Chemical evolution of the Moon:A review
SUN Wei-dong1,2,LING Ming-xing1,3 and Qing-Zhu YIN4 1.CAS Key Laboratory of Isotope Geochronology and Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China;2.Research Center for Mineral Resources,School of Earth and Space Sciences,University of Science and Technology of China,Hefei 230026,China;3.Graduate University of Chinese Academy of Sciences,Beijing 100049,China;4.Department of Geology,University of California,Davis,CA 95616,USA
The Moon has experienced chemical differentiation,and is composed of crust,mantle±a small metallic core.Multiple observational evidences show that there was a magma ocean on the Moon,and the crystallization,and differentiation of the magma ocean dominated the chemical evolution of the Moon.Presently,the most popular idea about Moon's origin,which is consistent with planet formation theory in a protoplanetary disk,is that a Mar-sized planetary embryo had a grazing impact with the proto-Earth with 90%accretion completed,induced total melting of the Earth,forming a global magma ocean.The ejected molten materials beyond the Roche limit accreted rapidly,forming the Moon,followed by crystallization and differentiation.Given that there was no plate tectonics and its small size,differentiation of magma ocean are the dominant pathway of chemical evolution in the Moon,augmented by subsequent high flux impact at around 3.8-4.0 Ga ago.About 80%-85%of the magma ocean of the Moon had solidified in 100 Ma after the giant impact,probably because of the small volume and lack of atmosphere.The Moon is rather poor in water,therefore plagioclase crystallized early as the magma ocean cooled down.Plagioclase floats to the surface of magma ocean,because of its lower density than basaltic magma,while other denser minerals like olivine accumulate at the bottom.As the crystallization and differentiation proceed,the residual magma was getting progressively enriched in incompatible elements,including K,U and other radioactive elements.At the same time,the denser ilmenite started to crystallize,which caused an unstable situation of denser ilmenite lying on top of olivine cumulates,which ensured subsequent mantle overturn and gravitationally a series of magmatic activities,with unique magnesian,alkaline series and other rock formation.Because of the low oxygen fugacity on the Moon,valence state of Eu is mainly in the form of +2;therefore highland plagioclase is highly enriched in Eu,whereas all of the other rocks are Eu depleted.At lower oxygen fugacity,Re behaves as a highly siderophile element similar to Os,such that Re/Os didn't fractionate during magma evolution of the Moon.Because the volume of the Moon is much smaller than the Earth,the timescale for the chemical and thermal evolution of the Moon was much shorter than the Earth,many original differentiation products were well preserved on the Moon.Hence the chemical evolution of the Moon serves as "fossil" record for early evolution of terrestrial planets.Although it is much different from modern Earth,the Moon plays an important role in our understanding of the early evolution of the Earth.
【CateGory Index】： P184
【CateGory Index】： P184