A simple three-dimensional model of diffusion-with-precipitation applied to localised pyrite formation in framboids, fossils and detrital iron minerals

Abstract

A three-dimensional diffusion-with-precipitation model is constructed to estimate radial variations in the amounts of pyrite which precipitate where a spherical mass of organic matter, producing H 2S by sulphate reduction, is enveloped in a dissolved-iron bearing porewater. The model indicates that higher rates of sulphate reduction (more readily metabolisable organic matter), and larger organic masses, require increasingly high dissolved iron concentrations in order to confine pyrite (or iron sulphide) precipitation to the decay site. The maximum size sphere of exceedingly metabolisable organic matter (equivalent to fresh planktonic material) which can be pyritised is about 50 μm radius, where decay occurs in porewaters with typical dissolved iron levels. This radius is close to the maximum radius of framboidal pyrite, the formation of which could involve model-type processes. Fossil carcases, although mainly composed of less readily metabolisable organic matter, may be orders of magnitude larger and the model demonstrates that their pyritisation requires unusually high porewater dissolved iron concentrations. These inferred chemical conditions are consistent with sedimentological observations of pyritisation in Beecher's Trilobite Bed (New York State). At greater depths within the sediment, pyritisation is controlled by the kinetics of iron mineral reactivity towards H 2S. Sediments vary widely in their exposure times to H 2S which can range at least from 50 to 10 6 years. At low exposure times only iron oxides are pyritised, whereas at high exposure times even the most refractory iron silicates can become pyritised.