Long Term Experimental Weathering in Granitoid Rocks: Rubidium-Potassium Ratio as an Indicator of Mineral Reaction Rates

U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA, 94025-3591, USA

Abstract

Experimental columns of fresh and exfoliated granitoid rocks and related soils were subjected to long term reactions with CO₂-charged water in order to examine changes in solute chemical ratios during phases of natural chemical weathering. The specimens were well-characterized rock and soil samples from research watersheds at Loch Vale, CO (Rocky Mountain National Park); Rio Icacos, Puerto Rico; Panola Mountain, Georgia; and the Merced River, CA (Yosemite National Park). In year-long laboratory experiments, deionized distilled H₂O, saturated with 5\% CO2 - 95\% air, was passed through columnar beds each containing 750 g of 0.25 - 0.85 mm sized samples at an average flow rate of ~10 ml/hr and ambient laboratory temperature. The Rb/K mole ratios, as measured by ICP-MS, for the effluents from the fresh granitoid rocks increased progressively from <0.0010 to between 0.0013 and 0.006. These values were compared to Rb/K ratios for K-feldspar which overlapped initial effluent ratios and to biotite ratios which approximated later effluent compositions. These results imply that the initial Rb-K releases are controlled by the rapid rate of stoichiometric dissolution of K-feldspar. Over time, this rate decreased as effluent compositions became dominated by rates of oxidation and cation release from biotite. In contrast, the ratios of weathered rock effluents were either in proximity of or less than those of the respective fresh rock effluents. Rb/K ratios of soil column effluents, in general, were lower than those from fresh rock effluents. These lower ratios from effluents of weathered rock and soils indicate effluent compositions resulting from K-feldspar dissolution. These preliminary results suggest that Rb/K ratios may be useful in identifying weathering and nutrient cycles in watersheds.

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