To investigate the chemical variation during space weathering of young mare basalts, here we report elemental, radiogenic Sr-Nd and stable Fe-Mg-Ca isotopic data of Chang’E-5 sieved soils and breccias. From the coarse fraction to the fine one, the sieved soils display increasing AlO (10.34 wt%–13.36 wt%) and Sr (248 ppm–307 ppm) but decreasing FeO (23.50 wt%–20.22 wt%), MgO (6.88 wt%–5.78 wt%), FeO/AlO (2.27–1.51) and MgO/AlO (0.67–0.43). The contents of rare earth elements (except Eu) and high field strength trace elements do not vary with particle size but correlate with PO contents. Given the limited contribution from contamination by meteorites and exotic materials ejected far away from the landing site, these elemental variations can be explained by differential comminution and distribution behaviors of plagioclase and mesostasis phases. These sieved soils yield a Sm-Nd isochron age (1.84 ± 0.83 Ga) comparable to that of basaltic clasts obtained by U-Pb dating (∼2.0 Ga). However, their Rb-Sr isotopic system is disturbed as indicated by their relatively homogeneous Sr/Sr (0.701425–0.701592) despite variable Rb/Sr (0.017–0.028). These results suggest the Sm-Nd isotopic system is more robust to impact disturbance during space weathering compared to the Rb-Sr isotopic system. Given that the bulk soil still plots on the 2.03 Ga Rb-Sr reference isochron from the pristine plagioclases in CE-5 basalts, this disturbance did not affect the Rb-Sr isotopic system on the bulk scale. The CE-5 bulk soil has higher Mg (33.6), Rb/Sr (0.06) and present-day Sr/Sr (0.701542) than the mean composition of reported basaltic clasts (Mg: ∼28; Rb/Sr: ∼0.038; Sr/Sr: ∼0.700941), possibly implying that the bedrocks in CE-5 landing site consist of multiple magma pulses. The δFe (0.122 ± 0.002 ‰ to 0.199 ± 0.008 ‰) and δMg (−0.204 ± 0.016 ‰ to −0.109 ± 0.006 ‰) of sieved CE-5 soils increase with decreasing particle sizes but their Ca (0.38 ± 0.04 ‰ to 0.44 ± 0.02 ‰) are relatively homogeneous. Mass balance modelling indicates that differential comminution has limited influence on the Fe-Mg-Ca stable isotopic compositions. We further dismiss the role of solar-wind sputtering, as Ca and Mg are more susceptible to sputtering and thus would be expected to show larger isotope fractionations compared to Fe, which is inconsistent with the observations. Free evaporation may explain the elevated δFe and δMg in fine fractions at given very limited depletion in FeO and MgO. The observed positive correlation between δFe and δMg, however, is much steeper than the slope expected for free evaporation, indicating also other mechanisms (e.g., Fe-Mg inter-diffusion). Since the CE-5 soil has a unique composition compared with Apollo and Luna soils, the chemical differentiation identified in this study provides new insights for establishing a connection between the chemistry and reflectance spectral properties of lunar soil. Our combined Fe-Mg-Ca isotopic study also provides a paradigm to distinguish the role of solar-wind sputtering and impact evaporation, and shows that the inter-particle diffusion process may be an important mechanism for the isotope fractionation among lunar soil components.

中文翻译：

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嫦娥五号月壤空间风化过程中的元素分异和同位素分馏


为了研究年轻母玄武岩在空间风化过程中的化学变化，我们在此报告了嫦娥五号筛分土壤和角砾岩的元素、放射性 Sr-Nd 和稳定 Fe-Mg-Ca 同位素数据。从粗粒到细粒，筛分的土壤显示 Al2O (10.34 wt%–13.36 wt%) 和 Sr (248 ppm–307 ppm) 增加，但 FeO (23.50 wt%–20.22 wt%)、MgO (6.88 wt%) 减少%–5.78 wt%)、FeO/Al2O (2.27–1.51) 和 MgO/Al2O (0.67–0.43)。稀土元素（Eu除外）和高场强微量元素的含量不随颗粒尺寸变化而变化，但与PO含量相关。鉴于远离着陆点的陨石和外来物质的污染贡献有限，这些元素变化可以通过斜长石和中稳态阶段的不同粉碎和分布行为来解释。这些筛分土壤得出的 Sm-Nd 等时线年龄 (1.84 ± 0.83 Ga) 与通过 U-Pb 测年获得的玄武岩碎屑的年龄 (∼2.0 Ga) 相当。然而，尽管 Rb/Sr (0.017–0.028) 存在差异，但它们的 Rb-Sr 同位素系统受到干扰，其相对均匀的 Sr/Sr (0.701425–0.701592) 表明了这一点。这些结果表明，与 Rb-Sr 同位素系统相比，Sm-Nd 同位素系统对空间风化期间的冲击扰动更加稳健。鉴于大块土壤仍然绘制在 CE-5 玄武岩中原始斜长石的 2.03 Ga Rb-Sr 参考等时线上，这种干扰不会影响大块尺度的 Rb-Sr 同位素系统。 CE-5 块体土壤的 Mg (33.6)、Rb/Sr (0.06) 和现今的 Sr/Sr (0.701542) 高于报道的玄武岩碎屑的平均成分（Mg：~28；Rb/Sr：~0.038； Sr/Sr: ∼0.700941)，可能意味着 CE-5 着陆点的基岩由多个岩浆脉冲组成。 δFe (0.122 ± 0.过筛CE-5土的002‰~0.199±0.008‰）和δMg（-0.204±0.016‰~-0.109±0.006‰）随着粒径的减小而增加，但Ca（0.38±0.04‰~0.44±0.02‰）相对较小同质。质量平衡模型表明，差别粉碎对 Fe-Mg-Ca 稳定同位素组成的影响有限。我们进一步忽略了太阳风溅射的作用，因为 Ca 和 Mg 更容易受到溅射的影响，因此与 Fe 相比，预计会显示出更大的同位素分馏，这与观测结果不一致。自由蒸发可以解释在 FeO 和 MgO 的消耗非常有限的情况下，细粒部分中 δFe 和 δMg 的升高。然而，观察到的 δFe 和 δMg 之间的正相关性比自由蒸发预期的斜率陡得多，这也表明了其他机制（例如 Fe-Mg 相互扩散）。由于与阿波罗和月球土壤相比，CE-5 土壤具有独特的成分，因此本研究中确定的化学差异为建立月球土壤的化学和反射光谱特性之间的联系提供了新的见解。我们的 Fe-Mg-Ca 同位素联合研究还提供了区分太阳风溅射和撞击蒸发作用的范例，并表明颗粒间扩散过程可能是月壤成分中同位素分馏的重要机制。