Lunar meteorites: new insights into the geological history of the Moon
Lunar meteorites: new insights into the geological history of the Moon
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DOI：
10.1093/astrogeo/att121
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发表时间：
2013-08
影响因子：
0.8
通讯作者：
K. Joy;T. Arai
中科院分区：
文献类型：
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作者：
K. Joy;T. Arai
4.28 A&G · August 2013 · Vol. 54月球具有科学意义，因为它保存了过去45亿年中类地行星的早期地质演化、内太阳系撞击轰击以及太阳和银河系环境的记录（NRC 2007，Crawford等人2012）。因此，它保存了影响地球历史的过程的证据（NRC 2012）。载人和无人月球飞行任务带回了约382公斤的月球岩石和土壤（Vaniman等人，1991年）（见表1月球样品摘要）。这些数据是由阿波罗任务从月球中央近侧的赤道纬度以内和周围收集的，由月球任务在月球近侧东部边缘收集的（图2）。遥感数据随后显示，许多着陆点实际上采样了一个地球化学异常区域，其中钾和钍等放射性元素的浓度增加。月球近侧的这一区域被称为Procellarum KREEP Terrane（KREEP是钾、稀土元素和富磷材料的首字母缩写）（Jolliff等人，2000年），而这种化学特征与大型撞击坑的撞击喷出物密切相关。（直径约1 200公里）、年轻（年龄估计在38.5亿至39.2亿年之间）、受撞击盆地（图2）。因此，尽管我们从阿波罗和月球样本的研究中了解了大量有关月球过去的信息，但我们还是从一个地理限制的数据集进行了解释，该数据集位于一个不一定最能代表月球全球地质构成的区域内。对阿波罗和月球样本的研究表明，月球在大约45亿年前形成后，被全球岩浆海洋所包围。岩浆海洋慢慢冷却，月球分化出一个小而致密的地核，一个由富含镁和铁的硅酸盐矿物组成的内地幔，以及一个由富含钙和铝的硅酸盐矿物组成的外地幔地壳（参见Shearer等人2006年及其参考文献的详细综述）。这种“原生”地壳构成了月球上大部分的白色高地。当月球高地地壳形成时，月球内部仍然很热，部分熔化。这些熔体侵入地壳，形成岩浆“次生”地壳岩石，称为高镁岩套和高碱岩套。小行星和彗星的撞击使整个月球重新出现，形成了直径大于300公里的大盆地和较小的陨石坑。撞击轰击在约37亿年前（Ga）月球盆地形成之前很高。这个盆地形成时期的持续时间和规模存在争议：阿波罗样本的年龄通常为3.9至3.8 Ga（图3），这表明此时的撞击增强期通常被称为“月球灾难”（Stöffler等人）。2006）。在3.8 Ga之后，火山活动是主要的地壳形成过程，当月海玄武岩喷发到表面时，熔岩流填充了许多较老的撞击盆地：月球近侧的大部分被这些暗色月海玄武岩覆盖，尽管只有少数玄武岩在远侧露出（图2：注意月海玄武岩表面暴露仅占全球月球表面的15%）。
4.28 A&G • August 2013 • Vol. 54 The Moon is of scientific significance because it preserves a record of the early geological evolution of a terrestrial planet, inner solar system impact bombardment, and the solar and galactic environment throughout the last 4.5 billion years (NRC 2007, Crawford et al. 2012). It thus preserves evidence of the processes that affected the history of the Earth (NRC 2012). Manned and unmanned missions to the Moon returned ~382 kg of lunar rocks and soils (Vaniman et al. 1991) (see table 1 for a summary of lunar samples). These were collected by the Apollo missions from within and around equatorial latitudes on the central nearside of the Moon and by the Luna missions on the eastern nearside limb (figure 2). Remote-sensing data have subsequently shown that many of these landing sites actually sampled a geochemically unusual region, with enhanced concentrations of radioactive elements such as potassium and thorium. This region of the lunar nearside is known as the Procellarum KREEP Terrane (where KREEP is an acronym for potassium, rare earth elements and phosphorus-rich materials) (Jolliff et al. 2000) and this chemical signature is closely associated with impact ejecta from the large (~1200 km diameter), young (age estimates range from ~3.85 to 3.92 billion years old) Imbrium impact basin (figure 2). Therefore, although we have learnt a great amount about the Moon’s past from studies of Apollo and Luna samples, interpretations have been made from a geographically restricted dataset within a region that does not necessarily best represent the Moon’s global geological makeup. Studies of Apollo and Luna samples suggest that, after the Moon formed at about 4.5 billion years ago, it was encased by a global magma ocean. The magma ocean slowly cooled and the Moon differentiated with a small dense core, an inner mantle made of Mgand Fe-rich silicate minerals, and an outer feldspathic crust formed from silicate minerals rich in Ca and Al (see Shearer et al. 2006 and references therein for a detailed review). This “primary” crust makes up much of the white highland areas of the Moon. When the lunar highland crust had formed, the lunar interior was still hot and partially melted. These melts were intruded into the crust and formed magmatic “secondary” crustal rocks known as the high-magnesian suite and the high-alkali suite. The whole of the Moon was resurfaced by impacting asteroids and comets that formed large basins >300 km in diameter and smaller craters. Impact bombardment was high before ~3.7 billion years ago (Ga) when the lunar basins were formed. The duration and magnitude of this basin-forming epoch is debated: Apollo samples often have ages of 3.9 to 3.8 Ga (figure 3), suggesting an enhanced period of bombardment at this time often called the “lunar cataclysm” (Stöffler et al. 2006). After 3.8 Ga, volcanism was the dominate crust-forming process when mare basalts were erupted onto the surface as lava flows infilling many of the older impact basins: much of the lunar nearside is covered by these dark mare basalts, although only a few basalts outcrop on the farside (figure 2: note that mare basalt surface exposure constitutes only about 15% of the global lunar surface).