华北中元古代鱼骨状方解石: 成因机制和古环境意义*

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。文中首次在华北地台中元古界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。笔者认为鱼骨状方解石的沉淀抑制剂为Fe²⁺和Mn²⁺,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。

Mesoproterozoic herringbone calcite from North China Platform:Genesis and paleoenvironmental significance

Herringbone calcite(HC),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 HC is a kind of inorganic chemical precipitation,formed in anoxic,calcium carbonate supersaturated,and precipitation inhibitor (Fe²⁺,Mn²⁺)￣rich seawater. Therefore,it has been used as an indicator of anoxic seawater conditions,and its abundance decreases over geologic time is thought to have reflected the increasing oxidation of the ocean. However,recent studies suggest that the genesis of HC might have been variable. For the first time,we have found HC in the microbial reef of the Mesoproterozoic Gaoyuzhuang Formation in the North China Platform. Macroscopically,these HCs mainly occurred in the voids of microbial reef as void-filling,distinct from their Archean counterparts which largely occurred as seafloor precipitation. Microscopically,the HC fiber shows a characteristic of rotated distinction along the crystal growth direction,indicating that the internal crystals of each HC fiber was once rotated from the bottom to the top. This is in accordance with the spherulitic growth pattern,therefore,requiring the participation of the calcite precipitation inhibitors. The secular decrease in HC abundance and the changes in their occurrence pattern from seafloor precipitation to void-filling,suggest that HC precipitation inhibitors should be redox-sensitive elements,which have been continuously removed from seawater with the oxidation of the ocean. Thus,we think that the major inhibitors for HC precipitation are Fe²⁺ and Mn²⁺,and HC can be used as a mineral indicator for anoxic environment,which is consistent with the Ce anomaly results(without obvious anomalies)in the microbial reef and the presence of strong inhibitor effect of Fe²⁺. In addition,both microscopic and ultramicroscopic observations revealed that there are a large number of bacterial filamentous relics and closely associated organominerals concentrated along the axis of HC fibers,indicating that the microbes have provided favorable sites for the initial nucleation and subsequent growth of HC crystals.