Sample Study,Remote Sensing,and Magmatic Evolution Of Lunar Mare Basalts

Lunar mare basalts represent magma sourced from partial melting of the lunar mantle and eruption to the lunar surface,although occupying minor volumes(<1%)in the lunar crust,their temporal and spatial distributions and petrogenesis indirectly record the geochemical signatures of deep reservoirs and related basaltic magmatic evolution processes.Current knowledges of lunar basaltic magmatic evolution history are constrained by petrologic models of basalt sampled by Apollo and Luna missions.However,these samples are returned from sparse sites,which only cover a relatively small area(?4.7%)of the lunar nearside,and are not representative of lunar global basaltic magmatism and the relationship between extrusive mare volcanism and intrusive basaltic magmatism.Basaltic lunar meteorites provide an important supplement to the inventory of lunar basalts and improve our understanding of the mineralogy,geochemistry,and chronology of lunar basaltic magmatic evolution.Lunar meteorites sampling random and unknown regions of the Moon have contributed to build connections between samples and lunar remote sensing.Basaltic samples and meteorites provide important ground-truth for remote sensing data and allow extended calibrations of other regions which have been not yet sampled and enable us to interpret lunar global basaltic magmatism.Remote sensing of lunar mare has revealed diversity and complexity of basaltic magmatism whose interpretations require systematic integration of datasets and petrologic knowledges from samples and meteorites.Therefore,the best understanding of lunar basaltic magmatic evolution should be from the comprehensive investigations of returned samples,meteorites,and remote sensing.The divergences between accurate analyses of basaltic samples/meteorites in terrestrial labs and large scale lunar orbital observations of mare regions still exist owing to the geographic limitation of returned samples and rarity of basaltic lunar meteorites.In recent years,more and more newly found basaltic lunar meteorites and newly released lunar orbital remote sensing products provide valuable opportunity for integrated studies of lunar basaltic magmatic evolution.This work applies mineralogical,geochemical,and petrological analyses of four basaltic lunar meteorites(i.e.,NWA 4734,NWA 10597,NWA 10985,and Swayyah 001)to compare their igneous texture,modal mineralogy,mineral compositions,major and trace elemental concentrations,and metamorphic characteristics,discuss the potential paring relationships among them and other basaltic lunar meteorites.NWA 4734,NWA 10597,and LAP mare basalts probably originate from the same volcanic episode(similar ages)and crystallized at subtly different cooling rates(various grain sizes and silica polymorphs).Their source lava flows may have been emplaced on the same region of lunar surface and these mare basalts underwent the same impact events on the Moon(similar shock-induced partial melting).The lithologies(e.g.,olivine gabbro,anorthositic gabbro,and gabbro),modal mineralogy,and mineral compositions of most lithic clasts in NWA 10985 resemble those in NWA 773 clan.Mineral compositions of the prominent gabbro clast in NWA 10985 are in accordance with the mineral compositional evolution trend of NWA 773 clan,while the bulk ITE concentrations of NWA 10985 gabbro do not fit into the chemical trend of NWA 773 clan,representing more complex crystallization regime.These meteorites are corresponding to different episodes of lunar basaltic magmatic evolution.Diverse lithic clasts in NWA 10985 and NWA 773 clan present the evolution of lunar crustal magma chamber:the sinking of early crystallized Mg-rich olivine and pyroxene formed the olivine websterite/olivine gabbro cumulates at the bottom of magma chamber,eruption of some early stage melts to the lunar surface formed the olivine phyric basalt,the rising of plagioclase formed the anorthositic gabbro cumulates at the top of magma chamber,evolved gabbro/ferroan gabbro formed in pockets of Fe-rich late stage melt,some of which erupted to the lunar surface and formed mafic(black)glass beads.The compositional inconsistency between gabbro and ferroan gabbro probably indicates an open system:replenishment of primitive magma during the evolution of magma chamber,that are supported by oscillatory zoning pyroxene phenocrysts in gabbro.The remarkable texture of Swayyah 001 demonstrates the picture of solidification in lunar crustal magma chamber:equigranular Mg-rich pyroxene and plagioclase crystallized under approximately equilibrium condition formed gabbro cumulates,the contraction of late stage residual melts formed Fe-rich pockets in Mg-rich cumulates,accelerated cooling in these melt pockets resulted in compositionally zoned mafic minerals and mesostasis.Mare basalt meteorites(NWA 4734,NWA 10597,and LAPs)represent the final phase of lunar basaltic magmatic evolution(ascent,eruption).The crystallization conditions(e.g.,liquidus temperature)and evolved nature of these mare basalts are consistent with fractionation of early formed chromite and olivine,which is corresponding to density-driven fractionation likely occurred in some form of lunar crustal magma chamber.Evolved mineral(e.g.,olivine and pyroxene)compositions were derived from fast cooling of the ascent and erupted magmas.Dense sampling quantitative EPMA maps of NWA 4734 and NWA 10597 highlight multi-stage evolution of their parental magma:crystallization of Mg-rich olivine and pyroxene with compositional zoning,resorption of olivine by melt and rising growth rate of pyroxene,plunge of pyroxene growth rate caused by contraction of pyroxene field in MgO-FeO-SiO2 phase diagram and reappearance of Fe-rich olivine in the liquid line of descent.The various textures of mesostais regions in these mare basalts present the role of silicate liquid immiscibility in the evolution of late stage melt.The petrogenetic relationships between basaltic plutons and mare basalts indicate that regularity of lunar magmatism revealed by returned samples(volcanism is typically younger than plutonic magmatism)is an illusion owing to the variation of impact flux.Extrusive volcanism could occur simultaneously with intrusive magmatism,however ancient volcanics couldn't be reserved due to heavy bombardments and few young plutons could be exposed owing to the lack of impact events with deep excavations.According to literature review of mineralogy,geochemistry,and chronology of mare basalt samples and meteorites,this work proposes a classification strategy of basalts based on mare lava flow units.Combined with compiled modal age datasets of mare units,the temporal and spatial distributions of mare basalts were mapped.By considering compositional and geochronological analyses of paired basaltic meteorites and orbital remote sensing measurements,their potential source regions on the lunar surface were constrained.NWA 4734,NWA 10597,and LAP mare basalts are likely from young mare units in Oceanus Procellarum,Mare Imbrium,Mare Serenitatis,and Mare Insularum.It could be expected that these young mare basalts around PKT were mixed with non-mare materials by continuous impact gardening and a Th-rich surface regolith layer was developed.The potential source areas of NWA 10985 and NWA 773 clan are constrained by the extrusive lithologies(basalt clasts)within them,and are likely young lava flows in Oceanus Procellarum and Mare Imbrium adjacent to non-mare ejecta and pyroclastic deposits.Although with unknown crystallization age,Swayyah 001 is possibly from pluton related to young(3.2-2.17 Ga)units in Oceanus Procellarum according to compositional matches.The ITE compositions of Swayyah 001(as a plutonic sample)are distinct from KREEP,but the related lava flows could incorporate KREEPy non-mare components during their transportation in the lunar crust or flowing across the lunar surface.With integration of basaltic lunar meteorite analyses and mare basalt unit remote sensing,this work transfers petrologic characteristics observed in samples/meteorites to interpret mineralogical and geochemical evolution observed in orbital datasets.The trend of SiO2 undersaturation and FeO enrichment of young mare basalts could be originated from silicate liquid immiscibility in late stage basaltic magmas,that is further supported by the geographical proximity of Fe-rich mare basalts and non-mare silicic volcanism.Young lava flows in Oceanus Procellarum and Mare Imbrium with high TiO2 contents couldn't be generated by fractionation of low-TiO2 parental liquid,and thus require contribution from more ilmenite-rich cumulates for their petrogenesis.The partial melting of these late stage cumulates from lunar magma ocean could accounts for the enrichment of ITE in young mare units.Although the youngest mare basalts are located within PKT,KREEP is not a prerequisite for the trigger of the youngest lunar volcanism.The ITE enrichment in young lava flow units also could be involved with contaminant of non-mare materials(assimilation,impact mixing,etc.).According to the different enrichment mechanisms of olivine with different solid solution compositions,the fayalitic olivine-rich spectral and mineralogical features in young mare basalt units are indicative of pyroxene field contraction in phase diagram or breakdown of metastable pyroxferroite.Remote sensing applications of petrochemical indices also support higher differentiation degrees for younger mare basalts.By integrating basaltic lunar meteorites and remote sensing of mare basalt units,this work establishes the connection between basaltic plutonic magmatism and effusive/explosive volcanism,contributes to understand lunar basaltic magmatic evolution,and presents paradigm of integrated studies of sample analyses and remote sensing investigations of landing,roving,and sampling sites for future lunar explorations(e.g.,Chang' e-5?Chang' e-8).