The role of disseminated calcite in the chemical weathering of granitoid rocks

Accessory calcite, present at concentrations between 300 and 3000 mg kg(-1) , occurs in fresh granitoid rocks sampled from the Merced watershed in Yose mite National Park, CA, USA; Loch Vale in Rocky Mountain National Park CO U SA; the Panola watershed, GA USA; and the Rio Icacos, Puerto Rico. Calcite occurs as fillings in microfractures, as disseminated grains within the sil icate matrix, and as replacement of calcic cores in plagioclase. Flow-throu gh column experiments, using de-ionized water saturated with 0.05 atm. CO2, produced effluents from the fresh granitoid rocks that were dominated by C a and bicarbonate and thermodynamically saturated with calcite. During reac tions up to 1.7 yr, calcite dissolution progressively decreased and was sup erceded by steady state dissolution of silicates, principally biotite. Mass balance calculations indicate that most calcite had been removed during th is time and accounted for 57-98% of the total Ca released from these rocks. Experimental effluents from surfically weathered granitoids from the same watersheds were consistently dominated by silicate dissolution. The lack of excess Ca and alkalinity indicated that calcite had been previously remove d by natural weathering. The extent of Ca enrichment in watershed discharge fluxes corresponds to th e amounts of calcite exposed in granitoid rocks. High Ca/Na ratios relative to plagioclase stoichiometries indicate excess Ca in the Yosemite, Loch Va le, and other alpine watersheds in the Sierra Nevada and Rocky Mountains of the western United States. This Ca enrichment correlates with strong prefe rential weathering of calcite relative to plagioclase in exfoliated granito ids in glaciated terrains. In contrast, Ca/Na flux ratios are comparable to or less than the Ca/Na ratios for plagioclase in the subtropical Panola an d tropical Rio Icacos watersheds, in which deeply weathered regoliths exhib it concurrent losses of calcite and much larger masses of plagioclase durin g transport-limited weathering. These results indicate that the weathering of accessory calcite may strongly influence Ca and alkalinity fluxes from s ilicate rocks during and following periods of glaciation and tectonism but is much less important for older stable geomorphic surfaces. Copyright (C) 1999 Elsevier Science Ltd.