Ion Useful Yields and Matrix Effects in ²⁶Al-²⁶Mg Dating by Secondary Ion Mass Spectrometry

Secondary ion mass spectrometry (SIMS) has been widely applied to ²⁶Al-²⁶Mg dating of the earliest Solar System solids. Both ¹⁶O^- and ¹⁶O₂ ^- primary beams have been used for such analyses, yet the influence of primary ion beam species on secondary ion useful yields and matrix effects has not been systematically evaluated. A better understanding of these effects is essential for improving the accuracy and precision of ²⁶Al-²⁶Mg dating.

For standard materials used in ²⁶Al-²⁶Mg dating, we determined sputter rates, useful yields, and matrix effects on the relative sensitivity factors (RSFs) of ²⁷Al/²⁴Mg and the instrumental mass fractionations (IMFs) of Mg isotopes under the ¹⁶O^- and ¹⁶O₂ ^- primary beam conditions using SIMS. The sputter rates were evaluated using a rastered beam, whereas the other parameters were obtained under the actual analytical conditions employed for ²⁶Al-²⁶Mg dating.

The ¹⁶O₂ ^- primary beam condition generally provided higher useful yields than the ¹⁶O^- condition, except for anorthite and melilite analyzed under low ion current conditions. Variations in RSFs among samples of the same mineral species but with different compositions were either larger or smaller between the ¹⁶O^- and ¹⁶O₂ ^- conditions, depending on the mineral species, whereas IMF variations tended to be slightly larger under the ¹⁶O₂ ^- condition than under the ¹⁶O^- condition.

Our results highlight the necessity of carefully evaluating useful yields, along with the behaviors of RSF and IMF, when optimizing analytical conditions for SIMS-based ²⁶Al-²⁶Mg dating, as well as for other isotopic systems that can be analyzed using SIMS.