Preferential Weathering of Calcic Plagioclase

Weathering studies based on catchment solute fluxes use either average cation ratios of bedrock or the stoichiometry of mineral phases in order to partition carbonate and silicate sources of major cations, particularly calcium. Both methods basically rely on the assumption that Ca is released during weathering in proportion to the Ca/Na ratio of average unweathered plagioclase. However, the dissolution rate of anorthite (Ca end member) is known to be faster than that of albite (Na end member), and preferential weathering of calcic plagioclase cores has been observed in the field (Clayton, 1986). For the first time, we have made quantitative estimates of the impact of preferential anorthite dissolution on catchment weathering fluxes. Substantial preferential loss of anorthite has occurred in two weathering profiles from the Himalayas, both developed on schist bedrock to a depth >1m. Both profiles have plagioclase with a range of compositions (Ca/Na = 0.03 - 0.23 and 0.02 - 0.36), and show significant decrease in average plagioclase Ca/Na through the weathering profile (e.g., from Ca/Na = 0.31 at the base to Ca/Na = 0.15 at the top in one profile). We attribute this to preferential anorthite weathering, and model the changing distribution as a function of plagioclase depletion, as inferred from major element chemistry of the soils. Fitting for the dependence of plagioclase weathering rate (R) on anorthite content, so that R = a + b*[An], the model gives b/a ~ 15+/-5. This is in the same order of magnitude as the composition dependence of plagioclase weathering observed in laboratory dissolution experiments (e.g., Stillings and Brantley, 1995). The range of our current estimates would imply a substantial impact of preferential plagioclase weathering on catchment solute budgets (on the order of 50% increase in the calculated flux of Ca from silicate weathering for the Himalayan catchments studied here).