Despite the growing interest and application of the analyses of silicon isotopes in geologic studies, the silicon isotopic budget and cycle in ancient oceans is still poorly constrained. This study uses silicon isotopic data from silicified carbonate rocks to investigate their paragenesis and discusses the implications of the data to the silicon isotopic budget and overall silicon cycle in pre-diatom oceans. Petrographic study of silicified coral fossils from the mid-Paleozoic Cutter and Fusselman formations from New Mexico and from the mid-Cenozoic Tampa Formation and Suwannee Limestone from Florida demonstrated that microquartz and chalcedonic microspherulites comprising the septal walls conform to accommodation space left adjacent to the megaquartz void-fill. In situ analyses shows that δ30Si(NBS28) compositions of silica defining the septal walls and silica that fills the septal voids range from - 5.23 ‰ to + 1.03 ‰. Silicon isotopic data is consistent with fractionation of a finite silica reservoir, with favored incorporation of lighter isotopes through kinetic fractionation. Void-filling megaquartz was composed of isotopically lighter Si than adjacent septal wall microquartz, confirming petrographic evidences of early silicification of the voids compared to microquartz comprising the septal walls. Application of the isotopic data from silicified coral fossils were used to estimate pre-diatom ocean water silicon isotopic compositions, and indicate the need for changes in the model used to calculate dissolved silica δ30Si of pre-diatom oceanic environments of silicification. A correction is proposed to the fractionation factor of sponge spicules used in the model. The correction is based on a Michaelis–Menten enzyme kinetics model and estimates ancient sponge fractionation factor values of -4.68 ‰. Using this correction, the dissolved Si was estimated at +1.99 ‰ to +2.28 ‰ δ30Si(NBS28) for Paleozoic ocean waters. These values are representative of an open-shelf marine environment, in contrast with previous values which were calculated for restricted marine paleoenvironments. This allows for a modelling of how buffering by marine water affects the isotopic compositions of different marine environments. Clumped isotope data from the dolostone hosting the ribbon chert nodules of the Paleozoic Aleman Formation of New Mexico indicates diagenetic temperatures of 28.4 to 40.0oC. Si isotopic compositions corroborates petrographic evidence of silicification in an essentially closed system in the mixing zone (meteoric and marine water) of a supratidal environment. Si isotopic data also indicates early silicification of the nodules. Moreover, the Si isotope composition of the marine water in this depositional environment was estimated at +2.28 ‰ to +2.43 ‰, in accordance with previously published values for pre-diatom open ocean water δ30Si(NBS28) of dissolved silicic acid. Further work is suggested to better constrain the temperature of silicification through the use of techniques such as triple O thermometry, and more detailed investigation of the Si isotopic fractionation amongst different fabrics of chert.



silicon isotopes, carbonate rocks, sedimentology, diagenesis, isotopic geochemistry


Portions of this document appear in: Paragenesis of silicified mid-Paleozoic and mid-Cenozoic corals based on petrography and silicon isotopic analyses” Andrade, C. N., Chafetz, H. S., Lapen, T. J. – Chemical Geology, vol. 538.,