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 δ 18 O and Δ' 17 O compositions.The subsequent replacement of carbonate layers is mimicked by decreasing δ 18 O and δ 30 Si.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 30/28 Si during silicification, conforming with predictions for isotope distribution at chemical equilibrium from ab-initio models.Application of the modelled Ɛ 30 Si-T relationship yields silicification temperatures of approx.50 • C. To reconcile the δ 18 O chert composition with these temperatures, the δ 18 O 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 δ 18 O composition of the fluida highly desired value needed for accurate reconstructions of the temperature-and δ 18 O histories of the oceans.