Ion useful yields and relative sensitivity factors in Mn–Cr isotope measurements of carbonates by secondary ion mass spectrometry

Abstract

The ⁵³Mn–⁵³Cr chronometer applied to carbonates in chondrites and asteroidal samples provides key insights into aqueous alteration processes on planetesimals in the early Solar System. In this study, we evaluated the useful yields of secondary ions and relative sensitivity factors (RSFs) of Cr and Mn for calcite and dolomite in Mn–Cr isotope measurements by secondary ion mass spectrometry (SIMS) with two different primary ion beam species, ¹⁶O^– and ¹⁶O₂^–. The choice of primary beam species exerts only minor effects on the chemical composition dependences of the ⁵⁵Mn/⁵²Cr RSFs among carbonate standards. In contrast, the ¹⁶O^– beam gives ~42% and ~6–11% higher useful yields of ⁵²Cr⁺ than the ¹⁶O₂^– beam for calcite and dolomite, respectively. Because higher useful yields directly improve the analytical precision of Cr-isotope ratios and the resulting formation ages, the use of the ¹⁶O^– primary beam is advantageous for ⁵³Mn–⁵³Cr dating of carbonates by SIMS.