We present new experimental results of Cr isotope fractionation during degassing of silicate melts. Our experiments at oxidizing conditions (in air) at 1 bar total pressure show that evaporative loss of Cr is substantial and depends on run duration and on temperature (between 1350 and 1500 °C). The stable Cr (δ⁵³Cr) isotope compositions of the evaporation residues were analyzed by thermal ionization mass spectrometry using the double spike method. Our results indicate that during degassing the lighter isotope ⁵²Cr is enriched in the gas phase and consequently the residual melt becomes enriched in the heavier ⁵³Cr isotope in a mass-dependent fashion. The kinetic isotopic fractionation factor, α, derived from our experimental data is 0.9995 ± 0.0001, which is significantly different from the fractionation factor predicted for an ideal gas composed of CrO₃, α_kin = 0.995. Our experimentally-determined value of α is consistent with diffusive transport of CrO₃ (g) away from the melt surface, proportional to the inverse cube-root of the reduced masses of the two isotopologues. We hypothesize that Cr loss by evaporation may be recorded by Cr isotopes in terrestrial magmas with sufficiently high oxygen fugacities erupting near the surface, which are plausibly found in lava lakes or erupting arc magmas.

我们提出了在硅酸盐熔体脱气过程中 Cr 同位素分馏的新实验结果。我们在总压力为 1 bar 的氧化条件下（空气中）的实验表明，Cr 的蒸发损失很大，并且取决于运行时间和温度（1350 到 1500 °C 之间）。采用双尖峰法通过热电离质谱分析蒸发残渣的稳定Cr( δ53Cr )^{同位素组成。}我们的结果表明，在脱气过程中，较轻的同位素⁵² Cr 在气相中富集，因此残余熔体在较重的^{53中富集。}Cr同位素以质量依赖的方式。_{从我们的实验数据得出的动力学同位素分馏因子 α 为 0.9995 ± 0.0001，这与为由 CrO 3}组成的理想气体预测的分馏因子α _kin  = 0.995 显着不同。我们通过实验确定的 α 值与 CrO ₃ (g) 远离熔体表面的扩散传输一致，与两种同位素体的减少质量的立方根倒数成正比。我们假设蒸发造成的 Cr 损失可能由在地表附近喷发的具有足够高氧逸度的陆地岩浆中的 Cr 同位素记录，这在熔岩湖或喷发的弧形岩浆中很可能发现。