NEUTRON-POOR NICKEL ISOTOPE ANOMALIES IN METEORITES
NEUTRON-POOR NICKEL ISOTOPE ANOMALIES IN METEORITES
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DOI：
10.1088/0004-637x/758/1/59
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发表时间：
2012-09
期刊：
影响因子：
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通讯作者：
R. Steele;C. Coath;M. Regelous;S. Russell;T. Elliott
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作者：
R. Steele;C. Coath;M. Regelous;S. Russell;T. Elliott
我们提供了一系列大块球粒陨石的新的、与质量无关的镍同位素数据。数据报告为 ε60Ni58/61、ε62Ni58/61 和 ε64Ni58/61，或 58Ni/61Ni 内部归一化的 60Ni/61Ni、62Ni/61Ni 和 64Ni/61Ni 比率与地面参考（NIST SRM 986 标准）的万分之一偏差。相对于典型样品精度 0.03、0.05 和 0.08 (2 s.e.)，球粒陨石在 ε60Ni58/61、ε62Ni58/61 和 ε64Ni58/61 中的精度范围分别为 0.15、0.29 和 0.84。碳质球粒陨石显示出最大的正异常，顽火辉石球粒陨石具有近似地球的比例，尽管只有EH在不确定范围内与地球的成分相匹配，而普通球粒陨石则显示出负异常。陨石数据显示ε62Ni58/61和ε64Ni58/61之间存在很强的正相关性，其推断在先前富含钙、铝的包裹体测量的平均值的误差范围内。此外，该块状陨石阵列的斜率是 3.003 ± 0.166，这在仅 58Ni 异常的预期误差范围内。我们还以高精度（每 AMU 约 10 ppm）确定了两个陨石样品的质量相关分馏，其范围涵盖 ε62Ni58/61 和 ε64Ni58/61。这些分析表明，58Ni/61Ni 的“绝对”比率在这两个样品之间有所不同，而 62Ni/61Ni 和 64Ni/61Ni 的“绝对”比率则没有。因此，Ni 同位素差异似乎最有可能由贫中子 58Ni 的变异性来解释，而不是富中子同位素 62Ni 和 64Ni 中的相关异常。这与之前对 Ni 和其他过渡元素进行与质量无关的测量得出的推论形成了鲜明对比，这些测量引发了富中子成分的可变贡献。我们检查了不同的核合成环境，以确定导致在批量样品中观察到的镍和其他过渡元素中观察到的同位素变化的异常材料的可能来源。我们发现，太阳系的 Ni 同位素变异性不能通过与 SN II、沃尔夫-拉叶或渐近巨型分支源的大量恒星喷射物成分混合来解释，并且不太可能由来自 SN Ia 的物质的大量混合造成。然而，SN II 的 Si/S 区材料的可变混合物可以产生太阳系材料中镍同位素变化的所有特征。此外，这些特征也可以由质量范围为 15 到 40 M☉ 的 SN II 提供，这表明 SN II 的输入是太阳系中镍同位素变化的可靠来源。 Ni同位素异常与块状陨石中O、Cr、Ti同位素比值以及Pb/Yb的相关性表明，早期太阳系中同位素异常的不均匀分布可能是由于在原太阳星云塌陷附近合成的化学或物理上不同的材料的星云分类造成的，这些材料具有不同数量的同位素。
We present new, mass-independent, Ni isotope data for a range of bulk chondritic meteorites. The data are reported as ε60Ni58/61, ε62Ni58/61, and ε64Ni58/61, or the parts per ten thousand deviations from a terrestrial reference, the NIST SRM 986 standard, of the 58Ni/61Ni internally normalized 60Ni/61Ni, 62Ni/61Ni, and 64Ni/61Ni ratios. The chondrites show a range of 0.15, 0.29, and 0.84 in ε60Ni58/61, ε62Ni58/61, and ε64Ni58/61 relative to a typical sample precision of 0.03, 0.05, and 0.08 (2 s.e.), respectively. The carbonaceous chondrites show the largest positive anomalies, enstatite chondrites have approximately terrestrial ratios, though only EH match Earth's composition within uncertainty, and ordinary chondrites show negative anomalies. The meteorite data show a strong positive correlation between ε62Ni58/61 and ε64Ni58/61, an extrapolation of which is within the error of the average of previous measurements of calcium-, aluminium-rich inclusions. Moreover, the slope of this bulk meteorite array is 3.003 ± 0.166 which is within the error of that expected for an anomaly solely on 58Ni. We also determined to high precision (∼10 ppm per AMU) the mass-dependent fractionation of two meteorite samples which span the range of ε62Ni58/61 and ε64Ni58/61. These analyses show that “absolute” ratios of 58Ni/61Ni vary between these two samples whereas those of 62Ni/61Ni and 64Ni/61Ni do not. Thus, Ni isotopic differences seem most likely explained by variability in the neutron-poor 58Ni, and not correlated anomalies in the neutron-rich isotopes, 62Ni and 64Ni. This contrasts with previous inferences from mass-independent measurements of Ni and other transition elements which invoked variable contributions of a neutron-rich component. We have examined different nucleosynthetic environments to determine the possible source of the anomalous material responsible for the isotopic variations observed in Ni and other transition elements within bulk samples. We find that the Ni isotopic variability of the solar system cannot be explained by mixing with a component of bulk stellar ejecta from either SN II, Wolf–Rayet or, an asymptotic giant branch source and is unlikely to result from bulk mixing of material from an SN Ia. However, variable admixture of material from the Si/S zone of an SN II can create all the characteristics of Ni isotope variations in solar system materials. Moreover, these characteristics can also be provided by an SN II with a range of masses from 15 to 40 M☉, showing that input from SN II is a robust source for Ni isotope variations in the solar system. Correlations of Ni isotope anomalies with O, Cr, and Ti isotope ratios and Pb/Yb in bulk meteorites suggest that the heterogeneous distribution of isotopic anomalies in the early solar system likely resulted from nebular sorting of chemically or physically different materials bearing different amounts of isotopes synthesized proximally to the collapse of the protosolar nebula.