The occurrence of microfossiliferous, early diagenetic chert in Proterozoic successions is broadly restricted to peritidal marine environments. Such coastal environments are amongst the most environmentally variable of marine environments, experiencing both enhanced evaporation and potential influx of terrestrial freshwaters. To better understand potential conditions under which silicification occurs, we focus on microfossiliferous early diagenetic chert from the Mesoproterozoic Bylot Supergroup, northern Baffin Island. Spectacular preservation of silicified microbial mats, their associated mineral phases, and the petrographic fabrics of the chert itself require that silicification occurred at the sediment–water interface, penecontemporaneously with mat growth. In some cases, silica is the primary precipitated mineral phase and is not associated with replacement of precursor mineral phases. In other cases, silica deposition includes the mimetic replacement of carbonate, gypsum, and halite mineral phases. These petrographic constraints suggest that silicification potentially occurred under a range of fluid chemistries associated with environmental variability in nearshore peritidal environments. Here we provide the first direct thermodynamic modeling of hypothetical Proterozoic seawater solutions, seawater‐derived brines, and mixed seawater‐freshwater solutions, and demonstrate that peritidal environments are capable of providing a wide range of fluid chemistries under which early diagenetic silica can both precipitate and replace primary mineralogical phases. Despite the thermodynamic potential for silica deposition under a wide range of fluid compositions, chert is not ubiquitous in Proterozoic nearshore environments, suggesting that the kinetics of silica polymerization exert a primary control over deposition.

中文翻译：

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元古代微生物垫及其对硅化环境的限制。

在元古代，微化石，早成岩的硅质t石的发生被广泛地限制在潮汐海洋环境中。这种沿海环境是海洋环境中环境变化最大的环境之一，其蒸发作用增强，陆地淡水可能大量涌入。为了更好地了解发生硅化作用的潜在条件，我们集中研究了北巴芬岛中元古代Bylot超群的微化石早成岩硅质岩。硅化微生物垫及其相关的矿物相以及石本身的岩相织物的壮观保存要求硅化作用发生在沉积物与水的界面处，同时随着垫的生长而渗透。在某些情况下，二氧化硅是主要的沉淀矿物相，与替代前体矿物相无关。在其他情况下，二氧化硅沉积包括碳酸盐，石膏和岩盐矿物相的模拟替代。这些岩相学约束表明硅化作用可能发生在与近岸蠕动环境中的环境变化有关的一系列流体化学作用下。在这里，我们提供了假设性的元古代海水溶液，海水衍生的盐水和海水淡水混合溶液的第一个直接热力学模型，并证明了蠕变环境能够提供广泛的流体化学性质，在这些化学性质下早期成岩的二氧化硅既可以沉淀也可以沉淀。取代主要的矿物相。