Paragenesis of silicified mid-Paleozoic and mid-Cenozoic corals based on petrography and silicon isotopic analyses

Abstract

Petrographic analyses of silicified coral fossils from the Upper Ordovician Cutter Formation and Silurian Fusselman Formation of New Mexico, and the Paleogene-Lower Neogene Tampa Member and Suwannee Limestone of Florida show that septal voids of these coral fossils are filled with megaquartz and septal walls are replaced by chalcedonic microspherulites and microquartz. The boundaries of the microquartz and spherulites of the septal walls conform to accommodation space left by the megaquartz that filled the septal voids. These textures suggest that megaquartz filled the septal voids prior to silicification of the septal walls. The measured δ30Si compositions of silica defining the septal walls and silica that fills the septal voids range from −5.23‰ to +1.03‰. The void-filling silica has isotopically light Si and the septal wall replacement Si is isotopically heavier. The silicon isotopic data are consistent with distillation of a finite silica reservoir which favors incorporation of lighter silicon isotopes in earlier-formed silica by Rayleigh fractionation. Distillation of light isotopes during progressive silica precipitation is also documented in vug-filling silica. Thus, silica replacement textures and silica isotopic compositions of samples from different geologic ages and locations, regardless of crystal sizes or degree of cementation of the primary porosity in the corals, support early void-filling silica precipitation followed by septal wall silica precipitation in essentially closed system environments.