Abstract

Weathering profiles in regolith record the long-term interaction between meteoric water and bedrock. Here we investigate the distribution of pyrite and its weathering products in a first-order catchment (0.08 km2 ) at the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania, USA. The well-studied catchment is developed on gray shale of the Silurian-aged Rose Hill Formation, which, when unweathered, contains 0.2-0.3 wt.% pyrite.  More than 90% of the pyrite is depleted in samples recovered
from cuttings down to a depth of 16-23 m under catchment ridges and to 8-9 m under the channel. These values, based on total sulfur analysis on bulk samples, represent averages for cuttings recovered from 0.3 m depth intervals. Under the ridge, pyrite depletion is one of the deepest weathering reactions, and roughly coincides with the water table; under the channel, deeper oxidation is attributed to deeply flowing oxygenated waters in highly fractured rock. In the unweathered bedrock, pyrite is observed as framboidal
crystals (microns to tens of microns in size). Above the oxidized zone, iron (hydr)oxide pseudomorphs after pyrite are observed. Micro-Raman spectroscopy suggests that these iron (hydr)oxides are goethite. In addition, where rocks are fractured beneath the water table under the ridge, pyrite is also observed to be depleted around fractures. Some euhedral goethite grains near the fractures retain the pyrite cores, clearly demonstrating the pseudomorphic transformation of pyrite into goethite. We are pursuing FIB-SEM and TEM studies of the pyrite-goethite interface to determine the mechanism of the pseudomorphic transformation. We observed similar pyrite-goethite transformations in two other shale weathering profiles in California, USA and Taiwan. The pyrite-goethite transformation might be an important biogeochemical pathway for pyrite weathering.

Citation

Xin Gu*, Peter Heaney, and Susan L. Brantley (2018): Pyrite oxidation during shale weathering: from catchment – to nanometer – scale. Abstract 12e:306 presented at 2018 Goldschmidt Meeting, Boston, MA 12-17 August .

This Paper/Book acknowledges NSF CZO grant support.