Constraints on the Moon's origin from the partitioning behaviour of tungsten
Constraints on the Moon's origin from the partitioning behaviour of tungsten
复制标题
钨的分配行为对月球起源的限制
作者：
H. Newsom;M. Drake
月球、地球和欧氏陨石显示W/La比，对应于W相对于球粒陨石的损耗因子为~ 19（参考文献1）。这种枯竭可归因于金属/硅酸盐分馏。普通球粒陨石和Cl球粒陨石的金属含量和氧化态差异很大，但它们的W/La比相似，这表明星云分选并不重要。因此，假设W和La都是难熔且不相容的，那么在地球、月球和长闪岩中W相对于La的消耗可能是由于在行星尺度的火成岩事件中金属与硅酸盐的分离。对于月球，Rammensee和Wänke2concluded认为，任何物理上合理的金属月球核都太小，无法解释月球上W的大量消耗。因此，有人提出2月球在非月球的火成岩事件中获得了W的消耗，这支持了月球是由原地球裂变形成的假设，原地球的上部由于地球核的形成而在W中消耗殆尽3 - 5。我们在这里表明，这一结论是隐含的，基于一个特殊情况，即金属和硅酸盐相之间的平衡与硅酸盐的完全熔化。在地质学上更普遍的情况下，金属在相对较低程度的部分熔融下从硅酸盐中分出，W的不相容性质可能会否定这一结论。地球物理学上合理的金属地核可以解释月球表面和月球地幔岩石中W的枯竭。月球的低W/La比值不能作为月球起源于陆地的无条件证据。
The Moon, Earth and eucritic meteorites display W/La ratios which correspond to depletions of W relative to chondrites by factors of ∼19 (ref. 1). This depletion can be attributed to metal/silicate fractionation. Ordinary chondrites and Cl chondrites with widely varying metal contents and oxidation states have similar W/La ratios, suggesting that nebular fractionations were unimportant. Therefore, assuming that both W and La are refractory and incompatible, the depletions of W relative to La in the Earth, Moon and eucrites are presumably due to separation of metal from silicate during planetary-scale igneous events. For the Moon, Rammensee and Wänke2concluded that any geophysically reasonable metallic lunar core would be too small to explain the large lunar depletion of W. It was therefore suggested2that the Moon acquired its depletion of W in a non-lunar igneous event, supporting the hypothesis that the Moon formed by fission from the proto-Earth, the upper portion of which was depleted in W due to terrestrial core formation3–5. We show here that this conclusion is implicity based on a special case, namely equilibrium between metal and silicate phases with total melting of silicates. In the geologically more general case where metal fractionates from silicate at relatively low degrees of partial melting, the incompatible nature of W may negate this conclusion. A geophysically plausible metallic core can account for the depletion of W in lunar surface and, presumably, lunar mantle rocks. The low W/La ratio in the Moon cannot be used as unconditional evidence for a terrestrial origin of the Moon.
DOI：
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发表时间：
2007
期刊：
Lunar and Planetary Science Conference XXXVIII 38
影响因子：
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作者：
原田 知明;保科 洋介;吉村 英恭;C.Okamoto
通讯作者：
C.Okamoto