月球的化学演化
Chemical evolution of the Moon: A review

孙卫东 ¹ ^* 凌明星 ¹ Qing-Zhu YIN ²

文摘	月球是一个发生了化学分异的星球,它由月壳、月幔±一个小的金属月核组成.大量观察事实显示月球曾经有过岩浆洋,岩浆洋的结晶分异主导了月球的化学演化.目前主流观点认为,月球是在太阳系演化的早期,至少45亿年前,一个火星大小的星球,与即将完成原始吸积的地球胚胎发生偏心撞击,造成地球的熔融,形成岩浆洋,飞溅出来的物质迅速吸积形成绕地球运动的月球,并且在月球上形成了全球规模的岩浆洋,进而发生了结晶分异.由于月球上没有海洋和板块俯冲,岩浆洋分异是其化学演化的主要途径.月球岩浆洋的80%～85%在大撞击后的100Ma内已经固化,这可能是由于月球体积小、表面没有大气包裹所致.月球极贫水,因此在岩浆结晶过程中斜长石首先结晶.斜长石由于密度小于玄武质岩浆而漂浮在岩浆洋的表层,橄榄石等密度大的矿物则堆积在岩浆洋的底部.随着结晶分异的进行,残余岩浆不断富集不相容元素,包括K、U等放射性元素;与此同时,密度较大的钛铁矿开始结晶,造成高钛堆晶岩密度大于其下的橄榄石堆晶岩的不稳定结构,进而发生月幔翻转,引发一系列岩浆活动,进而形成月球上特有的镁质系列、碱质系列等岩石.由于月球氧逸度较低,Eu主要以+2价形式存在,因此斜长石高度富集Eu,相应地除高地斜长岩外,其他岩石均表现为Eu高度亏损的特点.与此同时,Re在低氧逸度下表现为强亲铁元素的特点,Re/Os在月球岩浆过程中不发生分异.月球的体积远小于地球,因而其演化时间远远短于地球,很多原始的分异被完整地保留下来.因此月球的化学演化是类地行星早期演化过程的"化石",尽管与现代的地球存在较大差异,但是对于认识地球早期演化具有借鉴意义.
其他语种文摘	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.
来源	地球化学 ,2010,39(2):131-141 【核心库】
关键词	月球 ; 化学演化 ; 水 ; 主元素 ; 微量元素
地址	1. 中国科学院广州地球化学研究所, 中国科学院同位索年代学和地球化学重点实验室, 广东, 广州, 510640 2. Department of Geology, University of California, 95616
语种	中文
文献类型	研究性论文
ISSN	0379-1726
学科	天文学;地质学
基金	国家自然科学基金国家杰出青年科学基金
文献收藏号	CSCD:3865491