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 + delta O-17) versus ln(1 + delta O-18) space, with anomalous depletion of O-17 in the solid and equivalent enrichment of O-17 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 (Delta'O-17) 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 Delta.' O-17 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 Delta'O-17 = -0.367 +/- 0.004 parts per thousand (standard error). The comparable figure from rhombohedral examples was -0.317 +/- 0.010 parts per thousand, whereas from the sole monoclinic (synthesised) specimen it was -0.219 +/- 0.011 parts per thousand. 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 O-17 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 O-17 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 Delta'O-17 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.

Automated summary

The study reveals that thermal decomposition of carbonates with different crystal structures can lead to anomalous oxygen isotope effects, and the specific values are influenced by the crystal structure. The results also suggest that the generation of oxygen isotope anomalies may be related to hyperfine coupling of unpaired electrons.