Isotopic evidence for chondritic Lu/Hf and Sm/Nd of the Moon
Isotopic evidence for chondritic Lu/Hf and Sm/Nd of the Moon
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DOI：
10.1016/j.epsl.2013.08.018
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发表时间：
2013-10-15
影响因子：
5.3
通讯作者：
Scherer, Erik E.
中科院分区：
文献类型：
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作者：
Sprung, Peter;Kleine, Thorsten;Scherer, Erik E.
d难熔亲石元素通常被认为在类地行星中以球粒质相对丰度出现。这一假设构成了使用同位素系统（如176Lu-176Hf和146147 sm_142143nd）作为行星演化示踪剂的基础。然而，基于高精度的142测量，最近提出了高于球粒的Sm/Nd和Lu/Hf值，用于地球，月球和火星。这一假设可以用月球的147sm_l43nd和176Lu-176Hf同位素系统来验证。在这里，我们表明月球样品的Hf同位素组成受到宇宙射线照射在月球上产生的次级中子捕获的强烈影响，并提出了一个模型来纠正这些影响。在对中子俘获效应进行校正后，具有球粒Lu/Hf的月球形成KREEP的Lu-Hf模型年龄约为4.35-4.43 Ga，与月球分化时间的其他独立约束条件很好地吻合。低钛和高钛海玄武岩以及富含kreep样品的SmNd和Lu-Hf组合同位素系统为月球中Sm/Nd和Lu/Hf的球粒质提供了强有力的证据，并且不支持这两个比值高于球粒质值。考虑到地球和月球之间强烈的遗传联系，这一发现意味着地球的整体也以球粒状的Lu/Hf和Sm/Nd为特征。地球和月球的粒子体Sm/Nd意味着它们的Nd-142/ 144nd可以从月幔146sm_Nd-142等时线的粒子体Sm/Nd推断出来。由此产生的Nd-142/144Nd值将比普通球粒陨石高12ppm。由于地球、月球和球粒陨石具有相同的Sm/Nd，这种差异不可能是由146Sm衰变造成的，而一定是核合成的。推断出的月球和硅酸盐地球的Nd-142/ 144nd仅略低于现代陆地地幔的Nd-142/ 144nd，并且不能明确地与现代陆地地幔的Nd-142区分，这表明可接近的硅酸盐地球和硅酸盐地球之间的Nd-142差异比以前认为的要小。(C) 2013 Elsevier B.V.版权所有
d Refractory lithophile elements are generally considered to occur in chondritic relative abundances in terrestrial planets. This assumption forms the basis for using isotope systems such as 176Lu-176Hf and 146,147 sm_142,143Nd as tracers of planetary evolution. However, on the basis of high-precision 142Nd measurements, higher-than-chondritic Sm/Nd and Lu/Hf values have been recently proposed for the Earth, Moon, and Mars. This hypothesis can be tested using the combined 147 sm_l 43 Nd and 176Lu-176Hf isotope systematics of the Moon. Here we show that the Hf isotope compositions of lunar samples are strongly affected by capture of secondary neutrons produced during cosmic ray exposure on the Moon, and present a model to correct these effects. After correction for neutron capture effects, the Lu-Hf model age for the formation of KREEP from a Moon having chondritic Lu/Hf is ca. 4.35-4.43 Ga, in good agreement with other independent constraints on the timing of lunar differentiation. The combined SmNd and Lu-Hf isotope systematics of low- and high-Ti mare basalts and KREEP-rich samples provide powerful evidence for chondritic Sm/Nd and Lu/Hf in the Moon and do not support higher-thanchondritic values for these two ratios. Given the strong genetic link between the Earth and Moon, this finding implies that the bulk Earth is also characterized by chondritic Lu/Hf and Sm/Nd.A chondritic Sm/Nd in the Earth and Moon means that their Nd-142/144Nd may be inferred from the Nd-142/144Nd of the lunar mantle 146sm_Nd-142 isochron at a chondritic Sm/Nd. The resulting Nd-142/144Nd value would be 12 ppm higher than that of ordinary chondrites. Because the Earth, Moon, and chondrites share a common Sm/Nd, this difference cannot result from 146Sm decay, but must be nucleosynthetic in origin. The deduced Nd-142/144Nd of the Moon and the bulk silicate Earth is only slightly lower than and not unequivocally resolved from that of the modern terrestrial mantle, indicating that any Nd-142 difference between the accessible silicate Earth and the bulk silicate Earth is smaller than previously thought. (C) 2013 Elsevier B.V. All rights reserved.