月球的化学演化

发布时间：2018-08-09 18:46

【摘要】：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。
[Abstract]:The moon is a chemically differentiated planet consisting of a lunar shell, mantle, and a small metal moon nucleus. A large number of observations show that there were magmatic oceans on the moon, and the crystallization of magmatic oceans dominated the chemical evolution of the moon. The current mainstream view is that the moon was in the early stages of the solar system evolution, at least 4.5 billion years ago, a planet the size of Mars, and is about to complete the original accretion of the Earth's embryo eccentrically, resulting in the melting of the Earth, forming magmatic oceans. The spattered matter rapidly accretes to form the moon, which moves around the earth, and forms a magmatic ocean on the moon on a global scale, thus resulting in crystalline differentiation. Because there is no ocean and plate subduction on the moon, magmatic ocean differentiation is the main way of chemical evolution. Eighty percent of the lunar magma ocean has been solidified in the 100Ma after the big impact, which may be due to the small size of the moon and the absence of atmospheric enclosing on the surface. The moon is extremely water-poor, so plagioclase crystallizes first during magmatic crystallization. Plagioclase floats on the surface of the magmatic ocean because its density is less than that of the Black Tortoise magma and olivine deposits at the bottom of the magmatic ocean. With the crystallization differentiation, the residual magma continuously enriched incompatible elements, including radioactive elements such as Ku, and at the same time, the densified ilmenite began to crystallize. The unstable structure of olivine heap rocks with density greater than that of high titanium heap rocks is caused, and a series of magmatic activities are initiated, resulting in the formation of special magnesite series, alkali series and other rocks on the moon. Because the low oxygen fugacity of the moon mainly exists in the form of bivalent, the plagioclase is highly Eu-enriched, and the other rocks, except the highland plagioclase, are characterized by EU high depletion. At the same time, re / Os does not differentiate during the lunar magma process due to its strong ferriphilic element at low oxygen fugacity. The moon is much smaller than the earth, so its evolution time is much shorter than that of the earth, and many primitive differences have been completely preserved. Therefore, the chemical evolution of the moon is a "fossil" in the early evolution process of Earth-like planets. Although there is a great difference with the modern earth, it is useful for us to understand the early evolution of the Earth.
【作者单位】：中国科学院广州地球化学研究所同位素年代学和地球化学重点实验室;中国科学技术大学地球和空间科学学院矿产资源研究室;中国科学院研究生院;Department
【基金】：国家杰出青年科学基金(40525010)
【分类号】：P184

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