The chondritic neodymium stable isotope composition of the Earth inferred from mid-ocean ridge, ocean island and arc basalts
The chondritic neodymium stable isotope composition of the Earth inferred from mid-ocean ridge, ocean island and arc basalts
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DOI：
10.1016/j.gca.2020.09.038
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发表时间：
2021
影响因子：
5
通讯作者：
A. McCoy-West;K. Burton;M. Millet;Peter A. Cawood
中科院分区：
文献类型：
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作者：
A. McCoy-West;K. Burton;M. Millet;Peter A. Cawood
准确了解地球主要化学储层的组成是限制所有现代地球化学循环的基础。玄武岩提供了一种直接的方法来对地球内部难以接近的成分进行采样。在这里，我们首次使用双尖刺技术对全球汇编的洋中脊、洋岛、大陆板内和岛弧玄武岩进行了全面的钕（Nd）稳定同位素分析。在这些原始岩浆成分中，岩浆分异对δ146/144Nd没有可分辨的影响。洋中脊玄武岩δ146/144Nd极为均匀，平均δ146/144Nd = −0.025 ± 0.013‰（±2 s.d;n= 33）。洋岛和陆相板内岩浆具有更多的可变成分（δ146/144Nd = 62 ppm），这与其源区循环成分的可变掺入有关。新不列颠岛弧玄武岩（δ146/144Nd = 61 ppm）反映了源区成分、熔融程度和板-流体输入之间复杂的相互作用。δ146/144Nd的变化与岩浆分异或板块-流体加成的指标无关，而随着板块深度的增加，δ146/144Nd的增加是由于熔融区内交代弧下地幔比例增加所致。利用Born-Lande近似计算的Nd单键do力常数，建立了Nd稳定同位素的部分熔融模型。典型地幔橄榄岩的熔融将产生不可分解的Nd稳定同位素分馏（Δ146/144Ndmelt-mantle< 0.003‰，1200 °C）。原始岩浆岩在部分熔融过程中没有分馏作用，这意味着可以用原始岩浆岩计算出整体硅酸盐土（BSE）的平均组成：δ146/144Nd = −0.024 ± 0.031‰（±2 s.d;n= 80）。在95%的置信水平上，BSE的成分与球粒陨石的成分难以区分，球粒陨石是地球的基石。因此，大量的Nd没有被隔离到硫化物中，这与最近没有Sm-Nd分馏的实验证据相结合，意味着硫化物不能被认为是地球和球粒陨石之间的142 /144Nd偏移的合理解决方案。尽管典型值δ146/144Nd的可分辨变化在陆地物质中广泛存在，但它们不足以产生BSE和球粒陨石之间放射性成因Nd同位素比率的差异。
Accurate knowledge of the composition of Earth’s major chemical reservoirs is fundamental for constraining all modern geochemical cycles. Basaltic rocks provide a direct way of sampling the composition of Earth’s inaccessible interior. Here, we present the first comprehensive neodymium (Nd) stable isotope analyses for a global compilation of mid-ocean ridge, ocean island, continental intraplate and island arc basalts using a double-spike technique. In these primitive magma compositions magmatic differentiation has no resolvable effect on δ146/144Nd. Mid-ocean ridge basalts possess an extremely homogenous δ146/144Nd with an average composition of δ146/144Nd = −0.025 ± 0.013‰ (±2 s.d.;n= 33). Ocean island and continental intraplate magmas possess more variable compositions (δ146/144Nd = 62 ppm) that are related to the variable incorporation of recycled components in their source regions. Island arc basalts from New Britain (δ146/144Nd = 61 ppm) reflect the complex interplay between source composition, degree of melting and slab-fluid inputs. Variations are uncorrelated with indicators of magmatic differentiation or slab-fluid addition, rather increasing δ146/144Nd with slab depth is attributed to a higher proportion of metasomatized sub-arc mantle in the melting region. A partial melting model for Nd stable isotopes has been constructed using Ndsingle bondO force constants calculated using the Born-Lande approximation. Melting of typical mantle peridotite will induce no resolvable fractionations of Nd stable isotopes (Δ146/144Ndmelt-mantle< 0.003‰ at 1200 °C). The lack of fractionation upon partial melting means primitive magmatic rocks can be used to calculate the average composition of the bulk silicate Earth (BSE), which is δ146/144Nd = −0.024 ± 0.031‰ (±2 s.d.;n= 80). This BSE composition is indistinguishable at the 95 % confidence level from that of chondritic meteorites, the building blocks of Earth. Therefore, sequestration of significant quantities of Nd into the sulfide matte did not occur, this combined with recent experimental evidence for no Sm-Nd fractionation means the sulfide matte cannot be considered a plausible solution for the142Nd/144Nd offset between the Earth and chondrites. Despite resolvable variations in δ146/144Nd from the canonical value being widespread in terrestrial materials, they are not large enough to generate the difference in radiogenic Nd isotope ratios between the BSE and chondrites.