A Mercury-like component of early Earth yields uranium in the core and high mantle (142)Nd.
A Mercury-like component of early Earth yields uranium in the core and high mantle (142)Nd.
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DOI：
10.1038/nature14350
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发表时间：
2015-04-16
期刊：
影响因子：
64.8
通讯作者：
Wood, Bernard J.
中科院分区：
文献类型：
--
作者：
Wohlers, Anke;Wood, Bernard J.
最近的142 Nd同位素数据表明，硅酸盐地球（地壳加上地幔）的钐钕元素比（Sm/Nd）大于假设的地球的南极洲组成部分。这种升高的Sm/Nd被归因于地球中的“隐藏”水库，或早期形成的地壳的影响消融的损失。由于通过烧蚀去除地壳也会去除产生热量的元素钾、铀和钍，这种去除将使地球产生的热量与观测到的热流保持平衡变得极其困难。在“隐藏”水库的替代，一个补充低Sm/Nd层通常被认为是居住在硅酸盐下地幔未观察到。然而，我们以前已经表明，该核心是一个可能的水库，一些亲石元素，如铌。因此，我们解决的问题，核心的形成是否可以分馏钕钐，也作为一个散热器的发热元素。我们在这里表明，在早期地球中加入一个还原的富含硫的类汞体（或者，一个类似顽火辉石-球粒陨石的体），将在地幔中产生一个超地幔Sm/Nd和一个相对于球粒陨石约为+14ppm的142 Nd/144 Nd异常。此外，富含硫的地核会强烈地分离铀和钍，为地球发电机提供了相当大一部分“缺失”的热源。
Recent142Nd isotope data indicate that the silicate Earth (its crust plus the mantle) has a samarium to neodymium elemental ratio (Sm/Nd) that is greater than that of the supposed chondritic building blocks of the planet. This elevated Sm/Nd has been ascribed either to a ‘hidden’ reservoir in the Earth,or to loss of an early-formed terrestrial crust by impact ablation. Since removal of crust by ablation would also remove the heat-producing elements—potassium, uranium and thorium—such removal would make it extremely difficult to balance terrestrial heat production with the observed heat flow. In the ‘hidden’ reservoir alternative, a complementary low-Sm/Nd layer is usually considered to reside unobserved in the silicate lower mantle. We have previously shown, however, that the core is a likely reservoir for some lithophile elements such as niobium. We therefore address the question of whether core formation could have fractionated Nd from Sm and also acted as a sink for heat-producing elements. We show here that addition of a reduced Mercury-like body (or, alternatively, an enstatite-chondrite-like body) rich in sulfur to the early Earth would generate a superchondritic Sm/Nd in the mantle and an142Nd/144Nd anomaly of approximately +14 parts per million relative to chondrite. In addition, the sulfur-rich core would partition uranium strongly and thorium slightly, supplying a substantial part of the ‘missing’ heat source for the geodynamo.
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