Earth's chondritic light rare earth element composition: Evidence from the Ce–Nd isotope systematics of chondrites and oceanic basalts
Earth's chondritic light rare earth element composition: Evidence from the Ce–Nd isotope systematics of chondrites and oceanic basalts
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DOI：
10.1016/j.epsl.2018.12.004
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发表时间：
2019-03
影响因子：
5.3
通讯作者：
M. Willig;A. Stracke
中科院分区：
文献类型：
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作者：
M. Willig;A. Stracke
组合的Ce和Nd同位素比值提供了其来源物质的轻稀土元素(LREE)丰度的时间综合记录。在这里，我们提供了来自大洋岛屿(OIB)和大洋中脊(MORB)的球粒陨石和玄武岩的高精度Ce同位素数据。球粒陨石中新的Ce同位素比值定义了一个精确的新CHUR参考值。在Ce-ND同位素空间中，MORB和OIB形成了与Ce-ND球粒陨石参考值相交的清晰阵列。最简单的一级解释是块状硅酸盐地球(BSE)具有球粒陨石LREE和Ce-Nd同位素比值。然而，我们发现Ce-ND同位素地幔阵列的截距和斜率取决于几个因素。也许最重要的是BSE是否是球粒陨石，以及MORB和OIB数据中反映的地幔平均值与BSE的Ce-ND同位素比率的对应程度有多高。可到达地幔的平均Ce-ND同位素比值与BSE的平均Ce-ND同位素比值之间的显著差异可能是因为地球总LREE预算的相当大一部分永久储存在大陆地壳或潜在的孤立储集层中。结果表明，孤立油气藏的形成对平均地幔的Ce-Nd平均组成影响不大，或者在地球化学和地球动力学上是不可信的。如果由于陆壳的形成，平均地幔的Ce-ND同位素组成相对于BSE发生了显著的偏移，这种偏移与Ce-ND地幔阵列平行，并且不影响其球粒截距。因此，Ce-ND同位素地幔阵列的球粒陨石截距是BSE相对的LREE和Ce-ND同位素组成为球粒陨石的有力证据。然而，如果MORB和OIB没有以具有代表性的方式采样可访问地幔，那么可访问地幔的平均Ce-ND同位素比值与BSE的平均Ce-ND同位素比值之间也可能产生明显的差异。尽管我们不能完全排除后者，但它需要多种因素的偶然组合才能导致观测到的Ce-ND同位素地幔阵列的球粒状截获。因此，我们得出结论：块状硅酸盐地球具有球粒陨石LREE和Ce-Nd同位素比值。
Combined Ce and Nd isotope ratios provide a time-integrated record of the light rare earth element (LREE) abundances of their source materials. Here, we present new high precision Ce isotope data for chondrites and basalts from ocean islands (OIB) and mid ocean ridges (MORB). The new Ce isotope ratios in chondritic meteorites define a precise new CHUR reference value. In Ce–Nd isotopic space, the MORB and OIB form a well-defined array that intersects with the Ce–Nd chondritic reference value. The simplest first-order explanation is that the bulk silicate Earth (BSE) has chondritic LREE and Ce–Nd isotope ratios. We show, however, that the intercept and slope of the Ce–Nd isotope mantle array depend on several factors. Perhaps most important are whether the BSE is chondritic and how closely the mantle average reflected in the MORB and OIB data corresponds to the Ce–Nd isotope ratio of the BSE. A significant difference between the accessible mantle's average Ce–Nd isotope ratio and that of BSE could result from the permanent storage of a considerable proportion of Earth's total LREE budget in the continental crust or potential isolated reservoirs. We show that the formation of isolated reservoirs either has a minor effect on the average Ce–Nd composition of the average mantle, or is geochemically and geodynamically implausible. If, due to formation of the continental crust, a significant shift in the average mantle's Ce–Nd isotope composition relative to BSE occurs, this shift is parallel to the Ce–Nd mantle array, and does not affect its chondritic intercept. The chondritic intercept of the Ce–Nd isotope mantle array therefore is strong evidence that BSE's relative LREE and Ce–Nd isotope composition is chondritic. However, an apparent difference between the accessible mantle's average Ce–Nd isotope ratio and that of BSE could also result if MORB and OIB do not sample the accessible mantle in a representative manner. Although we cannot entirely exclude the latter, it would require a fortuitous combination of factors to cause the observed chondritic intercept of the Ce–Nd isotope mantle array. We therefore conclude that the bulk silicate Earth has chondritic LREE and Ce–Nd isotope ratios.