Silicon and oxygen isotope fractionation in a silicified carbonate rock

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 δ¹⁸O and Δ’¹⁷O compositions. The subsequent replacement of carbonate layers is mimicked by decreasing δ¹⁸O and δ³⁰Si. 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 Ɛ^30/28Si during silicification, conforming with predictions for isotope distribution at chemical equilibrium from ab-initio models. Application of the modelled Ɛ³⁰Si-T relationship yields silicification temperatures of approx. 50 °C. To reconcile the δ¹⁸O_chert composition with these temperatures, the δ¹⁸O 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 δ¹⁸O composition of the fluid – a highly desired value needed for accurate reconstructions of the temperature- and δ¹⁸O histories of the oceans.