Calculation of radiogenic 26Mg of CAI minerals under high precision isotope measurement by SIMS

High precision Mg isotope measurements have been performed to determine radiogenic 26Mg of CAI minerals by secondary ion mass spectrometry using Faraday cup multi-collection system. Terrestrial samples of spinel and augite, and synthetic glasses of melilite and fassaite, were prepared to correct instrumental mass fractionation. Reproducibility of Mg isotope measurements for each standard were limited to ∼0.4‰ (2σ). On the other hand, the standard errors for one spot are ∼0.05‰ (2σ). The poor reproducibility resulted from variations of instrumental mass fractionation among individual measurement spots. We propose novel calculation method of radiogenic 26Mg considering instrumental and natural mass fractionation for each mineral. The overall measurement error of radiogenic 26Mg of the minerals can be calculated less than 0.2‰ (2σ). This provides that the time resolution of early solar system chronometer was improved up to 100K year for ∼20 μm scale objects formed in the early solar system.