High precision Mg isotope measurements of meteoritic samples by secondary ion mass spectrometry

Abstract

The possibility of establishing an accurate relative chronology of the early solar system events based on the decay of short-lived ²⁶Al to ²⁶Mg (half-life of 0.72 Myr) depends on the level of homogeneity (or heterogeneity) of ²⁶Al and Mg isotopes. However, this level is difficult to constrain precisely because of the very high precision needed for the determination of isotopic ratios, typically of ±5 ppm. In this study, we report for the first time a detailed analytical protocol developed for high precision in situ Mg isotopic measurements (²⁵Mg/²⁴Mg and ²⁶Mg/²⁴Mg ratios, as well as ²⁶Mg excess) by MC-SIMS. As the data reduction process is critical for both accuracy and precision of the final isotopic results, factors such as the Faraday cup (FC) background drift and matrix effects on instrumental fractionation have been investigated. Indeed these instrumental effects impacting the measured Mg-isotope ratios can be as large or larger than the variations we are looking for to constrain the initial distribution of ²⁶Al and Mg isotopes in the early solar system. Our results show that they definitely are limiting factors regarding the precision of Mg isotopic compositions, and that an under- or over-correction of both FC background instabilities and instrumental isotopic fractionation leads to important bias on δ²⁵Mg, δ²⁶Mg and Δ²⁶Mg values (for example, olivines not corrected for FC background drifts display Δ²⁶Mg values that can differ by as much as 10 ppm from the truly corrected value). The new data reduction process described here can then be applied to meteoritic samples (components of chondritic meteorites for instance) to accurately establish their relative chronology of formation.