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.

很少有物理或化学过程违反质量相关的同位素分馏的公认规律。二十年前发现的一个令人惊讶的例子是，碳酸钙和碳酸镁矿物的热分解（在真空中进行，以最大限度地减少逆反应和同位素交换）导致所得固体氧化物和 CO ₂的氧三同位素组成适合在 ln(1 + δ ¹⁷ O) 与 ln(1 + δ ¹⁸ O) 空间的平行质量相关分馏线上，固体中¹⁷ O异常消耗，CO ₂中¹⁷ O等量富集. 通过研究其他天然二价金属碳酸盐的热分解和一个合成实例，在类似条件下，我们发现异常同位素效应在所有情况下都会发生，并且异常 (Δ' ¹⁷ O) 的大小似乎取决于房间碳酸盐的温度晶体结构。较低的阳离子配位数（与较小的阳离子半径相关联）与更接近于零的Δ' ¹⁷ O值相关。因此，局部对称性考虑可能有影响。相对于斜率为 0.524 并通过 VSMOW 的参考分馏线，斜方碳酸盐热分解产生的固体氧化物的特征为 Δ' ¹⁷O = -0.367 ± 0.004‰（标准误差）。菱面体样品的可比数字为 -0.317 ± 0.010‰，而来自唯一单斜（合成）样品的为-0.219 ± 0.011‰。数值在某种程度上取决于实验程序的细节。我们讨论了同位素异常的潜在起源，包括¹⁷ O 核与瞬态自由基的未配对电子之间超精细耦合的可能性（“磁同位素效应”）。提出了一种基于后一种过程的新机制。相关的过渡态与最近方解石和菱镁矿热分解的量子化学和动力学研究表明的相一致。较早的基于磁同位素效应的建议被证明与磁同位素的产生不相容¹⁷ O 异常，与碳酸盐的身份无关。在某些情况下，我们不能排除导致解离的状态之间的费米共振可能另外影响 Δ' ¹⁷ O大小的可能性。我们的发现具有宇宙化学意义，碳酸盐的热处理为原行星系统中与质量无关的氧同位素分馏提供了潜在的机制。