WATER-ROCK INTERACTIONS IN TROPICAL CATCHMENTS - FIELD RATES OF WEATHERING AND BIOMASS IMPACT

The knowledge of dissolution and precipitation rates of rocks and mine rals is important to understand the effect of natural as well as human influences on the soil system. The combination of mass-balance calcul ations and water sample geochemistry allows us the computation of weat hering rates for small watersheds. The basaltic area of Ribeirao Preto (Parana, Brazil) provides an opportunity to study solute acquisition by meteoric waters in basaltic terranes, to quantify the water-basalt interaction, to address the impact of biomass on the weathering and to estimate the age of the weathering processes affecting the basalts un der humid tropical climate conditions. The mass-balance approach is ex tended to Si and trace elements as well as neoformed minerals. The com putations are showing that the transformation rates calculated with a biomass factor are 1.3-5 times faster than the rates achieved for the model not taking into account the biomass. The rates of release of Si (log k, k in mol m(-2) s(-1) Si) computed for augite and labradorite h ave the same magnitude: -12.6 and are two orders of magnitude smaller than the experimental rates. The rate of precipitation of kaolinite (l og k, k in mol m(-2) s(-1) Si) is -15.2. The mean rate of lowering of the weathering front has a value of around 40 +/- 10 mm kyr(-1) under humid tropical climate. A mean age of 700 +/- 200 kyr is calculated fo r the weathering processes affecting the basalts of the Parana basin.