A systematic rare-earth elements and yttrium study of Archean cherts at the Mount Goldsworthy greenstone belt in the Pilbara Craton: Implications for the origin of microfossil-bearing black cherts
A systematic rare-earth elements and yttrium study of Archean cherts at the Mount Goldsworthy greenstone belt in the Pilbara Craton: Implications for the origin of microfossil-bearing black cherts
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DOI：
10.1016/j.precamres.2009.10.005
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发表时间：
2010-02
影响因子：
3.8
通讯作者：
H. Sugahara;K. Sugitani;K. Mimura;Fumiaki Yamashita;Koshi Yamamoto
中科院分区：
文献类型：
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作者：
H. Sugahara;K. Sugitani;K. Mimura;Fumiaki Yamashita;Koshi Yamamoto
对西澳大利亚州皮尔巴拉克拉通东北部 Goldsworthy 山绿岩带的太古宙（约 3.0Ga）燧石进行了系统的 REE+Y 研究，以了解其起源和沉积环境。分析的样品包括来自 Farrel 石英岩的含微化石的黑色燧石和来自下方 Warrawoona 群的黑色脉燧石，以及层压到带状燧石，包括来自上覆、向上加深的 Cleaverville 地层的碳质燧石、jaspilite 和带状铁地层。来自 Cleaverville 组的层状燧石显示出明显的地层趋势向上剖面，Y/Ho 增加，Eu 正异常，同时具有 HREE 富集和 La 正异常。这些数据包括新提出的 Y/Ho-Eu 异常图上两个端元分量之间的混合阵列。一个端元被解释为太古代海水，具有超球粒状 Y/Ho 比值 (∼100) 和弱正 Eu 异常 (∼3)。具有球粒状 Y/Ho 比和可忽略不计的 Eu 异常的另一个端元被假定为非海洋水，例如大陆径流、地下水或地热水。法雷尔石英岩中含有微化石的黑色燧石的特征是正 La 异常、HREE 富集、可忽略至轻微正 Eu 异常以及球粒状至轻微超球粒状 Y/Ho 比率。它们与具有明显 MREE 富集的脉状燧石和具有明显 Eu 异常的同期热液燧石不同，并且在 Y/Ho-Eu 异常图上绘制的图接近假定的非海相端元成分。因此，黑色燧石是从受大陆径流、地下水和/或地热水等显着影响的水体中沉淀出来的，而不是从高温热液中沉淀出来的，这增加了其中所含微化石的可信度。
A systematic REE+Y study of Archean (ca. 3.0Ga) cherts from the Mount Goldsworthy greenstone belt in the northeastern Pilbara Craton, Western Australia was performed in order to understand their origin and depositional environment. Analyzed samples include microfossil-bearing black cherts from the Farrel Quartzite and a black vein chert from the underlying Warrawoona Group, and laminated to banded chert including carbonaceous chert, jaspilite and banded iron-formation from the overlying, deepening-upward Cleaverville Formation. Laminated to banded cherts from the Cleaverville Formation show a clear stratigraphic trend upsection of increasing Y/Ho and positive Eu-anomalies, with HREE-enrichment and positive La-anomalies. The data comprise a mixing array between two end-member components on a newly proposed Y/Ho–Eu-anomaly diagram. One end-member is interpreted to be Archean seawater, with a super-chondritic Y/Ho ratio (∼100) and a weak positive Eu-anomaly (∼3). The other end-member with a chondritic Y/Ho ratio and a negligible Eu-anomaly is assumed to be non-marine water such as continental run-off, ground water, or geothermal water. Black cherts containing microfossils in the Farrel Quartzite are characterized by a positive La-anomaly, HREE-enrichment, negligible to a slight positive Eu-anomaly, and a chondritic to slightly super-chondritic Y/Ho ratio. They are distinct from vein cherts with a distinct MREE-enrichment and contemporaneous hydrothermal cherts with pronounced Eu-anomaly, and plot close to the supposed non-marine end-member component on the Y/Ho–Eu-anomaly diagram. The black cherts were thus precipitated from a water mass influenced significantly by for example continental run-off, ground water and/or geothermal water, but not from high-T hydrothermal solution, increasing the credibility of microfossils contained in them.