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Geochemistry of carbonate rocks of the Chilpi Group, Bastar Craton, India: Implications on ocean paleoredox conditions at the late Paleoproterozoic Era
Precambrian Research ( IF 3.8 ) Pub Date : 2020-12-19 , DOI: 10.1016/j.precamres.2020.106023
Prasanta Kumar Mishra , Sarada Prasad Mohanty

The chemogenic sediments of the Chilpi Group (~2.0–1.8 Ga) of the Bastar Craton have been analysed for the sedimentary environment, geochemistry and redox evaluation of seawater towards the end of the Paleoproterozoic Era. Fabric-retentive fine-grained micritic and oolitic carbonates suggest preservation of primary texture and precipitation from contemporaneous seawater in a shallow water tidal environment. Reducing condition of shallow sea for the Chilpi Group is also recognized by the presence of chamosite associated with Mn-ankerite. The Post-Archean Australian Shale (PAAS) normalized positive Gd and La anomalies, Lu/La, Y/Ho molar ratios and the Y anomaly indicate the preservation of pristine marine nature of the carbonates. Evaluation of multi-element concentration indicates no significant effects of detrital contamination, diagenesis, metamorphism and contributions from hydrothermal sources. Positive Ce anomalies observed in all the samples, with maximum Ce/Ce* up to 2.83, and redox responsive trace element ratio patterns in association with enrichment of Mn and other redox responsive elements such as Zn, Fe, Mo and U indicate a reducing marine environment of deposition of the Chilpi carbonates. Display of positive europium anomalies (Eu/Eu* = 1.02 – 1.56) can be attributed to the reducing environment as contributions of hydrothermal fluids are ruled out for lack of LREE enrichment compared to HREE. We infer suboxic to anoxic conditions of shallow sea in the Chilpi basin towards the end of the Paleoproterozoic Era, with estimated atmospheric oxygen level of ~10-3 PAL. Reducing conditions in shallow sea indicate that the Great Oxidation Event did not affect seawater composition during late Paleoproterozoic.



中文翻译:

印度Bastar Craton的Chilpi集团碳酸盐岩的地球化学:古元古代晚期对海洋古土壤的影响

到古元古代末期,已经对Bastar Craton的Chilpi组(〜2.0–1.8 Ga)的化学成因沉积物进行了沉积环境,地球化学和海水氧化还原评估的分析。保留织物的细粒微碳酸盐和橄榄酸盐碳酸盐表明,在浅水潮汐环境中,保留了同期海水的主要质地和降水。对于奇尔皮集团,浅海条件的减少也被锰锰铁矿伴生的绿铁矿的存在所认识。后Archean澳大利亚页岩(PAAS)归一化了正Gd和La异常,Lu / La,Y / Ho摩尔比和Y异常表明碳酸盐的原始海洋性质得以保留。对多元素浓度的评估表明,碎屑污染,成岩作用,热液源的变质作用和贡献。在所有样品中均观察到正Ce异常,最大Ce / Ce *高达2.83,并且氧化还原响应痕量元素比率模式与Mn和其他氧化还原响应元素(如Zn,Fe,Mo和U)的富集有关,表明海洋生物含量降低。 Chilpi碳酸盐的沉积环境。positive异常的显示(Eu / Eu * = 1.02-1.56)可以归因于环境的减少,因为与HREE相比,由于缺少LREE富集而排除了热液的贡献。我们推论到古元古代末期奇尔皮盆地浅海的低氧至缺氧条件,估计大气中的氧气水平约为10 Ce / Ce *最高可达2.83,氧化还原响应痕量元素比率模式与Mn和其他氧化还原响应元素(如Zn,Fe,Mo和U)的富集有关,表明Chilpi碳酸盐沉积的海洋环境正在降低。positive异常的显示(Eu / Eu * = 1.02-1.56)可以归因于环境的减少,因为与HREE相比,由于缺少LREE富集而排除了热液的贡献。我们推论到古元古代末期奇尔皮盆地浅海的低氧至缺氧条件,估计大气中的氧气水平约为10 Ce / Ce *最高可达2.83,氧化还原响应痕量元素比率模式与Mn和其他氧化还原响应元素(如Zn,Fe,Mo和U)的富集有关,表明Chilpi碳酸盐沉积的海洋环境正在降低。positive异常的显示(Eu / Eu * = 1.02-1.56)可以归因于环境的减少,因为与HREE相比,由于缺少LREE富集而排除了热液的贡献。我们推论到古元古代末期奇尔皮盆地浅海的低氧至缺氧条件,估计大气中的氧气水平约为10 Mo和U表示Chilpi碳酸盐沉积的海洋环境正在减少。positive异常的显示(Eu / Eu * = 1.02-1.56)可以归因于环境的减少,因为与HREE相比,由于缺少LREE富集而排除了热液的贡献。我们推论古元古代时代末期奇尔皮盆地浅海的缺氧至缺氧条件,估计大气中的氧气水平约为10 Mo和U表示Chilpi碳酸盐沉积的海洋环境正在减少。positive异常的显示(Eu / Eu * = 1.02-1.56)可以归因于环境的减少,因为与HREE相比,由于缺少LREE富集而排除了热液的贡献。我们推论到古元古代末期奇尔皮盆地浅海的低氧至缺氧条件,估计大气中的氧气水平约为10-3 PAL。浅海的还原条件表明,大元氧化事件并没有影响古元古代晚期的海水成分。

更新日期:2020-12-20
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