月海玄武岩与月球演化

Mare basalts and lunar evolution

月海玄武岩主要产于月球近边的盆地中,覆盖面积为月球表面的1%,其形成年龄多在39～31亿年之间,是各类月岩中最年轻的。与地球玄武岩相似,月海玄武岩由斜长石、辉石和橄榄石组成,但它们比地球玄武岩具有更低的Mg#、Al2O3、K和Na含量,高的FeO含量(大于16%)和变化范围大的TiO2含量(小于1%到大于13%)。根据TiO2含量的变化,月海玄武岩分成高Ti(>6%),低Ti(1.5%<TiO2<6%)以及极低Ti(<1.5%)三类。所有月海玄武岩都具有Eu负异常,并亏损挥发性元素和亲铁元素。月海玄武岩的同位素特征指示其至少为三个组分混合的产物:(1)高238U/204Pb、高87Sr/86Sr和负εNd组分,可能是岩浆海分异的残余岩浆即KREEP;(2)低238U/204Pb、低87Sr/86Sr和正εNd组分,来源于原始月幔,其熔融产物为低Ti玄武岩;(3)中等87Sr/86Sr和εNd组分,位于月幔的顶部,经历了岩浆海(洋)过程中形成的堆晶物质的再熔融,还可能受到了陨击事件的影响,其熔融产物是高Ti玄武岩。月海玄武岩的元素和同位素地球化学性质支持岩浆海的假说,其源区的形成与岩浆海的分异密切相关,并经历了三个阶段:(a)岩浆海阶段,通过岩浆海的结晶分异形成顶部为斜长岩月壳,中间为高Ti、富钛铁矿层,底部为巨厚的硅酸盐低Ti层的三层壳幔结构;(b)富钛铁矿堆晶岩(携带少量残余熔体)因密度大而下沉至下部的硅酸盐月幔(400km以下);(c)月幔中这些不同源区的岩石发生减压熔融。早期由较浅的低熔点组分熔融形成低K高Ti玄武岩,之后形成来源较深的高Ti玄武岩和低Ti玄武岩。

Mare basalts are mainly distributed in basins in the near-earth side, occupying 〈1% of the volume of the Moon. Most mare basalts were emplaced during 3.9- 3.1 Ga, thus being young by lunar standards. Like basaltic rocks on Earth, mare basalts are composed of plagioclase, pyroxene and olivine. However, they are lower in Mg#, AlzOB, Na and K, higher in FeO contents than the terrestrial basalts. TiO2 contents in mare basalts are extremely varied, ranging fi＇om 〈 1% to 〉 13%. Based on the TiO2 variation, mare basalts are classified into three groups, i.e., highTi （〉 6%）, lowTi （1.5% 〈TIO2〈6%） and extremely lowTi （〈1.5%） basalts. All the mare basalts are characterized by negative Eu anomaly and are deficient in volatile and siderophile elements. Isotopic systematics of mare basalts indicate at least three components： （1） a component with high 238U/204pb, high 87Sr/86Sr and negative end values, likely represented by KREEP residuum of a global lunar magma ocean; （2） a ＂primordial＂ deep mantle source with 23sU/204pb typical of Earth, low 87Sr/86Sr and positive end values; and （3） a shallow mantle reservoir that has similar 238U/2044pb values to the second, intermediate end and low to intermediate 87Sr/86Sr values. The typical melting products of the second and third components are low-Ti and high-Ti basalts, respectively. Elemental and isotopic characteristics of mare basalts are in support of the lunar magma ocean hypothesis and suggest a close relationship between their source formation and differentiation of magma ocean. The sources for the mare basahs were likely fomled in a three-stage process. First, a global lunar magma ocean differentiated into a plagioclase-rich flotation crust, a thin high-Ti source and a thick low-Ti source consisting mostly of variable proportions of olivine and orthopyroxene with minor clinopyroxene. Second, the denser ilmenite-rich layer, probably accompanied by variable amount of KREEP-type magma residuum, sank into the lower silicate layer over time below 400 km. Third, mare basahs are generated from these sources by decompression mehing. Earliest mehs （low K, high Ti basahs） were likely derived from the low-temperature components in the relatively shallow mantle. This was followed by both high-Ti and low-Ti basahs generated at greater depths.