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

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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. In particular, 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.