Isotopic mass fractionation laws for magnesium and their effects on 26Al–26Mg systematics in solar system materials. (1st June 2015)

• Record Type: Journal Article
• Authors: Davis, Andrew M.; Richter, Frank M.; Mendybaev, Ruslan A.; Janney, Philip E.; Wadhwa, Meenakshi; McKeegan, Kevin D.
• Is Part Of: Geochimica et cosmochimica acta. Volume 158(2015:Jun. 01)
• Page Start: 245
• Page End: 261
• Publication Date: 2015-06-01
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2015.01.034
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Magnesium isotope ratios are known to vary in solar system objects due to the effects of 26 Al decay to 26 Mg and mass-dependent fractionation, but anomalies of nucleosynthetic origin must also be considered. In order to infer the amount of enhancement of 26 Mg/ 24 Mg due to 26 Al decay or to resolve small nucleogenetic anomalies, the exact relationship between 26 Mg/ 24 Mg and 25 Mg/ 24 Mg ratios due to mass-dependent fractionation, the mass-fractionation "law", must be accurately known so that the 25 Mg/ 24 Mg ratio can be used to correct the 26 Mg/ 24 Mg 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 25 Mg/ 24 Mg vs. log 26 Mg/ 24 Mg 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 26 Al– 26 Mg chronology.