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 53Mn-53Cr ages of secondary fayalites (Fa90-100) in the Elephant Moraine (EET) 90161 (L3.05), Vicencia (LL3.2), Asuka 881317 (CV3) and MacAlpine Hills (MAC) 88107 (C3) chondrites (2.4-1.3+1.8, 4.0-1.1+1.4, 4.2-0.7+0.8 and 5.1-0.4+0.5 Myrs after CV CAIs, respectively) are ∼3 Myr older when using an RSF measured on a matrix-matched (Fa99) standard, rather than on a San Carlos olivine. The inferred 53Mn-53Cr ages of fayalite formation are consistent with the ages reported for calcites in CM chondrites measured with similarly matrix-matched standards, suggesting an early onset of aqueous alteration on the ordinary and carbonaceous chondrite parent bodies heated by decay of 26Al.