Silicon isotope fractionation during silicification is poorly understood and impedes our ability to decipher paleoenvironmental conditions from Si isotopes in ancient cherts. To investigate isotope fractionation during silica-for‑carbonate replacement we analyzed the microscale Si and O isotope composition in different silica phases in a silicified zebra dolostone as well as their bulk δO and Δ’O compositions. The subsequent replacement of carbonate layers is mimicked by decreasing δO and δSi. The textural relationship and magnitude of Si and O isotope fractionation is best explained by near-quantitative silica precipitation in an open system with finite Si. A Rayleigh model for silicification suggests positive Si during silicification, conforming with predictions for isotope distribution at chemical equilibrium from ab-initio models. Application of the modelled Si-T relationship yields silicification temperatures of approx. 50 °C. To reconcile the δO composition with these temperatures, the δO of the fluid must have been between −2.5 and − 4 ‰, compositions for which the quartz phases fall close to the oxygen equilibrium fractionation line in three-isotope space. Diagenetic silica replacement appears to occur in O and Si isotopic equilibrium allowing reconstructions of temperatures of silicification from Si isotopes and derive the δO composition of the fluid – a highly desired value needed for accurate reconstructions of the temperature- and δO histories of the oceans.

中文翻译：

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硅化碳酸盐岩中的硅和氧同位素分馏


人们对硅化过程中的硅同位素分馏知之甚少，这阻碍了我们从古代燧石中的硅同位素破译古环境条件的能力。为了研究二氧化硅碳酸盐置换过程中的同位素分馏，我们分析了硅化斑马白云岩中不同二氧化硅相中的微尺度 Si 和 O 同位素组成及其本体 δ18O 和 Δ’O 组成。随后的碳酸盐层替换是通过减少 δO 和 δSi 来模拟的。 Si 和 O 同位素分馏的结构关系和大小可以通过在 Si 有限的开放系统中近定量的二氧化硅沉淀来最好地解释。硅化的瑞利模型表明硅化过程中 Si 为正，这与从头算模型对化学平衡时同位素分布的预测一致。应用模拟的 Si-T 关系可产生约 100°C 的硅化温度。 50°C。为了使 δ18O 组成与这些温度相一致，流体的 δ18O 必须在 -2.5 和 -4 ‰ 之间，石英相的组成接近三同位素空间中的氧平衡分馏线。成岩二氧化硅置换似乎发生在 O 和 Si 同位素平衡中，允许从 Si 同位素重建硅化温度并推导流体的 δ18O 组成——这是精确重建海洋温度和 δ18O 历史所需的高度期望值。