Pyrite is the most common sulfide mineral in rocks near Earth’s surface. The oxidative weathering of pyrite (OWP) is usually the earliest alteration reaction observed in many formations. In rocks with low porosity such as shale, pyrite is transformed to Fe oxide pseudomorphs through OWP. Therefore, the distributions of pyrite and the Fe oxide pseudomorphs with depth record the advance of weathering that reflect cumulative interactions among meteoric water, gas, and rock over time.<br/><br/>We investigated the Rose Hill shale weathering at the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania, USA. In that shale, oxidation mostly occurs above the water table under ridges, but pyrite oxidation is also occurring in haloes around deeper fractures in the saturated zone, presumably where those fractures are connected, allowing oxygenated water to flow. Near such fractures, only Fe oxide pseudomorphs were observed, and pyrite grains were observed only in the inward matrix of shale. Such patterns indicate the penetration of oxygen through matrix controls the OWP reaction at clast scale. Reaction fronts of OWP are observed in some partially pseudomorphed pyrite grains where the cores remain pyrite but are rimmed by Fe oxide. The sulfur in the crystalline and non-porous inner pyrite core progressively becomes replaced by oxygen to form ferrihydrite or feroxyhite across a sharp interface and the lattices are not coherent across the interface. No sulfate minerals were observed. The observations are consistent with an interfacial dissolution-reprecipitation mechanism at grain scale. We concluded that pyrite oxidation is rate-limited by diffusion of oxygen to the grain surface and regulated by large-scale erosion and clast-scale fracturing under the atmosphere today. We also revealed microbial activity does not control the oxidation rate of pyrite today, while microorganisms might accelerate the rate when the atmospheric oxygen concentrations were lower enough (e.g. before the Great Oxygen Event) that rate of pyrite oxidation was limited by the slower reaction kinetics.

Investigating weathering in shale across scales by tracking pseudomorphic transformation of pyrite

ACS Spring Meeting 2021

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