Taxa de Dissolução do Plagioclásio em uma Bacia de Drenagem Granito-Gnáissica de uma Floresta Ombrófila Densa Montana

Abstract

A 180 ha area in the headwater of Paquequer watershed, a drainage system located at the Teresopolis municipality, was selected for the evaluation of chemical weathering from collected river water samples in February and August 2013 in a granitic-gnaissic substrate free of direct anthropogenic influence for being located in a conservation unit (Serra dos Orgaos National Park). The study area has a native forest cover, an altitude range of 1100-1900 m and an average annual rainfall of 2800 mm. The dissolved elements (Na, Mg, Ca, K, Si, Al) were analyzed by inductively coupled plasma spectrometries and the anions Cl - and SO 4 2- , by ion chromatography. These results, combined with the concentration data of nitrogen and phosphorus compounds (Alvim, 2016), the atmospheric deposition (Rodrigues, 2006), and the litogeochemistry (Zorita, 1979; Tupinamba, 1999; Tupinamba et al., 2012) were used to generate the dissolution model of mineral, employing the PHREEQC software. The mass balance indicate Na as the main chemical specie due to weathering. Mineralogical data show plagioclase, quartz, k-feldspar and biotite as substrate’s most abundant minerals, among which only plagioclase has sodium in its composition. According to Goldich’s series (mineral stability to weathering), plagioclase was identified as the main weathered mineral in this assemblage, being consumed at a rate of 649 mol ha -1 year -1 and forming the minerals kaolinite and gibbsite, with the release of a minimum of 818 mols ha -1 year -1 and a maximum of 1490 mols ha -1 year -1 of dissolved silica in the river (measured value). Considering plagioclase as the main mineral responsible for Na flux in the basin, stoichiometric calculations in association with Si ratio, it was estimated that only from the plagioclase weathering, the soil should be composed by 14% of kaolinite and 86% of gibbsite in percentage relative of the two mineral phases. The quantitative analysis of the mineralogy (Rietveld method) confirmed the calculations, showing a soil composition 7% kaolinite e 93% gibbsite, and attested the importance of plagioclase weathering in fluvial hydrogeochemistry and soil composition.