Search for 60Ni excesses in MET-78008 ureilite: An ion microprobe study

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We have developed a technique for in-situ Ni isotopic analysis using the ion microprobe, in order to detect ^<60>Ni excess from the decay of the short lived nuclide ^<60>Fe (half life=1.5Ma) in ureilite samples. The silicate minerals from MET-78008 ureilite with an old U-Pb age of 4.563±0.006 Ga were analyzed. The ^<56>Fe/^<58>Ni ratios of olivine and orthopyroxene are between 2700 and 5400. In spite of the high Fe/Ni ratios, we could not observe any detectable ^<60>Ni excess. From the mean value of olivine core data, we obtain an upper limit of the ^<60>Fe/^<56>Fe ratio at the time of ureilite formation of 1.8×10^<-7>. The time difference between CAI formation and ureilite formation was estimated to be more than 4 million years, which is consistent with the UPb data from the same meteorite. We concluded that the impact event for the disruption of the ureilite parent body happened more than 4 million years after CAI formation. However, a large uncertainty in the initial ^<60>Fe/^<56>Fe ratio is introduced by the possibility that the ^<60>Ni excess observed in CAIs is of nucleosynthetic origin. Our conclusion may change if the initial ^<60>Fe/^<56>Fe ratio of the solar system using CAI data is too high.
国立極地研究所の論文