CAJ | 학술논문

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.
월구시일개발생료화학분이적성구,타유월각、월만±일개소적금속월핵조성.대량관찰사실현시월구증경유과암장양,암장양적결정분이주도료월구적화학연화.목전주류관점인위,월구시재태양계연화적조기,지소45억년전,일개화성대소적성구,여즉장완성원시흡적적지구배태발생편심당격,조성지구적용융,형성암장양,비천출래적물질신속흡적형성요지구운동적월구,병차재월구상형성료전구규모적암장양,진이발생료결정분이.유우월구상몰유해양화판괴부충,암장양분이시기화학연화적주요도경.월구암장양적80%～85%재대당격후적100Ma내이경고화,저가능시유우월구체적소、표면몰유대기포과소치.월구겁빈수,인차재암장결정과정중사장석수선결정.사장석유우밀도소우현무질암장이표부재암장양적표층,감람석등밀도대적광물칙퇴적재암장양적저부.수착결정분이적진행,잔여암장불단부집불상용원소,포괄K、U등방사성원소;여차동시,밀도교대적태철광개시결정,조성고태퇴정암밀도대우기하적감람석퇴정암적불은정결구,진이발생월만번전,인발일계렬암장활동,진이형성월구상특유적미질계렬、감질계렬등암석.유우월구양일도교저,Eu주요이+2개형식존재,인차사장석고도부집Eu,상응지제고지사장암외,기타암석균표현위Eu고도우손적특점.여차동시,Re재저양일도하표현위강친철원소적특점,Re/Os재월구암장과정중불발생분이.월구적체적원소우지구,인이기연화시간원원단우지구,흔다원시적분이피완정지보류하래.인차월구적화학연화시류지행성조기연화과정적"화석",진관여현대적지구존재교대차이,단시대우인식지구조기연화구유차감의의.
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.