Abstract Major phosphogenic events took place in the late Paleoproterozoic and the Ediacaran, both of which followed an episode of atmospheric oxygenation. These apparent coincidences have led to suggestions that the Precambrian phosphorus cycle was partly regulated by oceanic redox conditions, although direct evidence is lacking. In this paper, we report the discovery of glaucony in drill cores from the phosphorite-rich Ediacaran Doushantuo Formation at Weng'an, South China, which provides new insights into redox conditions and geochemical cycles during the deposition and early diagenesis of the lower Doushantuo Formation. The presence of flaser and lenticular bedding in the sandstones suggests deposition in a tidally-influenced shallow water environment. Apatite is present as millimeter-scale flasers in sandstones, but more commonly as intraclastic grains. Glaucony typically occurs as interstitial aggregates cementing and replacing detrital minerals and phosphatic grains, with an upsection decrease in abundance. The Doushantuo glauconies have consistently high K2O (7.6 – 9.6 wt%). In particular, phosphorite-hosted glaucony has generally higher total iron (TFe2O3) than sandstone-hosted glaucony, consistent with Al3+ - Fe3+ substitution in the octahedral site, as supported by Raman peaks slightly shifted towards lower wavenumbers for higher Fe content. A decrease in detrital input probably resulted in limited Al availability during early diagenesis, which in turn led to the waning of glaucony upsection. The phosphate grains do not show any negative Ce anomalies, consistent with precipitation of authigenic apatite in suboxic environments with insignificant preferential remobilisation of Ce during deposition and early diagenesis. REE + Y features show variable diagenetic REE enrichment in phosphorites, but identical concave-down patterns. The wide occurrence of glaucony cement further documents the presence of a generally suboxic early diagenetic environment on the seafloor, which was in active exchange with bottom seawater. Slight fluctuations of redox states in the porewater and bottom water would have promoted the cycling of Fe, enrichment of phosphate and REE, and extensive glaucony authigenesis. Therefore, the glaucony-bearing phosphorites in the Doushantuo Formation are a direct result of Fe-redox driven diagenesis. Apatite-glaucony association likely forms in suboxic environments and is considered as a useful mineralogical proxy for paleoredox conditions on the Precambrian ocean floors.

中文翻译：

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华南埃迪卡拉纪陡山沱组磷灰石-蓝藻组合及其对海洋氧化还原条件的影响

摘要 主要的磷生事件发生在晚古元古代和埃迪卡拉纪，两者都发生在大气氧合事件之后。这些明显的巧合导致人们认为前寒武纪磷循环部分受海洋氧化还原条件的调节，尽管缺乏直接证据。在本文中，我们报道了在华南瓮安富含磷矿的埃迪卡拉纪陡山沱组钻探岩芯中发现的蓝藻，为了解下兜山沱组沉积和早期成岩过程中的氧化还原条件和地球化学循环提供了新的见解。 . 砂岩中存在闪石和透镜状层理表明沉积在受潮汐影响的浅水环境中。磷灰石在砂岩中以毫米级闪烁体存在，但更常见的是碎屑内颗粒。Glaucony 通常发生在间隙聚集体胶结并取代碎屑矿物和磷酸盐颗粒时，随着丰度的上升而减少。豆山沱冰川的 K2O 含量一直很高（7.6 – 9.6 wt%）。特别是，磷矿承载的蓝藻通常比砂岩承载的蓝藻具有更高的总铁 (TFe2O3)，这与八面体位点中的 Al3+ - Fe3+ 取代一致，由拉曼峰的支持，为更高的 Fe 含量略微向低波数移动。碎屑输入的减少可能导致早期成岩过程中铝的可用性有限，进而导致蓝藻上部的减少。磷酸盐颗粒没有显示任何负的 Ce 异常，与自生磷灰石在低氧环境中的沉淀一致，在沉积和早期成岩过程中，Ce 的优先再动员不显着。REE + Y 特征显示磷矿中成岩 REE 富集可变，但下凹模式相同。海藻胶的广泛存在进一步证明了海底存在普遍缺氧的早期成岩环境，与底部海水进行活跃交换。孔隙水和底水中氧化还原状态的轻微波动会促进铁的循环、磷酸盐和稀土元素的富集以及广泛的蓝藻自生。因此，豆山沱组含蓝藻磷矿是铁氧化还原驱动成岩作用的直接结果。