Origin and significance of Si and O isotope heterogeneities in Phanerozoic, Archean, and Hadean zircon
Origin and significance of Si and O isotope heterogeneities in Phanerozoic, Archean, and Hadean zircon
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DOI：
10.1073/pnas.1808335115
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发表时间：
2018-09
期刊：
影响因子：
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通讯作者：
D. Trail;P. Boehnke;P. Savage;Ming‐Chang Liu;M. Miller;I. Bindeman
中科院分区：
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作者：
D. Trail;P. Boehnke;P. Savage;Ming‐Chang Liu;M. Miller;I. Bindeman
地壳或其化学风化衍生物可能是生命起源的基质，可能发生在4.1 Ga。然而，没有已知的真正的陆地岩石从这个时候仍然存在。因此，研究转向了从这个时候碎屑锆石的地球化学特征。虽然锆石不直接记录低温风化过程，但它们继承了沉积物再循环和重熔时的同位素信息。我们开发了一种方法来指纹的身份参与水-岩石相互作用>4 Ga，由更现代的锆石样品的大型Si和O同位素数据集支持。这里提供的数据提供了化学沉积物的证据，如地球上> 4Ga的硅质岩和带状铁建造。水圈的相互作用和地壳的变化可能在塑造地球表面方面发挥了关键作用，并促进了4.03 Ga（Hadean Eon）之前导致生命的前生物反应。水蚀变物质的性质强烈依赖于岩石圈中丰富的水溶性元素（如Si和O）的循环。然而，定义冥古宙沉积物的性质的直接限制是不存在的，因为从这个最早的时代的样品仅限于碎屑锆石（ZrSiO 4）。在这里，我们表明，同时测量的硅和O同位素比值在中生代和碎屑前3.0 Ga锆石约束含水蚀变前体的组成纳入其源熔体。（S）沉积型岩石中的早古生代锆石具有不均匀的δ 18 O和δ 30 Si值，与（I）片麻岩和（A）造山型岩石中锆石的同位素特征不同，这与变质物质同化作用一致。太古代碎屑锆石的δ ~（18）O值具有不均一性，但其Si同位素组成与幔源锆石相似。冥古宙晶体产生较高的δ 18 O值（相对于地幔锆石）和δ 30 Si值几乎涵盖了整个范围内观察到的古生代样品。耦合的Si和O同位素数据代表了一个约束Hadean风化和沉积输入长英质熔体，包括重熔的斜长角闪岩可能的玄武质起源，和化学沉积物，如硅质岩和/或带状铁的形成（BIFs）到源熔体的分数。这种沉积矿床的广泛性足以改变壳内熔体的化学特征，这表明它们可能早在4.1 Ga就已经成为（前）生物化学的合适生态位。
Significance The crust or its chemically weathered derivatives likely served as a substrate for the origin of life, which could have occurred by 4.1 Ga. Yet no known bona fide terrestrial rocks from this time remain. Studies have thus turned to geochemical signatures within detrital zircons from this time. While zircons do not directly record low-temperature weathering processes, they inherit isotopic information upon recycling and remelting of sediment. We developed a method to fingerprint the identity of material involved in water–rock interactions >4 Ga, bolstered by a large Si and O isotopic dataset of more modern zircon samples. The data presented here provide evidence for chemical sediments, such as cherts and banded iron formations on Earth >4 Ga. Hydrosphere interactions and alteration of the terrestrial crust likely played a critical role in shaping Earth’s surface, and in promoting prebiotic reactions leading to life, before 4.03 Ga (the Hadean Eon). The identity of aqueously altered material strongly depends on lithospheric cycling of abundant and water-soluble elements such as Si and O. However, direct constraints that define the character of Hadean sedimentary material are absent because samples from this earliest eon are limited to detrital zircons (ZrSiO4). Here we show that concurrent measurements of Si and O isotope ratios in Phanerozoic and detrital pre-3.0 Ga zircon constrain the composition of aqueously altered precursors incorporated into their source melts. Phanerozoic zircon from (S)edimentary-type rocks contain heterogeneous δ18O and δ30Si values consistent with assimilation of metapelitic material, distinct from the isotopic character of zircon from (I)gneous- and (A)norogenic-type rocks. The δ18O values of detrital Archean zircons are heterogeneous, although yield Si isotope compositions like mantle-derived zircon. Hadean crystals yield elevated δ18O values (vs. mantle zircon) and δ30Si values span almost the entire range observed for Phanerozoic samples. Coupled Si and O isotope data represent a constraint on Hadean weathering and sedimentary input into felsic melts including remelting of amphibolites possibly of basaltic origin, and fractional addition of chemical sediments, such as cherts and/or banded iron formations (BIFs) into source melts. That such sedimentary deposits were extensive enough to change the chemical signature of intracrustal melts suggests they may have been a suitable niche for (pre)biotic chemistry as early as 4.1 Ga.