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首页> 外文期刊>Contributions to Mineralogy and Petrology >Seawater-like trace element signatures (REE + Y) of Eoarchaean chemical sedimentary rocks from southern West Greenland, and their corruption during high-grade metamorphism
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Seawater-like trace element signatures (REE + Y) of Eoarchaean chemical sedimentary rocks from southern West Greenland, and their corruption during high-grade metamorphism

机译：西格陵兰南部的欧欧古纪化学沉积岩的海水状微量元素特征（REE + Y）及其在高级变质作用下的腐败

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摘要

Modern chemical sediments display a distinctive rare earth element + yttrium (REE + Y) pattern involving depleted LREE, positive La/La*_(SN), EU/EU*_(SN), and Y_(SN) anomalies (SN = shale normalised) that is related to precipitation from circumneutral to high pH waters with solution complexation of the REEs dominated by carbonate ions. This is often interpreted as reflecting precipitation from surface waters (usually marine). The oldest broadly accepted chemical sediments are c. 3,700 Ma amphibolite facies banded iron-formation (BIF) units in the Isua su-pracrustal belt, Greenland. Isua BIFs, including the BIF international reference material IF-G are generally considered to be seawater precipitates, and display these REE + Y patterns (Bolhar et al. in Earth Planet Sci Lett 222:43-60, 2004). Greenland Eoarchaean BIF metamorphosed up to granulite facies from several localities in the vicinity of Akilia (island), display REE + Y patterns identical to Isua BIF, consistent with an origin by chemical sedimentation from seawater and a paucity of clastic input. Furthermore, the much-debated magnetite-bearing siliceous unit of "earliest life" rocks (sample G91/26) from Akilia has the same REE + Y pattern. This suggests that sample G91/26 is also a chemical sediment, contrary to previous assertions (Bolhar et al. in Earth Planet Sci Lett 222:43-60, 2004), and including suggestions that the Akilia unit containing G91/26 consists entirely of silica-penetrated, metasomatised, mafic rock (Fedo and White-house 2002a). Integration of our trace element data with those of Bolhar et al. (Earth Planet Sci Lett 222:43-60, 2004) demonstrates that Eoarchaean siliceous rocks in Greenland, with ages from 3.6 to 3.85 Ga, have diverse trace element signatures. There are now geographically-dispersed, widespread examples with Isua BIF-like REE + Y signatures, that are interpreted as chemically unaltered, albeit metamorphosed, chemical sediments. Other samples retain remnants of LREE depletion but are beginning to lose the distinct La, Eu and Y positive anomalies and are interpreted as metasomatised chemical sediments. Finally there are some siliceous samples with completely different trace element patterns that are interpreted as rocks of non-sedimentary origin, and include metasomatised mafic rocks. The positive La/La*_(SN), Eu/ Eu*_(SN) and Y_(SN) anomalies found in Isua BIFs and other Eoarchaean Greenland samples, such as G91/26 from Akilia, suggests that the processes of carbonate ion complexation controlling the REE - Y patterns were already established in the hydrosphere at the start of the sedimentary record 3,600-3,850 Ma ago. This is in accord with the presence of Eoarchaean siderite-bearing marbles of sedimentary origin, and suggests that CO_2 may have been a significant greenhouse gas at that time.
机译：现代化学沉积物表现出独特的稀土元素+钇（REE + Y）模式，涉及贫化的LREE，正La / La * _（SN），EU / EU * _（SN）和Y_（SN）异常（SN =页岩）归一化），这与从中性到高pH值的水的沉淀有关，其中以碳酸根离子为主的REE发生溶液络合。通常将其解释为反映了地表水（通常是海洋）的降水。最早被广泛接受的化学沉积物是c。格陵兰岛伊苏阿苏-早断裂带中3,700 Ma的角闪岩相带状铁形成（BIF）单元。 Isua BIF，包括BIF国际参考材料IF-G，通常被认为是海水沉淀物，并显示出这些REE + Y模式（Bolhar等人，Earth Planet Sci Lett 222：43-60，2004）。格陵兰Eoarchaean BIF变质到Akilia（岛）附近几个地方的粒状相，显示出与Isua BIF相同的REE + Y图案，这与海水化学沉积和少量碎屑输入的来源一致。此外，来自Akilia的“最早的”岩石（样品G91 / 26）中含有大量磁铁矿的硅质单元具有相同的REE + Y模式。这表明样品G91 / 26也是一种化学沉积物，这与先前的说法相反（Bolhar等人在Earth Planet Sci Lett 222：43-60，2004），并提出了包含G91 / 26的Akilia单元完全由以下成分组成的建议：二氧化硅渗透，交代化的镁铁质岩石（Fedo和White-house 2002a）。将我们的微量元素数据与Bolhar等人的数据进行整合。 （Earth Planet Sci Lett 222：43-60，2004）证明，格陵兰岛的太古宙硅质岩年龄从3.6到3.85 Ga不等。现在，在地理上分散且分布广泛的例子都带有类似Isua BIF的REE + Y签名，这些例子被解释为化学上未发生变化的化学沉积物。其他样品保留了LREE耗尽的残留物，但开始失去明显的La，Eu和Y正异常，并被解释为交代化的化学沉积物。最后，有些硅质样品的痕量元素形态完全不同，被解释为非沉积成因的岩石，包括交代的镁铁质岩石。在Isua BIF和其他Eoarchaean格陵兰岛样本（例如来自Akilia的G91 / 26）中发现的La / La * _（SN），Eu / Eu * _（SN）和Y_（SN）正异常在3,600-3,850 Ma以前的沉积记录开始时，已经在水圈中建立了控制REE-Y模式的络合物。这与沉积起源的含欧亚弧形菱铁矿的大理石的存在是一致的，并且表明当时CO_2可能是重要的温室气体。

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