The neodymium stable isotope composition of the silicate Earth and chondrites
The neodymium stable isotope composition of the silicate Earth and chondrites
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DOI：
10.1016/j.epsl.2017.10.004
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发表时间：
2017-12
影响因子：
5.3
通讯作者：
A. McCoy-West;M. Millet;K. Burton
中科院分区：
文献类型：
--
作者：
A. McCoy-West;M. Millet;K. Burton
硅酸盐地球的非球粒状钕(Nd) 142 Nd/144 Nd 比率可能对地球的吸积和早期演化提供关键约束。然而，这种偏移是否是由于地球是由富含 s 过程 Nd 同位素的物质形成的，还是由于早期分化过程的结果，例如地核形成过程中晚期硫化物锍的分离、碰撞侵蚀或这些过程的某种组合，尚有争议。钕稳定同位素可能对早期硫化物偏析到地核敏感，这一过程无法使用放射性同位素来解决。这项研究首次提供了球粒陨石和陆地岩石的 Nd 稳定同位素综合数据。使用双尖峰技术结合热电离质谱法进行稳定的 Nd 测量。所有三个主要类别的球粒陨石（碳质、顽火辉石和普通球粒陨石）都具有大致相似的同位素组成，允许计算球粒陨石平均值 δ 146/144 Nd=− 0.025±0.025‰（±2 sd；n= 39）。顽辉石球粒陨石产生最均匀的稳定同位素组成 (Δ 146/144 Nd= 26 ppm)，在普通陨石 (Δ 146/144 Nd= 72 ppm) 和碳质陨石 (Δ 146/144 Nd= 143 ppm) 中观察到的变异性要大得多。陆地风化、核合成变异和母体热变质作用似乎对球粒陨石中的 δ 146/144 Nd 几乎没有可测量的影响。普通球粒陨石群之间观察到的微小变化很可能反映了母体之间遗传的成分差异，而在碳质球粒陨石中观察到的较大变化与富含钙铝包裹体的不同模式比例有关。这里分析的陆地样本包括成分从玄武岩到流纹岩的岩石、MORB 玻璃和残余地幔岩性。所有这些陆地岩石都具有大致相似的 Nd 同位素组成，使得大块硅酸盐地球的平均组成为 δ 146/144 Nd=− 0.022±0.034‰(n= 30)。在此处的样品中，岩浆分异似乎仅对高度演化的岩浆中的稳定 Nd 产生影响，在 SiO 2 含量 > 70 wt% 的样品中观察到具有较重的 δ 146/144 Nd 值。球粒陨石和块状硅酸盐地球的平均稳定 Nd 同位素组成在 95% 的置信水平下无法区分。然而，地幔样品确实具有可变的稳定 Nd 同位素组成（Δ 146/144 Nd= 75 ppm），平均 δ 146/144 Nd 值为− 0.008‰。如果这些较重的值代表了原始地幔的真实成分，那么就不可能完全排除地核形成在解释地幔和球粒陨石之间的某些偏移方面的某些作用。总体而言，这些结果表明，地球和球粒陨石之间 142 Nd 的不匹配最好的解释是地球中 s 过程 Nd 的比例较高，而不是在核心中分配为硫化物或富硫金属。
The non-chondritic neodymium (Nd) 142 Nd/144 Nd ratio of the silicate Earth potentially provides a key constraint on the accretion and early evolution of the Earth. Yet, it is debated whether this offset is due to the Earth being formed from material enriched in s-process Nd isotopes or results from an early differentiation process such as the segregation of a late sulfide matte during core formation, collisional erosion or a some combination of these processes. Neodymium stable isotopes are potentially sensitive to early sulfide segregation into Earth's core, a process that cannot be resolved using their radiogenic counterparts. This study presents the first comprehensive Nd stable isotope data for chondritic meteorites and terrestrial rocks. Stable Nd measurements were made using a double spike technique coupled with thermal ionisation mass spectrometry. All three of the major classes of chondritic meteorites, carbonaceous, enstatite and ordinary chondrites have broadly similar isotopic compositions allowing calculation of a chondritic mean of δ 146/144 Nd=− 0.025±0.025‰(±2 sd; n= 39). Enstatite chondrites yield the most uniform stable isotope composition (Δ 146/144 Nd= 26 ppm), with considerably more variability observed within ordinary (Δ 146/144 Nd= 72 ppm) and carbonaceous meteorites (Δ 146/144 Nd= 143 ppm). Terrestrial weathering, nucleosynthetic variations and parent body thermal metamorphism appear to have little measurable effect on δ 146/144 Nd in chondrites. The small variations observed between ordinary chondrite groups most likely reflect inherited compositional differences between parent bodies, with the larger variations observed in carbonaceous chondrites being linked to varying modal proportions of calcium–aluminium rich inclusions. The terrestrial samples analysed here include rocks ranging from basaltic to rhyolitic in composition, MORB glasses and residual mantle lithologies. All of these terrestrial rocks possess a broadly similar Nd isotope composition giving an average composition for the bulk silicate Earth of δ 146/144 Nd=− 0.022±0.034‰(n= 30). In the samples here magmatic differentiation appears to only have an effect on stable Nd in highly evolved magmas with heavier δ 146/144 Nd values observed in samples with> 70 wt% SiO 2. The average stable Nd isotope composition of chondrites and the bulk silicate Earth are indistinguishable at the 95% confidence level. However, mantle samples do possess variable stable Nd isotope compositions (Δ 146/144 Nd= 75 ppm) with an average δ 146/144 Nd value of− 0.008‰. If these heavier values represent the true composition of pristine mantle then it is not possible to completely rule out some role for core formation in accounting for some of the offset between the mantle and chondrites. Overall, these results indicate that the mismatch of 142 Nd between the Earth and chondrites is best explained by a higher proportion of s-process Nd in the Earth, rather than partitioning into sulfide or S-rich metal in the core.