Contact metamorphism of the Mancos shale: impacts on solute release and weatherability in the East River Valley, Gothic, CO

Abstract

Alteration of rocks by contact metamorphism directly impacts rock pore structure and mineralogy, potentially decreasing weathering susceptibility and altering water residence times and release rates of aqueous solutes. Mountainous catchments, which can often be metamorphosed, are important for water and solute supply. Changes in the underlying lithology of a catchment can therefore directly impact water quality and solute release rates, imparting watershed scale changes in flow regimes and rate of flow within hydrologic systems. Here we aim to quantify differences in solute release rates from Mancos shale samples as a function of degree of metamorphism. Variable contact metamorphism of Mancos shale bedrock in the East River provides a natural test bed to investigate the role of underlying geology on weathering susceptibility, solute release rates and changing river chemistry in metamorphosed and unmetamorphosed sections of the Mancos shale. To analyze the relationship between rate of solute release and degree of metamorphism, three trials of column scale experiments were conducted for unmetamorphosed and metamorphosed Mancos shale samples collected in the East River, Colorado. Steady state calcium release rates from metamorphosed samples were found to be an order of magnitude higher than the release of calcium from unmetamorphosed samples. These solute release rates will help constrain watershed scale weathering and solute sources in the East River to better understand the relationship between rates of rock weathering and catchment solute release. Different solute release rates calculated from metamorphosed and unmetamorphosed samples at the column scale can be used to further constrain the spatial variation of weatherablity in metamorphosed and unmetamorphosed shale. Changes in release rates from unmetamorphosed and metamorphosed samples could be linked to differences present in sample mineralogy.