摘要:
目的Girvanella(葛万菌)是一种常见于寒武系碳酸盐岩中的蓝细菌化石,它对许多微生物岩的形成起到了极其重要的作用。尽管人们目前对于钙化Girvanella及其钙化机制有了较为深入的认识,然而, 关于对铁矿化Girvanella及其铁矿化机制的理解仍然比较模糊。方法本文通过沉积学、矿物学和地球化学的综合研究,探讨了河南卫辉地区寒武系苗岭统馒头组三段中铁质微生物岩中Girvanella的结构特征及其铁矿化作用。结果研究结果表明,研究区铁质微生物岩的纹层或凝块均由密集分布的Girvanella组成。Girvanella管芯由亮晶方解石晶体组成,而Girvanella管壁由大量直径0.1-0.5μm、杂乱分布的片状赤铁矿晶体组成。结论Girvanella通过二氧化碳浓缩机制,在其周围产生大量阴离子,吸附阳离子Fe3+和Al、Ca 、Mg和Si在其胞外聚合物（EPS)上成核,从而形成了隐晶质的赤铁矿。这种微生物成因的赤铁矿在矿物形态及元素组成上比化学沉淀形成的赤铁矿明显要复杂一些。Girvanella铁矿化作用形成的隐晶质赤铁矿和化学沉淀形成的自形晶体鲕绿泥石在铁质微生物岩中的共同发育表明,蓝细菌微生物席（膜）内存在一个氧化还原梯度的波动和含铁矿物的相变,也存在三价铁与二价铁矿物的共存。
Abstract:
[Objective] Girvanella, a cyanobacterial fossil commonly found in Cambrian carbonates, plays a crucial role in the formation of many microbialites. Despite considerable advancements in elucidating calcifying Girvanella and its biomineralization mechanisms, the understanding of iron-mineralized Girvanella and its iron-mineralization mechanism is still relatively vague. [Methods]This paper explores the structural characteristics of Girvanella and its iron mineralization in ferruginous microbialites from the Mantou Formation Member Ⅲ (Cambrian Miaolingian Series) of Weihui area, northern Henan through an integrated analysis of sedimentology, mineralogy and geochemistry. [Results]The results indicate that the lamina or clots of ferruginous microbialites in study area are dominated by densely distributed Girvanella. The tube core of Girvanella is primarily composed of crystalline calcite crystals, whereas the tube wall of Girvanella is composed of a large number of sheet-like hematite crystals with diameter of 0.1-0.5 μm and random distribution. [Conclusions]Through CO2-concentrating mechanism, Girvanella generates a large number of anions in its surroundings，which adsorb Fe3? cations along with Al、Ca 、Mg and Si, facilitating nucleation on Girvanella EPS, leading to the formation of cryptocrystalline hematites. This kind of microbially induced hematite is significantly much complex in mineral morphology and elemental composition than chemically precipitated hematite. The co-occurrence of cryptocrystalline hematite minerals formed by the iron mineralization of Girvanella and idiomorphic chamosite crystals formed by chemical precipitation within ferruginous microbialites indicate that there are fluctuating changes in the redox gradient and iron mineral phase, as well as involving the coexistence of ferric and ferrous minerals within the cyanobacterial microbial mat（film）.
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