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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Characterisation of early Archaean chemical sediments by trace element signatures
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Characterisation of early Archaean chemical sediments by trace element signatures

机译：微量元素特征表征古生代早期化学沉积物

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摘要

Rare earth element (REE) plus yttrium (Y) patterns of modern seawater have characteristic features that can be used as chemical fingerprints. Reliable proxies for marine REE+Y chemistry have been demonstrated from a large geological time span, including Archaean banded iron formation (BIF), stromatolitic limestone, Phanerozoic reef carbonate and Holocene microbialite. Here we present new REE+Y data for two distinct suites of early Archaean (ca. 3.7–3.8 Ga) metamorphosed rocks from southern West Greenland, whose interrelationships, if any, have been much debated in recent literature. The first suite comprises magnetite-quartz BIF, magnetite-carbonate BIF and banded magnetite-rich quartz rock, mostly from the Isua Greenstone Belt (IGB). The REE+Y patterns, particularly diagnostic anomalies (Ce/Ce~*, Pr/Pr~*), are closely related to those of published seawater proxies. The second suite includes banded quartz-pyroxene-amphibole±garnet rocks with minor magnetite from the so-called Akilia Association enclaves (in early Archaean granitoid gneisses) of the coastal region, some 150 km southwest of the IGB. Rocks of this type from one much publicised and highly debated locality (the island of Akilia) have been identified by some workers [Nature 384 (1996) 55; Geochim. Cosmochim. Acta 61 (1997) 2475] as BIF-facies, and their ~(13)C-depleted signature in trace graphite interpreted as a proxy for earliest life on Earth. However, REE+Y patterns of the Akilia Association suite (except for one probably genuine magnetite-rich BIF from Ugpik) are inconsistent with a seawater origin. We agree with published geological and geochemical (including REE) work [Science 296 (2002) 1448] that most of the analysed Akilia rocks are not chemical sediments, and that C-isotopes in such rocks therefore cannot be used as biological proxies. Application of the REE+Y discriminant for the above two rock suites has been facilitated in this study by the use of MC-ICP technique which yields a more complete and precise REE+Y spectrum than was available in many previous studies.
机译：现代海水的稀土元素（REE）加上钇（Y）模式具有可以用作化学指纹的特征。已经从一个大的地质时间跨度证明了海洋REE + Y化学的可靠代表，包括古生带状铁形成层（BIF），层间质石灰石，生代礁碳酸盐岩和全新世微辉石。在这里，我们提供了来自西格陵兰南部的两个不同的古生代早期（约3.7–3.8 Ga）变质岩组的新REE + Y数据，它们的相互关系（如果有的话）在最近的文献中已被争论不休。第一套包括磁铁矿-石英BIF，磁铁矿-碳酸盐BIF和带状富磁铁矿的石英岩，这些岩岩主要来自伊苏阿绿岩带（IGB）。 REE + Y模式，特别是诊断异常（Ce / Ce〜*，Pr / Pr〜*）与已发布的海水代理密切相关。第二套包括带状石英-辉石-斜方石-石榴石岩和少量磁铁矿，来自IGB西南约150公里处沿海地区的所谓的Akilia协会飞地（在古生古纪的花岗岩片麻岩中）。一些工人已经发现这种岩石是在一个广受争议的地方（Akilia岛）发现的[Nature 384（1996）55; Geochim。宇宙猫Acta 61（1997）2475]作为BIF相，其在痕量石墨中的〜（13）C耗尽签名被解释为地球上最早生命的代理。但是，Akilia Association套件的REE + Y模式（除了一个可能来自Ugpik的真正富含磁铁矿的BIF之外）与海水来源不一致。我们同意已发表的地质和地球化学（包括REE）工作[Science 296（2002）1448]，即分析的大多数Akilia岩石不是化学沉积物，因此此类岩石中的C同位素不能用作生物代理。通过使用MC-ICP技术，本研究促进了上述两种岩石套件中REE + Y判别的应用，该技术产生的REE + Y光谱比以前的许多研究都更完整和精确。

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