Impact melt sheet zircons and their implications for the Hadean crust
Impact melt sheet zircons and their implications for the Hadean crust
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DOI：
10.1130/g30251a.1
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发表时间：
2009-10
期刊：
影响因子：
5.8
通讯作者：
J. Darling;C. Storey;C. Hawkesworth
中科院分区：
文献类型：
--
作者：
J. Darling;C. Storey;C. Hawkesworth
撞击可能是地壳再分配和分化的重要机制，特别是在4.5 Ga至3.9 Ga之间强烈的增生后轰击期间。这一时期地壳作用的证据主要来自澳大利亚伊尔加恩克拉通的碎屑锆石。这些古锆石的微量元素组成、结晶温度和包裹体数量作为花岗岩岩浆主要来源的证据，表明地球早期广泛存在长英质大陆地壳。然而，对于冥古宙锆石的潜在来源——撞击熔体中形成的锆石，人们知之甚少。在这里，我们介绍了来自1.85 Ga萨德伯里撞击熔融板（加拿大安大略省）的锆石的Ti温度测量，微量元素和包裹体种群。研究结果表明，锆石结晶温度和组成的巨大变化是岩浆系统演化的必然结果。我们还发现，基性岩石中的锆石在花岗岩成分的残余液体中结晶，产生的包裹体组合与古伊尔冈颗粒的包裹体组合非常相似。因此，我们得出结论，冥古宙锆石的微量元素组成和包裹体数量与先前认识到的更多基性熔体的结晶一致，尽管萨德伯里锆石的高结晶温度分布表明，撞击熔体不是超过3.9 Ga的颗粒的主要来源。
Impacts may have been important mechanisms of crustal redistribution and differentiation, particularly during intense postaccretionary bombardment between 4.5 Ga and 3.9 Ga ago. Evidence of crustal processes during this period is largely provided by detrital zircons from the Yilgarn craton, Australia. Trace element compositions, crystallization temperatures, and inclusion populations of these ancient zircons have been taken as evidence for predominantly granitic source magmas, implying widespread felsic continental crust on the early Earth. However, there is little knowledge of zircons formed in impact melt sheets, a potential source for the Hadean zircons. Here we present Ti thermometry, trace elements, and inclusion populations of zircons from the 1.85 Ga Sudbury impact melt sheet (Ontario, Canada). Our results demonstrate that large variations in zircon crystallization temperature and composition will be an inevitable consequence of the evolution of such magmatic systems. We also show that zircons in mafic rocks crystallize in residual liquids of granitic composition, producing inclusion assemblages that are remarkably similar to those reported for the ancient Yilgarn grains. Thus, we conclude that the trace element compositions and inclusion populations of the Hadean zircons are consistent with crystallization from more mafic melts than previously recognized, although high crystallization temperature distributions of Sudbury zircons indicate that impact melt sheets were not a dominant source for the grains older than 3.9 Ga.