| Although mare basalts collected by the Apollo missions have established the petrologic model and magmatic evolution of mare basalts,limited area of the sampled zones make them difficult to represent the complete eruptive products of lunar volcanic activities.Lunar meteorites can provide important supplements to Apollo mare basalt samples by recording potentially new information about volcanic events far removed from the Apollo landing sites.In 2020,the Chang’e-5(CE-5)probe completed a historic delivery of lunar samples from the young mare basalt unit in the northern Oceanus Procellarum with a mass of 1731 g to the Earth,providing essential samples for investigations of lunar late-stage volcanism.Considering the CE-5 samples collected from young mare basalt unit and basaltic lunar meteorite samples ejected from surface regions far removed from the Procellarum KREEP Terrane,we can interpret the spatio-temporal extension of lunar volcanism.Additionally,the spectroscopic characterization of the CE-5 lunar soils can provide essential complement to chemical analyses on the samples and remote sensing,strengthening scientific understanding of the pedogenesis and space weathering in the past 2 billion years on the lunar surface.In this study,we investigated two basaltic breccia meteorites(NWA 7611 and NWA 10480)and three CE-5 samples(breccia CE5C0400-B006,scooped soil CE5C0600,and drilled soil CE5Z0107)to gain insights into the mineralogical,petrological,geochemical,and spectroscopic characteristics of lunar mare basalt units.The goal of this thesis is to better understand lunar volcanic activity and the evolutionary processes of basaltic magma based on both sample and remote sensing data.The main findings of this study are as follows:(1)Lunar ancient volcanism recorded by basaltic lunar breccia meteoritesThe lunar basaltic breccia meteorites NWA 7611 and NWA 10480 contain abundant very low-titanium volcanic materials and a series of highland rock fragments.Fine-grained basalt,coarse-grained gabbroic pluton,and volcanic glass are the products of magma differentiation with different emplacement depths and eruption style during the same magmatic activity.They provide insight into the evolution of the magma chamber in the shallow lunar crust,where magma formed through partial melting of the lunar mantle.During this magmatic activity,Mgrich olivine and pyroxene crystallized from the magma and sank to the bottom of the magma chamber,forming olivine gabbro cumulates.As plagioclase began to crystallize,some melts containing the early-formed phenocrysts erupted to the lunar surface,resulting in the formation of very low-titanium volcanic glass and porphyritic mare basalt.Within the magma chamber,plagioclase continued to crystallize and buoyantly rise towards the top,forming anorthositic gabbro cumulates.The remaining melts with highly evolved compositions were then fractionated through silicate liquid immiscibility,resulting in the formation of ferroan gabbro including symplectic mineral assemblages.After the cessation of basaltic magmatic activity,the gabbroic lithologies formed in the shallow lunar crust were transported by large impact events into the lunar cryptomaria,which had been covered by highland ejecta materials.Simultaneously,the local heating induced by impact events facilitated the decomposition of Fe-rich pyroxene rims,leading to the formation of various symplectites consisting of fayalite,silica,and pyroxene.However,the black vermicular intergrowths observed are void spaces formed due to the decrease in volume from the breakdown reaction.The continuous impacts on the lunar surface led to multiple small craters that caused vertical mixing between the mare basalt of the cryptomaria and highland ejecta materials.This mixture was cemented into breccia containing a significant amount(5866 wt.%)of mare component.These brecciated rocks were eventually ejected into space through small impact events during the Copernican period,forming the paired YQEND meteorites.The Cleomedes located in the north of Mare Crisium.as well as the southeastern region of Mare Marginis,are possible source regions of the YQEND meteorites.(2)Magmatic evolution and pedogenetic processes recorded by CE-5 soilsThe scooped and dilled soils may indicate the magmatic evolution and pedogenetic processes of Chang’e-5 basalt.Due to the rapid cooling of the surface lava flows corresponding to the scooped soil,the compositions of olivine and pyroxene crystals were highly evolved.In fact,most of the pyroxene compositions extend into the pyroxene "forbidden zone" and broke down into fayalite,silica,and Fs-rich augite.This contributed to the increased abundance of olivine in the CE-5 soils.The deeper lava flow corresponding to the dilled soil,on the other hand,maintained a higher temperature with a slower cooling rate,resulting in the formation of relatively Mg-rich mafic minerals.However,the highly differentiated residual melts led to the contraction of the pyroxene field in phase diagram,which resulted in the crystallization of fayalite.This ultimately contributed to the relatively abundant olivine found in the CE-5 drilled soil.Interestingly,the presence of multiple volcanic activities in the CE-5 landing region suggests that these Mg-rich soils could be sourced from different lava flows corresponding to various types of lunar soils.The CE-5 lunar soil samples provide insights into the impact history and space weathering of the lunar surface over the past 2 billion years.The occurrence of pressure-sensitive minerals(e.g.,quartz and feldspar glass)indicate that the pressure conditions during shock metamorphism at the sampling site varied between 17 and 25.8 GPa,with a transient shock temperature of 238-336℃ and a postshock temperature range of 168-200℃.The soil at the landing region is a complex mixture of local mare basalts and ejected feldspathic materials from large impact craters,with non-mare materials in breccia CE5-B006(～10%),scooped soil CE5C0600(5～7%),and drilled soil CE5Z0107(1～3%).The Aristarchus Plateau located in the Oceanus Procellarum is the best candidate source of exotic ejecta materials.The Eratoshenianaged mare basalts were ejected into the CE-5 landing site by the Xu Guangqi cratering event around 70-500 million years ago,and the predominant micrometeoroid bombardment in space weathering facilitated the pulverization and fragmentation of fresh basalt debris and coarse crustal materials,producing relatively mature lunar soils.However,the subsurface coarsegrained regoliths were transported to the landing site by lateral and vertical mixing from subsequent small impact events,forming soils scooped by CE-5 mission which contains an appreciable fraction of coarse rock fragments and mineral grains.The drilled sample CE5Z0107 was collected from the deep deposit of ejecta materials of the Xu Guangqi crater,and the fresh rock debris were deeply buried and fragmented.Smaller impacts are more frequent and further pulverize the coarse-grained regolith in the subsurface,facilitating the formation of relatively coarse drilled soils.(3)The evolution of basaltic magmatism on the MoonRemote sensing observations suggest that the spectral characteristics of small impact craters at the landing site are consistent with those of the CE-5 soils,characterized by a strong 1-μm absorption feature and relatively weak 1.2-μm and 2-μm absorption.The differences between scooped soil and drilled soil possibly result from different mineral phases.The absorption features in visible and near-infrared spectra of scooped soil are likely contributed by ferroaugite,Fe-rich glass,and fayalite,which are representative of predominant Fs-rich augite and Fe-rich basaltic glass.The spectral characteristics of CE-5 drilled soil are mainly contributed by pyroxene and olivine,with higher abundance of olivine causing more significant spectral absorption features.The variation in spectral properties between scooped soil and drilled soil is attributed to the effects of space weathering,in which suppression of spectra absorption features is incomplete due to low agglutinate contents of the dilled soils,and the absorption characteristics of mafic minerals are dominant.The basaltic volcanic activity on the lunar surface displays a chemical evolution pattern,starting with little-fractionated very low-titanium and/or high-aluminum(Al)basalts that are older in age.This trend continues with Imbrian-aged low-Ti mare plains and ends with young mare basalt with Fe-and Ti-rich compositions.Late-stage mare basalt has a different SiO2 concentration from the products of old lunar volcanism.The FeO enrichment is most likely from partial melting of late-stage ilmenite-rich cumulates of the lunar magma ocean in the mantle source and high-degree fractional crystallization of parent magma.Lunar lava flows tend toward a Fe-and Ti-rich composition instead of a Si-and Al-rich composition of magma.However,the decrease of SiO2 concentration in late-stage mare basalt is due to the melting of Ti-rich cumulate materials in the low-Ti mantle source.Highly fractional crystallization controls the evolution of lunar basaltic magma,and silicate liquid immiscibility fractionates various late-stage mineral assemblages with varying textures. |
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