Elemental differentiation and isotopic fractionation during space weathering of Chang'E-5 lunar soil

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 Al2O3 (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/Al2O3 (2.27-1.51) and MgO/Al2O3 (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 P2O5 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 87Sr/86Sr (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), 87Rb/86Sr (0.06) and present-day 87Sr/86Sr (0.701542) than the mean composition of reported basaltic clasts (Mg#: -28; 87Rb/86Sr: -0.038; 87Sr/86Sr: -0.700941), possibly implying that the bedrocks in CE5 landing site consist of multiple magma pulses. The delta 56Fe (0.122 +/- 0.002 %o to 0.199 +/- 0.008 %o) and delta 26Mg (-0.204 +/- 0.016 %o to -0.109 +/- 0.006 %o) of sieved CE -5 soils increase with decreasing particle sizes but their delta 44/42Ca (0.38 +/- 0.04 %o to 0.44 +/- 0.02 %o) 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 delta 56Fe and delta 26Mg in fine fractions at given very limited depletion in FeO and MgO. The observed positive correlation between delta 56Fe and delta 26Mg, 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.