华北地台中元古代鱼骨状方解石的成因机制和古环境意义

Herringbone calcite, characterized by the alternation of sub-millimetric light and dark jagged bands, is a special type of carbonate mineral phase that mainly developed during the Archean. Early studies suggest that herringbone calcite is inorganic chemical precipitation, formed in anoxic, calcium carbonate supersaturated, and precipitation inhibitor (Fe2+ or Mn2+) rich seawater. Therefore, it has been used as an indicator of anoxic conditions in seawater, and its abundance reduction over time has been thought to reflect the course of ocean oxidation. However, later studies suggest that the genesis of herringbone calcite might be variable. In recent years, abundant herringbone calcite was recognized in the Mesoproterozoic Gaoyuzhuang Formation of the North China Platform, providing an important opportunity for us to further study of its genesis and paleoenvironmental implications. The proposed study will select 2-4 potential sections in the platform, with specific emphases on the crystal growth model, genesis, and paleoceanic conditions for the precipitation of herringbone calcite. With the herringbone calcite samples silicified earlier in diagenesis and less alternated, we will use the technology of EBSD for crystal orientation study, EDS and ICP-MS for elemental mapping and composition analyses, as well as Mass Spectrum Instruments for C (organic)-C (carbonate) isotope detection. Furthermore, the program will focus on revealing the potential redox structures of North China epicontinental sea during the Mesoproterozoic, by elucidating the variations of abundance and precipitation forms of herringbone calcites in carbonates deposited at different water depth, and provide important information for in-depth understanding impacts from ocean chemistry on the early evolution of marine eukaryotes.

鱼骨状方解石是一种特殊的碳酸盐沉积，由锯齿状亮暗交互的亚毫米级条带组成，主要见于太古宙。早期的研究认为，鱼骨状方解石是形成于缺氧、碳酸钙过饱和、富Fe2+或Mn2+等碳酸钙沉淀抑制剂的水体中的无机化学沉淀，可指示沉积水体缺氧条件，而其丰度随时间的减少反映了海洋的氧化。但后期的研究表明其成因争议很大。申请者在华北地台中元古界发现了鱼骨状方解石（全球同时代首例）。立项拟选择2-4条重要剖面，以成岩改造较小或早期硅化标本为主要研究对象，运用EBSD、EDS、ICP-MS、同位素质谱仪，分别确定鱼骨状方解石的晶体生长取向、微区主-微量元素分布、有机-无机碳同位素特征，从而查明其晶体生长模式、成因机制及沉积海洋化学条件。我们还将进一步通过分析鱼骨状方解石在不同沉积水深条件下产出丰度和形式的变化，理清中元古代华北陆表海海水的化学结构，为深刻认识海洋化学条件对真核生物早期演化的影响机理提供重要信息。

鱼骨状方解石是一种特殊的碳酸盐沉积，由锯齿状亮暗交互的亚毫米级条带组成，主要见于太古宙。以往认为，鱼骨状方解石属无机化学沉淀成因，形成于水体缺氧、碳酸盐过饱和、富Fe2+、Mn2+等碳酸盐沉淀抑制剂的环境条件；在地质记录中其丰度随时间的减少反映了海洋的长期氧化趋势。但是最近的研究表明鱼骨状方解石的成因存在很大争议。本立项首次在华北地台中元古代高于庄组四段微生物礁内发现了大量鱼骨状方解石。宏观观察表明，这些鱼骨状方解石主要以微生物礁孔洞充填物形式产出，明显区别于太古宙以海底沉淀形式直接产出在海底的鱼骨状方解石。显微研究发现，鱼骨状方解石晶体纤维具有沿晶体生长方向旋转消光特征，证明其内部亚晶的光学C轴从纤维底部的随机排列逐步旋转至上部垂直纤维生长方向。这符合球状晶体生长模式，需要方解石沉淀抑制剂的参与。鱼骨状方解石产出具有丰度随时间减少以及产出形式由海底沉淀向孔洞胶结物转变的特征。这些特征与海洋氧化逐渐增强以及具氧化还原敏感属性的碳酸盐沉淀抑制剂逐渐从水体中移除相吻合。我们认为鱼骨状方解石的沉淀抑制剂为Fe2+，这与微生物岩无明显Ce异常和Fe2+极强的抑制能力相一致。因此，鱼骨状方解石可用于指示缺氧环境条件。此外，显微和超微研究也表明鱼骨状方解石晶体内存在有大量与其生长方向一致的菌丝体残余和与之密切伴生的有机矿物，表明微生物为鱼骨状方解石成核和初始沉淀提供了重要垫板。本立项进一步j将研究结果成功应用至高于庄组含鱼骨状方解石地层的古氧相分析中，得出了与有机碳同位素、铁组分以及氧化还原敏感元素分析一致的结论。本立项还将研究对象扩展到赤铁矿、海绿石、鲕绿泥石等矿物的研究中，并取得了一系列创新性的认识。