Isotopic mass fractionation laws for magnesium and their effects on ²⁶Al–²⁶Mg systematics in solar system materials

Magnesium isotope ratios are known to vary in solar system objects due to the effects of ²⁶Al decay to ²⁶Mg and mass-dependent fractionation, but anomalies of nucleosynthetic origin must also be considered. In order to infer the amount of enhancement of ²⁶Mg/²⁴Mg due to ²⁶Al decay or to resolve small nucleogenetic anomalies, the exact relationship between ²⁶Mg/²⁴Mg and ²⁵Mg/²⁴Mg ratios due to mass-dependent fractionation, the mass-fractionation “law” must be accurately known so that the ²⁵Mg/²⁴Mg ratio can be used to correct the ²⁶Mg/²⁴Mg ratio for mass fractionation. Mass-dependent fractionation in mass spectrometers is reasonably well characterized, but not necessarily fully understood. It follows a simple power fractionation law, sometimes referred to as the “exponential law”. In contrast, mass fractionation in nature, in particular that due to high temperature evaporation that likely caused the relatively large effects observed in calcium-, aluminum-rich inclusions (CAIs), is reasonably well understood, but mass-fractionation laws for magnesium have not been explored in detail. The magnesium isotopic compositions of CAI-like evaporation residues produced in a vacuum furnace indicate that the slope on a log ²⁵Mg/²⁴Mg vs. log ²⁶Mg/²⁴Mg plot is ∼0.5128, and different from those predicted by any of the commonly used mass-fractionation laws. Evaporation experiments on forsterite-rich bulk compositions give exactly the same slope, indicating that the measured mass-fractionation law for evaporation of magnesium is applicable to a wide range of bulk compositions. We discuss mass-fractionation laws and the implications of the measured fractionation behavior of magnesium isotopes for ²⁶Al–²⁶Mg chronology.