Ni,Sm Isotope Constraints On The Impact Process And Lunar Soil Formation Of Chang’e-5

The impact of asteroids on terrestrial planets is crucial to their evolution and intimately linked to their habitability.Due to its lack of atmosphere and surface processes,the moon effectively records the impact history of the Earth-Moon system.Impact events also play a critical role in driving young extraterrestrial geological processes on the lunar surface,contributing significantly to the formation of lunar regolith.The recent Chang’e-5 mission,which successfully returned samples of lunar regolith and basaltic debris,offers direct access to lunar materials for cosmochemistry study.The mantles of terrestrial planets and the moon are depleted in siderophile elements relative to chondrites.Therefore,late addition of chondrites or iron meteorites could shift the elemental concentration and isotopic composition of siderophile elements in mantles.Nickel(Ni)is a typical siderophile element,and its elemental concentration and isotopic composition could possibly indicate the presence of extraterrestrial materials.In addition,the samarium(Sm)isotope 149Sm has a large neutron capture cross section,so changes in its isotopic ratio are sensitive to neutron capture reactions.The cosmogenic Sm isotope abundance varies with distance from the exposure surface.Comparison of Sm isotopic composition between lunar regolith and model prediction has been used to indicate the impact process and provide chronological information on the lunar regolith formation.Therefore,in this study,we analyzed the stable isotopes of Ni,as well as the cosmogenic Sm isotopes,in samples from the Chang’e-5 lunar landing site and lunar meteorites to investigate the effects of impact processes on the lunar regolith formation.To investigate the impact-induced accretion processes affecting the lunar surface in the landing area of the Chang’e-5 mission,we utilized multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS)to determine Ni stable isotopic compositions of lunar regolith and basalt samples,and calculated the Ni content using isotope dilution method.Chang’e-5 lunar regolith samples exhibit significant elevated Ni abundances and high δ60Ni values(0.65-1.06‰)relative to the lunar basalt samples.The extremely high δ60Ni values are unlikely to be caused by magma evolution or simple mixing between lunar rocks and exotic materials.The most plausible explanation is that the hypervelocity impact volatilizes the impactor with high Ni content,resulting in the loss of Ni.This process is accompanied by the fractionation of Ni isotopes,which enriches the residual solids in heavy Ni isotopes.The addition of the residual solids is the reason for the heavy Ni isotopic composition of the Chang’e-5 lunar soil.This result shows that the impact of meteorites played an important role in the formation of lunar soil in the Chang’e-5 landing area.The Ni isotope compositions of Chang’e-5 lunar soil at different depths varied,indicating that different regions were differently affected by impact accretion and that Chang’e-5 lunar soil contains ejecta from different regions.Chang’e-5 lunar soil below 60 cm depth was directly ejected from the impact in which a large impactor was volatilized and mixed in.Furthermore,in order to investigate the age of lunar regolith disturbance in the landing area of the Chang’e-5 mission,this study used thermal ionization mass spectrometry(TIMS)to measure the cosmogenic Sm isotopes,149Sm and 150Sm,in lunar regolith.Within the depth range of 0-60 cm,there was no significant variation inε149Sm and ε150Sm in Chang’e-5 lunar regolith with depth.This relatively uniform Sm isotope composition reflects the disturbance caused by recent impact.According to the cosmogenic Sm isotope abundance,the deep lunar regolith deeper than 60 cm had an exposure age of approximately 255 million years,which is contemporaneous with the formation of two nearby large impact craters,IC-396(Xu Guangqi)and IC-265.Shallow regolith was from different impact craters as a result of several recent asteroid impact in the vicinity of Chang’e-5 landing site.Based on the analyses of Ni and Sm isotopes and previous research on the landing site of Chang’e-5,it is inferred that:about 255 million years ago,the vicinity of Chang’e5 landing area was hit by an iron meteorite with a diameter of about 17m.During this process,about 4.9%of the Ni in the impactor was volatilized and Ni isotopes were fractionated,and the residue of the impactor was mixed with the debris breccia generated by the impact.As a result,there is an ejecta blanket enriched in Ni elements and characterized by an extremely heavy Ni isotopic composition.These materials underwent further space weathering and deposited as the current deeper Chang’e-5 lunar regolith.Later some recent smaller impacts occurred in the vicinity of Chang’e-5 landing area,which caused the multi-stage deposition of the Chang’e-5 shallow lunar regolith from the ejecta blanket and other regions.