An effect of variations in relative sensitivity factors on Al-Mg systematics of Ca-Al-rich inclusions in meteorites with secondary ion mass spectrometry

Abstract

Instrumental mass fractionation of Mg-isotopes and the relative sensitivity factor (RSF) for 27Al/24Mg ratios for in situ analysis by secondary ion mass spectrometry were investigated for minerals in Ca-Al-rich inclusions (CAIs) of meteorites, to verify systematic errors in Al-Mg chronological systematics of CAIs. We synthesized seventeen glasses with different chemical compositions that imitate those for CAI minerals and measured their Al-Mg isotopic compositions. Inparticular, the variation range of RSFs for six melilite glasses almost covering chemical compositions of CAI melilite is 2.0 ± 0.3%, indicating that systematic error for 27Al/24Mg for melilite is less than 2.0 ± 0.3% and likely corresponds to that of an 26Al-26Mg relative age of ~0.01 Myr for melilite-rich CAIs. Our data strongly support the robustness of variations in initial 26Al/27Al ratios among CAIs, corresponding to a formation age spread of ~0.4 Myr at the very beginning of the Solar System formation.