High precision Mg isotopic measurement of CAI minerals with low Al/Mg ratio utilizing NanoSIMS 50L: Implications to thermal history of CAIs

Abstract

We report high-precision Mg isotopic measurements in melilite, spinel and fassaite in an Allende CAI by a NanoSIMS 50L with 4 Faraday cups in meltidetection in order to evaluate the chronology and thermal history of CAIs in the early solar system. We found an initial 26Al/27Al ratio of 4.67 (± 0.22, 2σ) x 10-5 and δ26Mg intercept of 0.18 (± 0.05, 2σ) ⁰/₀₀. We calculated an initial cooling rate for the CAI at 1450C using Mg diffusion kinetics in spinel. An initial cooling rate of at least 0.5 to 11C/h at 1450C is required to preserve the original 26Mg* in spinel through the formation process.