Читать книгу Isotopic Constraints on Earth System Processes - Группа авторов - Страница 26

The earliest report of measurable differences in the mobility of isotopes of a major element in a silicate liquid was reported by Richter et al. (1999) based on the results of a diffusion experiment involving an isothermal and isochemical CaO‐Al₂O₃‐SiO₂ melt with an initial step with different concentration of ⁴⁸Ca and ⁴⁰Ca but with the same isotopic ratio of ⁴⁸Ca/⁴⁰Ca on both sides of the step. When annealed, this setup would cause ⁴⁸Ca and ⁴⁰Ca to race each other as they diffused into the low concentration side of the couple. A well‐resolved isotopic fractionation in the glass recovered from this experiment showed that ⁴⁰Ca had diffused measurably faster than ⁴⁸Ca. The relative mobility of the calcium isotopes was reported as D₄₈ /D₄₀ = (40/48) ^β with β = 0.075 ± 0.025. Subsequent laboratory experiments by Richter et al. (2003; 2008; 2009b; 2014a) addressed the question of whether the relative mobility of isotopes seen in isochemical experiments would also arise in molten systems where chemical potential gradients are present and where there can be significant coupling between the various diffusing components of the melt.