Mn–Cr relative sensitivity factor in ferromagnesian olivines defined for SIMS measurements with a Cameca ims-1280 ion microprobe: Implications for dating secondary fayalite

The short-lived radionuclide 53Mn, which decays to 53Cr with a half-life of ∼3.7Myr, is useful for sequencing objects that formed within the first 20Myr of Solar System evolution. 53Mn–53Cr relative chronology enables aqueously formed secondary minerals such as fayalite and various carbonates in ordinary and carbonaceous chondrites to be dated, thereby providing chronological constraints on aqueous alteration processes. In situ measurements of Mn–Cr isotope systematics in fayalite by secondary ion mass spectrometry (SIMS) require consideration of the relative sensitivities of the 55Mn+ and 52Cr+ ions, for which a relative sensitivity factor [RSF=(55Mn+/52Cr+)SIMS/(55Mn/52Cr)true] is defined using appropriate standards. In the past, San Carlos olivine (Fa∼10) was commonly used for this purpose, but a growing body of evidence suggests that it is an unsuitable standard for meteoritic fayalite (Fa>90). Natural fayalite also cannot be used as a standard because it contains only trace amounts of chromium, which makes determining a true 55Mn/52Cr ratio and its degree of heterogeneity very difficult.