Mn/Cr relative sensitivity factors for synthetic calcium carbonate measured with a NanoSIMS ion microprobe

Abstract

Mn/Cr relative sensitivities for a synthetic calcite and a terrestrial olivine (San Carlos olivine) were measured using spot and rasterized modes with a NanoSIMS ion probe. Counting rates of ⁴³Ca⁺, ⁵²Cr⁺, ⁵³Cr⁺ and ⁵⁵Mn⁺ from the synthetic calcite by spot analysis mode decreased with time (and depth of crater). This may be interpreted as due to charge-up of the deep crater formed by primary ion bombardment because of poor conductivity of carbonate. The relative Mn/Cr sensitivity of carbonate measured by spot analysis mode decreased with time. It may be necessary to use a time-dependent relative sensitivity factor for chronological studies of meteoritic carbonates. The time-averaged RSF for the calcite spot analysis is significantly lower than that for San Carlos olivine. The instrumental isotopic fractionation of the ⁵³Cr/⁵²Cr ratio measured with the NanoSIMS 50 seems to be different between the calcite and San Carlos olivine.