A Mercury-like component of early Earth yields uranium in the core and high mantle ~(142)Nd

摘要

Anke Wohlers和Bernard Wood通过高压实验发现,将一个缩小的类似水星的天体加到富含硫的早期地球上,有可能产生在地球地幔中所观察到的超级球粒状陨石Sm/Nd比例和~(142)Nd/~(144)Nd同位素异常。这将意味着,并不像人们以前所提出的那样,地球深层地幔中必须有一个"隐藏''的硅酸盐库,或者说早期形成的一个地壳必须通过"撞击剥蚀"(impact ablation)而消失。另外,富含硫的地核可能含有更多的铀和钍,这有可能为驱动"地球发电机"提供很大一部分热源。%Recent ~(142)Nd 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 chon-dritic 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 an ~(142)Nd/~(144)Nd 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.