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.
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维普
