Quantifying early mineral weathering reactions in serpentinite bedrock

Abstract

Understanding early chemical reactions that initiate weathering in cohesive bedrock is crucial to predicting porosity development, nutrient availability, and trace element mobility in later stages of soil formation. We seek to characterize incipient chemical weathering in serpentinite bedrock containing primarily serpentine and augite from a former quarry on Little Deer Isle, ME, USA. Bedrock and a small amount of overlying soil were sampled along a quarry face to a depth of 6 m and analyzed using SEM, XRD, XRF, and electron microprobe for chemistry and primary and secondary mineralogy to determine chemical and mineralogical changes as a function of depth.Extensive chemical and mineralogical evidence of both dissolution and precipitation extend approximately 1 m into the bedrock. Approximately 50% of Ca is depleted to 1 m in the bedrock; additionally, we observed weathering textures including dissolution pits and denticulated edges within augite in the top 0.5 m of bedrock. Although serpentine shows evidence of weathering in the soils, we see minimal evidence of chemical or textural serpentine weathering within the bedrock. We observed small quantities of secondary Fe–Mn-(oxyhydr)oxides and possible Cr-bearing smectite, although analytical methods made clay identification difficult. The presence of barite and possible Cu-bearing secondary minerals along existing fracture networks suggests mobilization and reprecipitation to depths of 4 m. Based on both chemical and mineralogical evidence, it is likely that the dissolution of augite controls porosity development within the weathering profile. Augite dissolution along dissolution pits, denticulated fractures, and grain boundaries contributes to secondary porosity during incipient weathering of the bedrock, promoting further weathering of augite and accessory minerals and eventual dissolution and disaggregation of serpentine. This is confirmed by calculated field weathering rates for augite.