Impact glasses are abundant in the lunar regolith, and Mg isotopes have the potential to trace components from various lunar crustal reservoirs, which have recently been shown to exhibit large Mg isotopic fractionations. However, it remains unclear whether Mg isotopic fractionation occurs during the formation of impact glasses. In this study, we report Mg isotopic and elemental compositional data for agglutinatic glasses returned by the Chang’e 5 mission and obtained using the laser ablation split stream technique. Vesicular textures, Fe–Ni alloys, tiny Fe droplets, and high Ni contents suggest the studied agglutinatic glasses had an impact origin. The agglutinatic glasses exhibit large Mg isotopic fractionation, with δMg values ranging from −1.36 ‰ to −0.01 ‰. The lack of correlations between δMg values, Ni contents, and ratios between volatile and relatively refractory elements (K/La, Rb/Sr, and Ce/Pb) indicate the addition of a meteoritic component and evaporation was not the major process responsible for the measured Mg isotopic variations. In fact, the MgO profiles and correlations between δMg and MgO, NaO, Sc, Sr, CaO/AlO, and δEu reflect Mg isotopic fractionation caused by Mg diffusion from a region with high Mg contents (i.e., more melted pyroxene) to one with lower contents (i.e., more melted plagioclase). Diffusion modeling shows that the duration of diffusion was less than a fraction of a second. Our results indicate that chemical diffusion can produce large Mg isotopic fractionation in impact glasses on a scale of at least tens of microns, and that isotopic fractionation driven by chemical diffusion needs to be considered when the Mg isotopic compositions of impact glasses are used to identify different lunar rock reservoirs.

中文翻译：

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撞击引起的化学扩散引起月球凝集玻璃中的大量镁同位素分馏


月球风化层中富含撞击玻璃，镁同位素有可能追踪各种月地壳储层的成分，最近已证明这些储层表现出大量的镁同位素分馏。然而，目前尚不清楚在冲击玻璃的形成过程中是否发生镁同位素分馏。在这项研究中，我们报告了嫦娥五号任务返回的凝集玻璃的镁同位素和元素成分数据，并使用激光烧蚀分流技术获得。多孔结构、铁镍合金、微小的铁滴和高镍含量表明所研究的凝集玻璃具有撞击起源。凝集玻璃表现出较大的 Mg 同位素分馏，δMg 值范围为 -1.36 ‰ 至 -0.01 ‰。 δMg 值、Ni 含量以及挥发性元素和相对难熔元素（K/La、Rb/Sr 和 Ce/Pb）之间的比率之间缺乏相关性，表明陨石成分的添加和蒸发并不是造成这种现象的主要过程。测量镁同位素变化。事实上，MgO 分布以及 δMg 与 MgO、NaO、Sc、Sr、CaO/Al2O 和 δEu 之间的相关性反映了 Mg 同位素分馏，这是由于 Mg 从高 Mg 含量区域（即更多熔化的辉石）扩散到具有高 Mg 含量的区域（即熔化程度更高的辉石）引起的。含量较低（即更多熔化的斜长石）。扩散模型显示扩散持续时间不到一秒。我们的结果表明，化学扩散可以在冲击玻璃中产生至少数十微米尺度的大镁同位素分馏，并且当使用冲击玻璃的镁同位素组成来识别不同的镁同位素成分时，需要考虑由化学扩散驱动的同位素分馏。月球岩石储层。