地球科学Earth science
Giant impacts and the origin and evolution of continents
大碰撞及大陆的起源和演化
作者：Tim E. Johnson, Christopher L. Kirkland, Yongjun Lu et al.
链接：
https://www.nature.com/articles/s41586-022-04956-y
摘要：
地球是已知的唯一有大陆的行星，尽管大陆形成和演化的方式尚不清楚。在此，通过对岩浆锆石的氧同位素分析，我们发现西澳大利亚皮尔巴拉克拉顿是地球上保存最完好的太古宙（40 - 25亿年前）大陆遗迹，它的形成经历了三个阶段。第一阶段锆石（3.6 ~ 3.4 Ga）形成两个团簇，其中1 / 3记录为下地幔δ18O，表明岩浆来自于热液蚀变的玄武岩地壳（如今天的冰岛）。
浅层融化与代表地球最初10亿年历史的大碰撞是一致的。大碰撞为地壳破裂提供了动力，并通过与全球广泛的海洋的相互作用建立了长期的热液蚀变。大约发生在3.6 Ga的一次巨大撞击，与最古老的低δ18O锆石同时代，可能会触发大规模的地幔融化，产生厚的镁铁质-超镁铁质核。
另一个低δ18O锆石簇约为3.4 Ga，与球粒层同生，为地球上的巨大撞击提供了最古老的物质证据。第2阶段（3.4 ~ 3.0 Ga）锆石的δ18O多为类似地幔δ18O，主要来源于大陆核基底附近的母岩浆。第三阶段（<3.0 Ga）锆石δ18O具有上地幔特征，反映了表壳岩石的高效循环。最古老的长英质岩石形成于3.9-3.5 Ga，接近所谓的晚期重轰击末期，并非巧合。
Abstract：
Earth is the only planet known to have continents, although how they formed and evolved is unclear. Here using the oxygen isotope compositions of dated magmatic zircon, we show that the Pilbara Craton in Western Australia, Earth’s best-preserved Archaean (4.0–2.5 billion years ago (Ga)) continental remnant, was built in three stages. Stage 1 zircons (3.6–3.4 Ga) form two age clusters with one-third recording submantle δ18O, indicating crystallization from evolved magmas derived from hydrothermally altered basaltic crust like that in modern-day Iceland. Shallow melting is consistent with giant impacts that typified the first billion years of Earth history. Giant impacts provide a mechanism for fracturing the crust and establishing prolonged hydrothermal alteration by interaction with the globally extensive ocean. A giant impact at around 3.6 Ga, coeval with the oldest low-δ18O zircon, would have triggered massive mantle melting to produce a thick mafic–ultramafic nucleus. A second low-δ18O zircon cluster at around 3.4 Ga is contemporaneous with spherule beds that provide the oldest material evidence for giant impacts on Earth. Stage 2 (3.4–3.0 Ga) zircons mostly have mantle-like δ18O and crystallized from parental magmas formed near the base of the evolving continental nucleus. Stage 3 (<3.0 Ga) zircons have above-mantle δ18O, indicating efficient recycling of supracrustal rocks. That the oldest felsic rocks formed at 3.9–3.5 Ga, towards the end of the so-called late heavy bombardment, is not a coincidence.
生物学
