The Earth’s missing lead may not be in the core
The Earth’s missing lead may not be in the core
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DOI：
10.1038/nature07375
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发表时间：
2008-11
期刊：
影响因子：
64.8
通讯作者：
Markus Lagos;Chris Ballhaus;Carsten Münker;Cora C. Wohlgemuth-Ueberwasser;Jasper Berndt;D. V. Kuzmin
中科院分区：
文献类型：
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作者：
Markus Lagos;Chris Ballhaus;Carsten Münker;Cora C. Wohlgemuth-Ueberwasser;Jasper Berndt;D. V. Kuzmin
相对于CI球粒陨石类陨石(被广泛认为是类地行星的‘积木’)，地球上的挥发性元素是枯竭的。对于大多数元素来说，这种贫化被认为是太阳星云的特征，因为球粒陨石显示出与地球性质相似的贫化。另一方面，由于铅是一种挥发性元素，在沉积后可能也会损失一些铅。地幔独特的~(206)Pb/~(204)Pb和~(207)Pb/~(204)Pb比值表明，太阳系形成后约50~130 Myr，有部分铅丢失。通常的解释是，铅是通过硫化物熔体分离到地核而损失的，这假设铅与硫化物有亲和力。然而，一些模型已经调和了地球铅的短缺和挥发。无论选择哪一种模型，U-Pb模型年龄与月球年龄的广泛吻合表明，铅的损失可能与月球形成的撞击有关。本文报道了金属-硫化物-硅酸盐体系的分配实验。我们发现，铅既不亲铁，也不亲硫，不足以解释地幔高U/Pb值是铅泵入地核的结果。地球可能是从最初挥发的贫化物质中吸积而来的，一些铅可能在月球形成的巨大撞击后被脱气损失，或者在地幔深处存在隐藏的储集层，铅同位素组成与上地幔的值互补；尽管丢失的铅不太可能存在于地核。
Relative to the CI chondrite class of meteorites (widely thought to be the ‘building blocks’ of the terrestrial planets), the Earth is depleted in volatile elements. For most elements this depletion is thought to be a solar nebular signature, as chondrites show depletions qualitatively similar to that of the Earth. On the other hand, as lead is a volatile element, some Pb may also have been lost after accretion. The unique206Pb/204Pb and207Pb/204Pb ratios of the Earth’s mantle suggest that some lead was lost about 50 to 130 Myr after Solar System formation,,. This has commonly been explained by lead lost via the segregation of a sulphide melt to the Earth’s core,,, which assumes that lead has an affinity towards sulphide. Some models, however, have reconciled the Earth’s lead deficit with volatilization. Whichever model is preferred, the broad coincidence of U–Pb model ages with the age of the Moon,,suggests that lead loss may be related to the Moon-forming impact. Here we report partitioning experiments in metal–sulphide–silicate systems. We show that lead is neither siderophile nor chalcophile enough to explain the high U/Pb ratio of the Earth’s mantle as being a result of lead pumping to the core. The Earth may have accreted from initially volatile-depleted material, some lead may have been lost to degassing following the Moon-forming giant impact, or a hidden reservoir exists in the deep mantle with lead isotope compositions complementary to upper-mantle values; it is unlikely though that the missing lead resides in the core.