A search for nickel isotopic anomalies in iron meteorites and chondrites

摘要

We report Ni isotopic data, for 58.60-62N_, on (1) FeNi metal and sulfides in different groups of iron meteorites, (2) sulfides and a whole rock sample of the St. Séverin chondrite, and (3) chondrules from the Chainpur chondrite. We have developed improved, Multiple-Collector, Positive ion Thermal Ionization Mass Spectrometric (MC-PTIMS) techniques, with Ni~+ ion_ization efficiency at 1%~o, and chemical separation techniques for Ni which reduce mass interferences to the 1 ppm level, so that no mass interference corrections need be applied, except for ~(64)Ni (from ~(64)Zn, at the 0.1 %o level), for which we do not report results. We normalize the data to ~(62)Ni/~(58)Ni to correct for mass dependent isotope fractionation. No evidence was found for resolved radiogenic or general Ni isotope anomalies at the resolution levels of 0.2 and 0.5 cu (au = 0.01%) for ~(60)Ni/~(58)Ni and ~(6l)Ni/~(58)Ni, respectively. From the 56Fe/58Ni ratios and ε(~(60)Ni/~(58)Ni) values, we calculate upper limits for the initial value of (~(60)Fе/~(56)Fe)_0 of (a) 2.7 x 10~-7 for Chainpur chondrules, (b) <10~-8 for the St. Séverin sulfide, and (c) <4 x 10~-9 for sulfides from iron meteorites. We measured some of the same meteorites measured by other workers, who reported isotopic anomalies in Ni, using Multiple-Collector, Inductively-Coupled Mass Spectrometry. Our results do not support the previous reports of Ni isotopic anomalies in sulfide samples from Mundrabilla by Cook et al. [Cook D. L., Clayton R. N., Wadhwa M., Janney P. E., and Davis A. M. (2008). Nickel isotopic anomalies in troilite from iron meteorites. Geophy. Res. Lett. 35, L01203] and in sulfides from Toluca and Odessa by Quitté et al. [Quitté G., Meier M., Latkoczy C., Halliday A. N., and Gunther D., (2006). Nickel isotopes in iron meteorites-nucleosynthetic anomalies in sulfides with no effects in metals and no trace of ~(60)Fe. Earth Planet. Sci. Lett. 242, 16-25]. Hence, we find no need for specialized physical_chemical planetary processes for the preserva_tion of different Ni isotope compositions, between FeNi metal and sulfides in the same iron meteorites, as proposed by the above reports nor for complex astrophysical scenarios to provide the very peculiar Ni isotope anomalies reported by these workers for sulfides.