Trace-element fractionation in Hadean mantle generated by melt segregation from a magma ocean
Trace-element fractionation in Hadean mantle generated by melt segregation from a magma ocean
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DOI：
10.1038/nature03827
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发表时间：
2005-07-14
期刊：
影响因子：
64.8
通讯作者：
Corgne, A
中科院分区：
文献类型：
--
作者：
Caro, G;Bourdon, B;Corgne, A
对陆地吸积的能量计算表明，地球在其早期历史中广泛熔融（1）。使用短寿命Sm-146 - Nd-142测时法对西格陵兰早期岩石（3.6 - 3.8 Gyr）的研究表明，地幔分异发生在接近吸积结束时（4.46 +/- 0.11 Gyr ago）（2-4）。这产生了一个化学亏损地幔的Sm/Nd比高于南极洲的值。相比之下，Lu-176 - Hf-176系统学对西格陵兰3.6 - 3.8 Gyr-old锆石的应用表明其来自地幔源，具有南极Lu/Hf比值（5-7）。虽然早期Sm/Nd分馏可以用玄武岩地壳形成（8）、岩浆海洋结晶（2）或大陆地壳形成来解释，但同时代Lu/Hf分馏的缺失与地壳形成过程中Sm/Nd和Lu/Hf比值的众所周知的协变行为形成鲜明对比（5）。在这里，我们使用矿物熔体分配数据高压地幔矿物，所观察到的Nd和Hf的签名可能已经产生了分离的熔体从结晶岩浆海洋在上地幔压力在地球的历史早期。这些残余的熔融物会迅速上升，最终形成最早的陆地原地壳。
Calculations of the energetics of terrestrial accretion indicate that the Earth was extensively molten in its early history(1). Examination of early Archaean rocks from West Greenland (3.6 - 3.8 Gyr old) using short-lived Sm-146 - Nd-142 chronometry indicates that an episode of mantle differentiation took place close to the end of accretion (4.46 +/- 0.11 Gyr ago)(2-4). This has produced a chemically depleted mantle with an Sm/Nd ratio higher than the chondritic value. In contrast, application of Lu-176 - Hf-176 systematics to 3.6 - 3.8-Gyr-old zircons from West Greenland indicates derivation from a mantle source with a chondritic Lu/Hf ratio(5-7). Although an early Sm/Nd fractionation could be explained by basaltic crust formation(8), magma ocean crystallization(2) or formation of continental crust, the absence of coeval Lu/Hf fractionation is in sharp contrast with the well-known covariant behaviour of Sm/Nd and Lu/Hf ratios in crustal formation processes(5). Here we show using mineral - melt partitioning data for high-pressure mantle minerals that the observed Nd and Hf signatures could have been produced by segregation of melt from a crystallizing magma ocean at upper-mantle pressures early in Earth's history. This residual melt would have risen buoyantly and ultimately formed the earliest terrestrial protocrust.