Mn-Cr relative sensitivity Factors for Secondary Ion Mass Spectrometry analysis of Mg-Fe-Ca olivine and implications for the Mn-Cr chronology of meteorites

摘要

Secondary Ion Mass Spectrometry (SIMS) is used to date meteoritic olivine using the ~(53)Mn-~(53)Cr short-lived decay scheme. This involves simultaneously measuring the Cr-isotope composition and Mn/Cr ratio of a sample, but the relative sensitivity of SIMS to Cr and Mn over a compositionally varying olivine matrix is poorly understood. We have synthesized a range of olivine samples in Mg-Fe-Ca space, suitable for investigating Mn-Cr relative sensitivity with particular application to olivine in angrites and secondary fayalite in oxidised carbonaceous chondrites. These were analysed by SIMS (using Sensitive High-mass Resolution Ion Micro Probe Reverse Geometry; SHRIMP-RG) and Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS) to determine the relative sensitivity factors (RSF) necessary to correct SIMS ~(55)Mn~+/~(52)Cr~+ to atomic ~(55)Mn/~(52)Cr. We have found that RSF changes systematically as a function of the olivine matrix by several tens of percent, apparently being controlled by Fe content. Additionally, the RSF values found in this study are considerably different from most of those previously reported. A review of the literature shows that RSF for the Mg-rich San Carlos olivine differs widely between studies, and is probably controlled by SIMS instrumental settings. Our results show that the ~(55)Mn/~(52)Cr of olivine in some SIMS Mn-Cr studies has been accurately determined (e.g. for pallasites and angrites) but for secondary fayalite in carbonaceous chondrites it has probably been underestimated. If the relationship observed for our SIMS instrument holds true for others, then the initial ~(53)Mn/~(55)Mn of these materials (the abundance of ~(53)Mn with respect to ~(55)Mn at the time of crystallisation/isotopic closure) should be lower by a factor of ～1.6 and ages should be ～2.6Ma younger. This would imply that the initial ~(53)Mn/~(55)Mn for secondary fayalites in carbonaceous chondrites is in the range 0.7-1.7×10~(-6), equating to an age of 4555.2-4559.9Ma relative to the precisely dated D'Orbigny angrite.