Petrography and geochemistry of carbonate rocks of the Paleoproterozoic Zaonega Formation, Russia: Documentation of 13C-depleted non-primary calcite
Petrography and geochemistry of carbonate rocks of the Paleoproterozoic Zaonega Formation, Russia: Documentation of 13C-depleted non-primary calcite
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DOI：
10.1016/j.precamres.2013.10.005
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发表时间：
2014-01-01
影响因子：
3.8
通讯作者：
Brasier, A. T.
中科院分区：
文献类型：
--
作者：
Crne, A. E.;Melezhik, V. A.;Brasier, A. T.
俄罗斯西北部奥涅加盆地约2.0 Ga早奥涅加组是一个深水、含大量基性火山作用的硅-碳酸盐混合沉积体系。以极富有机质岩石（TOC为40 wt%）为代表，是已知最早的石油/沥青生成期之一。这些岩石被推断为地球历史上最大的负δ C-13偏移之一，它与全球范围内2.2-2.06 Ga Lomagundi-Jatuli碳酸盐岩碳同位素的高值偏移之后或部分重叠，被认为与古元古代地球表面环境的氧化作用有关。为了评价早奥涅加组碳酸盐岩碳同位素信号（δ C-13（碳水化合物））的沉积后完整性，我们对该组不同地层水平的8个碳酸盐岩层（厚度0.3 ~ 0.9 m）进行了详细的岩石学和地球化学研究。单个层的δ C-13值的范围可高达千分之17，相对于共存的白云岩，方解石中的C-13明显枯竭；贫c -13方解石的形成可能与有机质成岩和变质作用产生的碳有关，可能受到火山CO2的影响。方解石+/-滑石+/-绿云母+/-放线石的存在表明白云石与石英或可能是钾长石在水的存在下发生变质反应；通常伴随着富含C-13的二氧化碳脱气，这导致新形成的方解石进一步耗尽C-13。蚀变最小的白云岩出现在边缘钙化程度不同的厚白云岩层的中部。该白云岩被认为是最早的原生碳酸盐相，可能记录了周围海水的δ C-13信号。蚀变最小的白云岩分布在两个地层层段，地层中部δ C-13值为+8和+4‰，上部δ C-13值为-2和-4‰。所有其他地层，δ C-13在-19至+3‰之间，被认为在沉积后的过程中，其C-13含量已被不同程度地耗尽，因此不能可靠地用于评估古元古代海水的碳同位素组成。我们的研究结果强调了在用于重建全球环境变化的碳酸盐岩研究中区分原生和次生（或后期）同位素组成的重要性。(C) 2013 Elsevier B.V.版权所有
The ca. 2.0 Ga Zaonega Formation in the Onega Basin of NW Russia represents a deep-water, mixed siliciclastic-carbonate depositional system with voluminous mafic volcanism. It is typified by extremely organic-rich rocks (TOC > 40 wt%) and represents one of the earliest known episodes of oil/asphalt generation. These rocks have been inferred to archive one of the largest negative delta C-13 excursions in Earth history, one that followed and/or partially overlapped with the 2.2-2.06 Ga worldwide Lomagundi-Jatuli carbonate carbon isotopic excursion to high values and thought to be linked to the Paleoproteorozoic oxygenation of Earth's surface environments.In order to assess the post-depositional integrity of the carbonate carbon isotopic signal (delta C-13(carb)) of the Zaonega rocks, we examined in detail the petrography and geochemistry of eight carbonate beds (0.3-0.9 m thick) from different stratigraphic levels of the formation. The range of delta C-13 values for a single bed can be as much as 17 parts per thousand, with calcite being significantly depleted in C-13 relative to co-existing dolomite; the C-13-depleted calcite likely formed by involvement of carbon derived from diagenetic and catagenetic alteration of organic matter possibly abetted by volcanic CO2. The presence of calcite +/- talc +/- phlogopite +/- actinolite indicates metamorphic reaction of dolomite with quartz, or possibly K-feldspar, in the presence of water; commonly accompanied by degassing of C-13-enriched CO2, this caused further C-13 depletion of newly formed calcite. The least altered dolomite is documented in central parts of thick dolostone beds with variably calcitized margins. This dolomite is considered as the earliest and possibly primary carbonate phase, potentially recording the delta C-13 signal of the ambient seawater. The least-altered dolomite is found in two stratigraphic intervals exhibiting delta C-13 values of +8 and +4 parts per thousand for the middle part of the formation, and delta C-13 values of -2 and -4 parts per thousand for the upper part. All other beds, with delta C-13 ranging from -19 to +3 parts per thousand, are considered to have been variably depleted in their C-13 content by post-depositional processes and therefore cannot be reliably used for assessing the carbon isotope composition of Paleoproterozoic seawater. Our results emphasize the importance of distinguishing primary versus secondary (or later) isotopic compositions in studies of carbonate rocks used for reconstruction of global environmental change. (C) 2013 Elsevier B.V. All rights reserved.