Basalt and rhyo-dacite weathering and soil clay formation under subtropical climate in southern Brazil

International audienceThe mineralogy of highly weathered soils is dominated by kaolinite-group clay minerals. However in the highlands of southern Brazil the subtropical climate (mean annual temperature similar to 15 degrees C, mean annual precipitation 1900 mm/yr) favored accumulation of organic matter and occurrence of 2:1 clay minerals. The objective of this work was to compare the weathering of the two main rock types in the volcanic plateau in the Rio Grande do Sul in southern Brazil in order to consider soil fertility and sustainability in this intensively cropped area. Two weathering profiles were sampled on basalt and rhyo-dacite at elevations ranging between 760 and 850 m a.s.I. The soil minerals were analyzed by X-ray diffraction, chemical extractions, and FTIR spectroscopy. Weathering intensity was inferred from bulk chemical analyses and calculation of weathering indices (Chemical Index of Alteration, CIA; Weathering Intensity Scale, WIS) and elemental mass balance. Mineralogy was dominated by kaolinite and crystalline iron and aluminum oxy-hydroxides and 2:1 clay minerals. The surface horizons contained hydroxyl-Al-interlayered minerals originating from aluminization of smectite present in the saprolite under acidic conditions, with the important accumulation of organic matter. Results of mass balance calculation showed extensive loss of all elements (high Delta 4Si, WIS and CIA) with residual accumulation of Fe and Al oxy-hydroxides. The degree of weathering is higher on rhyo-dacite compared to basalt although rhyo-dacite rock is richer in SiO2. Given the importance of 2:1 clay minerals on soil properties (e.g. sorption, aggregation) a better understanding of their formation and transformation according to geochemical conditions is a key issue in the management of these sub-tropical soils