摘要： 海相碳酸盐的沉淀方式被认为与水体氧化还原条件密切相关,即太古宙至古元古代缺氧的铁化海水中碳酸盐沉淀抑制剂Fe²⁺和Mn²⁺强力抑制灰泥在水柱中成核,但允许文石直接在海底生长,从而导致大量文石以海底沉淀方式产出,而新元古代适度的氧化海水则有利于灰泥以水柱沉淀方式形成。然而,碳酸盐沉淀方式的长期变化还可能受控于其他因素,其与海水氧化还原条件之间的关系还需要通过大量具体实例来验证。针对上述科学问题,笔者选择碳酸盐沉淀方式尚处于过渡时期的华北中元古界碳酸盐岩为研究对象,开展碳酸盐沉淀方式及与之对应的氧化还原条件研究。结果表明,华北高于庄组三段(约1.56 Ga)、雾迷山组四段下部(约1.48 Ga)和铁岭组二段(约1.44 Ga)发育大量灰泥水柱沉淀,其Ⅰ/(Ca+Mg)值较高(普遍大于0.5 μmol/mol)、Ce负异常(低至0.8),指示适度氧化的条件;而高于庄组四段下部(约1.55 Ga)和雾迷山组二段中部(约1.50 Ga)则发育大量纤维状文石海底沉淀,其Ⅰ/(Ca+Mg)值约为0,指示次氧化至缺氧的环境。因此,本研究首次用大量实例证实了前寒武纪海水氧化还原条件对碳酸盐沉淀方式的重要调控作用,并且后者可作为海水氧化还原条件分析的重要指标,适用于高效开展长序列、多剖面的低氧背景下前寒武纪碳酸盐岩地层的氧化还原条件分析。

关键词: 碳酸盐海底沉淀, 灰泥水柱沉淀, 碳酸盐沉淀抑制剂, 二氧化碳浓缩机制, 稀土Ce异常, Ⅰ, /(Ca+Mg)值

Abstract: It has been considered that the precipitation pathways of marine carbonates are closely related to seawater redox conditions. Due to the presence of carbonate precipitation inhibitor(e.g.,Fe²⁺ and Mn²⁺)-rich shallow seawaters during Archean and Paleoproterozoic,nucleation of calcite mud in water column was inhibited but formation of seafloor aragonite precipitates was allowed. In contrast,the oxidative removal of carbonate precipitation inhibitors in Neoproterozoic shallow seawaters promoted the direct precipitation of carbonate mud from water column. However,it needs more detailed case studies to test the connection between seawater redox and pathways of carbonate precipitation,since the secular variation in the pathways of carbonate precipitation may controlled by other factors. This study focuses on the fabric and geochemistry of carbonates deposited during the Mesoproterozoic,a transitional period of carbonate precipitation. Abundant carbonate mud occurs in the Member Ⅲ of the Gaoyuzhuang Formation(~1.56 Ga),the Member Ⅳ of the Wumishan Formation(~1.48 Ga),and the Member Ⅱ of the Tieling Formation(~1.44 Ga)in North China. These water column precipitated carbonate mud has relatively high Ⅰ/(Ca+Mg)ratios(generally>0.5 μmol/mol)and negative Ce anomalies(down to 0.8),indicating moderately oxygenated conditions. In contrast,abundant seafloor precipitated aragonite fans occur in the lower Member Ⅳ of the Gaoyuzhuang Formation(~1.55 Ga)and the Member Ⅱ of the Wumishan Formation(~1.50 Ga). These seafloor precipitates have near zero Ⅰ/(Ca+Mg)ratios,suggesting suboxic to anoxic conditions. Therefore,this study,firstly using detailed cases,proves that the texture of Precambrian carbonates was largely controlled by the redox conditions of seawaters and could be used as a redox proxy to conduct long-term and multi-section study of marine redox conditions directly and efficiently based on outcrop observations in the field.

Key words: carbonate seafloor precipitate, carbonate mud water column precipitate, carbonate precipitation inhibitor, carbon dioxide concentration mechanism, Ce anomaly, Ⅰ/(Ca+Mg)

中图分类号: 

• P534.41