TERRAPUB Geochemical Journal
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Geochemical Journal, Vol. 44 (No. 3), pp. e11-e16, 2010

EXPRESS LETTER

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

N. Sugiura,1 K. Ichimura,1 W. Fujita1 and N. Takahata2

1Department of Earth and Planetary Science, The University of Tokyo, Tokyo 275-0005, Japan
2Ocean Research Institute, The University of Tokyo, Tokyo 164-8639, Japan

(Received March 15, 2010; Accepted May 13, 2010; Online published May 20, 2010)

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.
Key words: secondary ion mass spectrometry, Mn-Cr chronology, relative sensitivity, carbonate, isotope


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