SLOW CHEMICAL WEATHERING OF GRANITIC ROCKS, FRONT RANGE, COLORADO

The chemistry of weathered materials and solute export values from the Middle Boulder Creek catchment, Colorado, allow us to measure slow weathering processes that produced thick regolith that locally mantles the Front Range. In areas outside the glacial limit, the weathered profile consists of 4 to 8 m of oxidized bedrock, saprolite, and grus, capped by a thin soil. We measured the density and chemistry of ~120 fresh and weathered granitic samples and the chemistry of ~30 water samples collected from small drainages in alpine and forested areas. Thin sections show that plagioclase, which comprises 15 to 35% of fresh Boulder Creek Grandiorite and Silver Plume Granite, is the main altered mineral. In thin section, biotite shows slight bleaching and expansion along cleavage planes, but plagioclase is more weathered. Microcline and quartz are little altered, suggesting the weathering sequence plagioclase>biotite>microcline>quartz. Mass balance analyses demonstrate that elemental loss increases up through the weathering profile as Ca>Mg>>Na>Si>K. Solute export in surface waters demonstrates significant losses of Ca and Mg, in addition to SiO2 and Na. Depletion of Ca in weathering profiles and molar Ca/Na values > 1 in surface water, however, indicate loss of Ca over Na in excess of that supported by congruent weathering of plagioclase. Decreased Ca/Na in weathering profiles may record rapid dissolution of minor hornblende or trace phases. Excess Ca in drainage water may reflect rapid weathering of minor amounts of calcic minerals derived from bedrock or from dustfall. Local catchments export dissolved cations + silica at rates from about 2 to 5 T m-2 kyr-1 over a runoff range of 10 to nearly 160 cm; average cosmogenic radionuclide (CRN) erosion rates in the vicinity are ~2.2 cm kyr-1. Simple modeling shows that if 70% of the void space is created by volume expansion, CRN erosion rates could be balanced by grus + soil formation from saprolite if most of the annual solute loss comes from the soil zone. Rates of saprolite formation from oxidized bedrock, however, cannot balance soil and grus losses to erosion. Thick saprolite likely reflects a relict landscape that is eroding in the present climate. Better modeling of long-term processes requires clearer understanding of altering minerals and solute origin in the weathering profile.