Search for 60Ni excesses in MET-78008 ureilite: An ion microprobe study
Search for 60Ni excesses in MET-78008 ureilite: An ion microprobe study
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搜索 MET-78008 ureilite 中过量的 60Ni：离子微探针研究
DOI：
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发表时间：
1998
期刊：
影响因子：
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通讯作者：
H. Yurimoto
中科院分区：
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作者：
N. Kita;S. Togashi;Y. Morishita;S. Terashima;H. Yurimoto
我们发展了一种利用离子探针进行原位Ni同位素分析的技术，以探测铀矿样品中短寿命核素°Fe(半衰期=1.5 Ma)衰变过程中产生的~(60)Ni过剩。对MET-78008榴辉石的硅酸盐矿物进行了分析，其U-Pb年龄为4.563±0.006 Ga。橄榄石和斜方辉石的56Fe/58Ni比值在2700~5400之间。尽管有很高的Fe/Ni比，但我们没有观察到任何可检测到的60Ni过剩。根据岩芯数据的平均值，我们得到了辉绿岩形成时6°Fe/56Fe比值的上限为1.8×10~(-1)，估计蔡氏组与辉绿岩形成的时间相差超过400万年，这与同一陨石的U-Pb数据一致。我们认为，辉绿岩母体解体的冲击事件发生在CaI形成后400多万年。然而，由于在CAIS中观察到的MJNi过剩来自核合成的可能性，在初始6°Fe/56Fe比值中引入了很大的不确定性。如果用计算机辅助测量数据得到的太阳系初始6°Fe/56Fe比值过高，我们的结论可能会改变。
We have developed a technique for in-situ Ni isotopic analysis using the ion microprobe, in order to detect 60Ni excess from the decay of the short lived nuclide °Fe (half life= 1.5 Ma) 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 56Fe/58Ni ratios of olivine and orthopyroxene are between 2700 and 5400. In spite of the high Fe/Ni ratios, we could not observe any detectable 60Ni excess. From the mean value of oli vine core data, we obtain an upper limit of the 6°Fe/56Fe ratio at the time of ureilite formation of l.8x 10-1• The time difference between CAI formation and ureilite for mation was estimated to be more than 4 million years, which is consistent with the U Pb data from the same meteorite. We concluded that the impact event for the disrup tion of the ureilite parent body happened more than 4 million years after CAI forma tion. However, a large uncertainty in the initial 6°Fe/56Fe ratio is introduced by the possibility that the MJNi excess observed in CAis is of nucleosynthetic origin. Our conclusion may change if the initial 6°Fe/56Fe ratio of the solar system using CAI data is too high.