Magmatism And Thermal Evolution On The Moon Recorded In Mare Basalts

The Moon is the only satellite of the Earth,and it is also the first step of our exploration on the deep space.Studying the volcanism of the Moon can not only uncover the thermal history and the nature of mantle sources(e.g.,composition,structure,chronology,etc.),but also has implications on magmatism and thermal evolution of other terrestrial planets such as our Earth.Studying lunar volcanism and thermal evolution is also one of the main scientific objectives of China's lunar exploration — Chang'e program,both Chang'e-3 and Chang'e-5 landed on the lunar nearside where volcanism is active,raising needs for investigations of lunar volcanism and thermal history.As the product of partial melting of the lunar mantle,basalt is the best choice to study the nature of lunar mantle and thermal history of the Moon.Because of weak geological modification process on the Moon,lunar basalts recorded and preserved information for volcanism and thermal evolution of the Moon over the past 4 billion years.Basalts cover ~17% of the lunar surface,most of which are exclusively exposed within giant impact basins on the nearside,forming extensive lava plains — mare,these basalts are thus called “mare basalts”.In the 1960 s and 1970 s,USA's Apollo programme and the Soviet's Luna programme returned a large number of mare basalt samples,in addition,basalts of varied types are also found in many lunar meteorites,providing important materials for investigation of lunar volcanism and thermal history.On November 24 th,2020,China successfully launched Chang'e-5 mission,the first lunar sample return mission since Apollo era,~1.73 kg of lunar soil was returned.Chang'e-5 landed near Mons Rümker,in the northern part of the largest mare on the lunar nearside — Oceanus Procellarum,this region hasn't been visited by previous missions.Volcanism near the landing site is active and prolonged,some of which can even extend to ~1 Ga,generally younger than all of the existing basalt samples(older than 2.8 Ga).Chang'e-5 sample thus represents new basalt type that has never been sampled yet which will fill the gap of lunar young volcanism and update our knowledge about lunar volcanism and thermal evolution.In this respect,this research will also play important roles in analysis of returned samples for China's deep space exploration in the future.This thesis firstly gives a brief overview of the volcanism on the Moon,including(1)the formation of the Moon and the early "magma ocean" evolution and(2)the mare volcanism.Then,we summarize the recent progresses of lunar mare basalts and point out several relevant problems which are currently poorly understood,including(1)the classification of mare basalts,(2)progresses of Apollo basalt samples,(3)progresses of basalts from lunar meteorites,(4)main problems of the research on lunar mare basalt samples,and(5)quantitatively petrographic analysis of lunar mare basalts.Then,we briefly introduce the purpose,significance and content of this thesis,samples and methods are described as well.Samples in this thesis include Apollo 11 high-Ti basalts,Apollo 15 low-Ti basalts,and lunar meteorite Dhofar 1428,methods include petrographic analysis,crystal size distribution analysis,in situ major and trace element analysis of minerals,and in situ U-Pb isotope dating of apatite,etc.Finally,we use these samples and methods to explore some aspects concerning lunar volcanism and thermal history:1.Understanding the textures of Apollo 11 high-Ti mare basalts.Determination of crystal size distributions(CSDs)represents a powerful tool to investigate textures and crystallization histories of mare basalts.While all basalts returned by the Apollo 11(A-11)mission were high-Ti,they form five compositional groups.This paper represents the first comprehensive CSD study of ilmenite and plagioclase in twelve high-Ti basalts from different A-11 basalt groups,in an investigation their textural development and cooling regimes.Generally,Group A basalts show the largest ilmenite CSD slopes and intercepts among all the A-11 and Apollo 17 high-Ti basalts,consistent with the fastest cooling rates.Group B1 basalts record two different cooling regimes for ilmenite,consistent with ilmenite starting to crystallize prior to eruption,but have the lowest plagioclase CSD slopes(along with Group A basalt 10017,12 and Group U basalt 10062,45)and thus slowest cooling rates among all A-11 samples.High-Ti basalts from Groups B3,U and one Group A sample(10072,53)reflect ilmenite accumulation or textural coarsening.As a relatively late crystallization phase that probably formed after eruption,plagioclase in the high-Ti basalts always exhibits linear CSDs representing a single cooling regime in each sample,regardless of kinked/curved CSDs of cogenetic ilmenite.Textures of samples in this study can be indicated by nucleation density: As an early crystallizing phase in high-Ti basalts,the magma is typically saturated with ilmenite upon eruption(and for Group B1 ones,this happened before eruption),when the magma was interconnected.Unimpeded by other earlier crystalline phases,free nucleation and growth resulted in large number of small crystals for ilmenite.Plagioclase,however,formed much later after eruption and only from isolated magma pockets,nucleation and growth was therefore impeded by other existing crystalline phases.The latent heat of crystallization and insulating effects of these earlier phases resulted in slower cooling and lower nucleation densities leading to limited and relatively larger plagioclase crystals compared to the majority of ilmenite.This scenario is invoked for most A-11 samples in this study.The only exceptions are the Groups B2 and B3 basalts,with steeper CSDs(faster cooling rates)and higher nucleation densities for plagioclase than basalts from other groups,indicating relatively earlier crystallization of plagioclase on the lunar surface,where the remaining lava was still relatively interconnected,facilitating a larger number of smaller crystals of plagioclase.Our study shows that the dominant control on texture of plagioclase and ilmenite in A-11 high-Ti basalts is cooling rate.Bulk composition exerts an influence on the texture of phases that initially crystallize upon eruption-ilmenite,but this influence is subordinate for later crystallizing phases-plagioclase.2.The cooling regime of Apollo 15 low-Ti olivine-normative mare basalts.Determination of crystal size distributions(CSDs)represents a powerful tool to investigate textures and crystallization histories of mare basalts.Apollo 15(A-15)low-Ti mare basalts have traditionally been subdivided into olivine-and quartz-normative basalt types,based on their different whole-rock compositions.The A-15 olivine-normative basalts have abundant olivine crystals to be analysed texturally.CSDs have been conducted on olivines from 18 A-15 olivine-normative basalts of varied compositions and textures,in order to study their respective cooling regimes and crystallization histories.We found that CSD profiles of olivine from all A-15 samples studied here show general similarities: they all curved or kinked up and can be divided into “large” and “small” populations at about 0.25-0.4 mm,indicating their similar textures and cooling histories.The curved olivine CSDs indicate some A-15olivine-normative basalts have experienced olivine accumulation and/or textural coarsening,while kinked CSDs suggest two separate cooling events and mineral analyses will be conducted to investigate if these cooling events occurred in the same(phenocrysts and matrix)or separate magmas(magma mixing).Though with some overlaps,A-15 basalts generally have larger olivine CSD slopes and/or intercepts than basalts from other Apollo missions,suggesting the fastest cooling rates.Different from most A-15 samples in this study which are well in the region of endogenous basalts in the plot of olivine CSD slope-intercept,two distinctive samples(15676,12 and15557,94)with largest CSD slopes and intercepts are plotted with the impact melts,which means they may be of impact origin.However,whole rock siderophile element contents comfirm that they are endogenous mare basalts,indicating that fast cooling rate is the controlling of both impact and endogenous lunar melts,rather than formation mechanism.Futher chemical analyses of olivine can help interpret the origin and evolution of A-15 olivine basalts.3.Oldest high-Ti basalt in lunar meteorite Dhofar 1428.We conducted a thorough analysis of the breccia meteorite Dhofar 1428 with the aim of better understanding the magmatism and thermal evolution of the Moon,from which,basalt clasts are of the greatest concern,with the hope of new addition to Apollo mare basalt samples.This sample comprises a heterogeneous array of lithic fragments including magnesian and ferroan anorthositic granulites,mafic granulites,basalts,and different kinds of impact melt rocks.In which,an evolved high-Ti basalt clast comprising large zoned pyroxene was observed.Based on equilibrium melt calculations of mineral zonations from this basalt,Mg-pyroxene cores were interpreted to be formed from a light rare earth element(LREE)enriched liquid,whereas the Fe-pyroxene rims grew from an LREE-depleted magma.We propose that LREE-depleted signature of Fe-pyroxene results from co-crystallization with apatite.The Mg-pyroxenes suggest that enriched liquids with higher REE contents and different REE patterns relative to KREEP existed within lunar interior.Oscillating Ti/Al ratios across pyroxene in this basalt may indicate several magma recharge events or crystal movement within a zoned magma chamber.This feature illustrates that magmas were derived from a variety of sources around the time of formation of this basalt.In situ U-Pb dating was conducted on apatite grains within this basalt,the excellent consistence between the U-Pb Concordia age(3941±24 Ma;2?)and 207Pb/206 Pb isochron age(3934±24Ma;2?)indicates the most likely crystallization age of this high-Ti basalt at ~3940 Myr,making it one of the oldest high-Ti basalts observed on the Moon.In conclusion,the results presented in this thesis can improve our understanding of lunar mare volcanism and thermal evolution,including the cooling regime and evolution history of different Apollo mare basalts,compositions and chronology of basalt clasts in lunar meteorites and their implications to lunar volcanism.Our work can provide implications for probing the thermal evolutionary patterns of Earth and other terrestrial bodies,and comparative planetology studies.In addition,our work can also provide references to the scientific implementation of China's ongoing lunar exploration program — Chang'e missions.