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

Department of Chemistry and Biochemistry, University of California, San Diego, San Diego, California, United States
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 09/2002; 99(17):10988-93. DOI: 10.1073/pnas.172378499
Source: PubMed


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.

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    • "This line Fig. 4. Three-isotope plot (d 18 O vs. d 17 O) relative to Standard Mean Ocean Water (SMOW) of analyzed Ca-carbonate. TFL refers to the Terrestrial Fractionation Line (Miller et al., 2002). "
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    • "Other factors may be oxygen isotope exchange reactions between CO and CaO [67] or between CO 2 and CaO [68]. In case of a non-quantitative reaction of the carbonates, a recently reported mass-independent oxygen isotope fractionation during the thermal decomposition of carbonates [69] "
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