Pyrite oxidation in shales: Implications for palaeo-redox proxies based on geochemical and SEM-EDX evidence

Abstract

Pyrite oxidation in marine shales during weathering has been widely studied, however, the effects of this process on geochemical proxies commonly used to reconstruct ocean redox conditions, or the details of pyrite oxidation at the mineral grain scale, have not received much scientific attention. We conducted a four-week laboratory experiment designed to stimulate pyrite oxidation at shale outcrops, and to assess effects on the chemical phase of iron and sulphur in the samples both in bulk sediments (useful for comparison to palaeo-environmental geochemistry) and at a finer scale (to gain a better understanding of the nature of these changes). Geochemical and scanning electron microscopy (SEM) techniques provide evidence for pyrite oxidation, carbonate dissolution, and iron (oxyhydr)oxide formation during the experiment. The net effect of the experiment on the ratio of highly reactive iron phases (FeHR) to total iron (FeT) is minimal (≪0.03% difference), suggesting that this redox proxy behaves relatively conservatively during weathering. The effect of weathering on the ratio of pyrite-bound iron to highly reactive iron (FePY/FeHR), used to investigate the availability of sulphur, in contrast, is pronounced (up to 32.5% difference) due to the oxidation of pyrite and the precipitation of iron (oxyhydr)oxides in the shale samples. Electron microscopy provides evidence that iron (oxyhydr)oxides precipitated in situ as rims around cores of pyrite particles, “passivating” and protecting them from further oxidation. The quantification of these partly oxidised pyrite particles is now possible using a novel automated particle analysis method coupled to chemical mapping, developed in this study. We conclude that this method can be of wide use, both to quantify pyrite oxidation, and assess the significance of FePY/FeHR at measured in shale outcrop samples.