Trace element and stable (C, O) and radiogenic (Sr) isotope geochemistry of stromatolitic carbonate rocks of the Mesoarchaean Pongola Supergroup: Implications for seawater composition
Trace element and stable (C, O) and radiogenic (Sr) isotope geochemistry of stromatolitic carbonate rocks of the Mesoarchaean Pongola Supergroup: Implications for seawater composition
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DOI：
10.1016/j.chemgeo.2017.11.036
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发表时间：
2018-01
期刊：
影响因子：
3.9
通讯作者：
M. Siahi;A. Hofmann;S. Master;Allan H Wilson;C. Mayr
中科院分区：
文献类型：
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作者：
M. Siahi;A. Hofmann;S. Master;Allan H Wilson;C. Mayr
中太古代Pongola超群拥有最古老的保存完好的叠层石碳酸盐岩系列之一。通过对三个叠层碳酸盐岩地层剖面的主量和微量元素地球化学沿着以及C、O、Sr同位素分析，推断沉积环境，揭示同期海水的化学特征。碳酸盐岩主要由白云石和少量铁白云石组成。它们的可变Sr浓度为~ 95至450 mg/kg，被解释为在大气成岩作用过程中，亚稳碳酸盐（文石和/或高镁方解石）的早期至晚期结晶石化、相控制和钙化作用的结果。δ 18 O值介于− 19.6至− 12.0‰ VPDB之间，反映了成岩和变质作用期间高水-岩相互作用引起的碳酸盐重结晶。在受蚀变影响最小的碳酸盐岩样品中测得的δ 13 C值为2.5‰ VPDB，我们认为它近似于同时代海水中溶解无机碳的组成。样品显示可变的87 Sr/86 Sr同位素比值，从低至0.708至0.777。这些比率大大高于预期的在大洋大洋中的水质量缓冲热液与地幔的相互作用。我们解释了一些碳酸盐岩样品中的高放射性Sr同位素比值，以反映样品中的硅质岩/火山岩组分和富粘土层与碳酸盐岩夹层中的流体与放射性Sr的同位素交换。最少的放射性锶同位素比值可能近似于混合同位素组成的海水中的陆表海盆地和河流淡水排水Kaapvaal的湖泊。碳酸盐岩样品的页岩归一化的稀土元素和钇的分布模式，类似于那些积极的LaSN，GdSN和YSNanomalies，supercritical Y/Ho比值，一个小的负CeSNanomaly和亏损轻稀土元素相对于重稀土元素的现代海水。这些异常是不明显的预期为一个开放的海洋环境，这与锶同位素结果一起支持一个更受限制的设置与河流输入和海洋侵入的波动。一个凸向上的页岩标准化稀土元素和钇模式可能是几个因素，包括有机质络合，元素分馏在河口混合的结果。小的负CeSnanomaly可能表明由于局部产氧微生物活动，浅水沃茨中存在少量游离氧。
The Mesoarchaean Pongola Supergroup hosts one of the oldest well-preserved stromatolitic carbonate successions. Major- and trace-element geochemistry along with C, O, and Sr isotopic analyses of three stratigraphic sections of stromatolitic carbonate rocks were undertaken in order to infer the depositional environment, and to unravel the chemical characteristics of contemporaneous seawater. The carbonate rocks consist mainly of dolomite and, to a lesser extent, ankerite. Their variable Sr concentrations of ~ 95 to 450 mg/kg are interpreted as resulting from early to late dolomitization of metastable carbonates (aragonite and/or high-Mg calcite), facies control and calcitization during meteoric diagenesis. δ18O values range from − 19.6 to − 12.0‰ VPDB and reflect recrystallization of carbonates caused by high water-rock interactions during diagenesis and metamorphism. A δ13C value of 2.5‰ VPDB was measured in carbonate samples least affected by alteration, and we suggest that it approximates the composition of dissolved inorganic carbon in coeval seawater. The samples display variable87Sr/86Sr isotopic ratios, from as low as 0.708 to 0.777. These ratios are considerably higher than those expected in Archaean open-ocean water masses buffered by hydrothermal interaction with the mantle. We interpret the highly radiogenic Sr isotopic ratios in some carbonate samples to reflect isotopic exchange with radiogenic Sr from siliciclastic/volcaniclastic components in the samples and fluids from clay-rich layers intercalated with the carbonates. The least radiogenic Sr isotopic ratio may approximate the mixed isotopic composition of seawater in the epicontinental basin and of riverine freshwater draining the Kaapvaal Craton. The dolomitic carbonate samples show shale-normalized rare earth element and yttrium distribution patterns that resemble those of modern ocean water with positive LaSN, GdSNand YSNanomalies, superchondritic Y/Ho ratios, a small negative CeSNanomaly and depleted light rare earth elements relative to the heavy rare earth elements. These anomalies are not as pronounced as expected for an open marine setting, which together with the Sr isotopic results support a more restricted setting with riverine input and fluctuations in marine ingressions. A convex-upward shale-normalized rare earth element and yttrium pattern is probably a result of several factors, including organic matter complexation, and element fractionation during estuarine mixing. The small negative CeSNanomaly may suggest small amounts of free oxygen in shallow waters due to local oxygenic microbial activity.