permanently archived, so that improved measurements of the calculation parameters can be used to retrospectively improve production rates.
    Biogeosciences Discussions 01/2011; 8(2).
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report experimental observations of the vapor pressure isotope effect, including 33S/32S and 34S/32S ratios, for SF6 ice between 137 and 173 K. The temporal evolution of observed fractionations, mass-balance of reactants and products, and reversal of the fractionation at one temperature (155 K) are consistent with a subset of our experiments having reached or closely approached thermodynamic equilibrium. That equilibrium involves a reversed vapor pressure isotope effect; i.e., vapor is between 2‰ and 3‰ higher in 34S/32S than co-existing ice, with the difference increasing with decreasing temperature. At the explored temperatures, the apparent equilibrium fractionation of 33S/32S ratios is 0.551 ± 0.010 times that for 34S/32S ratios—higher than the canonical ratio expected for mass dependent thermodynamic fractionations (∼0.515). Two experiments examining exchange between adsorbed and vapor SF6 suggest the sorbate–vapor fractionation at 180–188 K is similar to that for ice–vapor at ∼150 K. In contrast, the liquid–vapor fractionation at 228–300 K is negligibly small (∼0.1‰ for 34S/32S; the mass law is ill defined due to the low amplitude of fractionation). We hypothesize that the observed vapor pressure isotope for SF6 ice and sorbate is controlled by commonly understood effects of isotopic substitution on vibrational energies of molecules, but leads to both an exotic mass law and reversed fractionation due to the competition between isotope effects on intramolecular vibrations, which promote heavy isotope enrichment in vapor, and isotope effects on intermolecular (lattice) vibrations, which promote heavy isotope enrichment in ice. This explanation implies that a variety of naturally important compounds having diverse modes of vibration (i.e., varying greatly in frequency and particularly, reduced mass) could potentially exhibit similarly non-canonical mass laws for S and O isotope fractionations. We examined this hypothesis using a density function model of SF6 vapor and lattice dynamic model of SF6(ice). These models support the direction of the measured vapor pressure isotope effect, but do not quantitatively agree with the magnitude of the fractionation and poorly match the phonon spectrum of SF6 ice. A strict test of our hypothesis must await a more sophisticated model of the isotopic dependence of the phonon spectrum of SF6 ice.
    Geochimica et Cosmochimica Acta 04/2013; 107:205–219. · 4.25 Impact Factor

Full-text (2 Sources)

Download
53 Downloads
Available from
May 30, 2014