Geochemical signatures and weathering rates in soils derived from different granites in contrasting climatic locations

ABSTRACT. We studied the mineralogical properties and chemical composition of different granites using energy dispersive X-ray spectroscopy coupled with scanning electron microscopy to understand the relationship between granite signatures and soil characteristics, including weathering patterns and soil fertility status. The discriminant analysis (DA) was based on soil physical and chemical properties and was used to differentiate soils developed from I- and S-type granites across contrasting climatic conditions in northeast Brazil. The DA identified the highest values of organic carbon, clay and plant nutrients as key variables to recognize soil profiles derived from I-type granites. In contrast, the greater sand contents and Al saturation distinguished soils developed from S-type granites. These results were related to the mineralogical characteristics of each granite type, such as a high percentage of silica-bearing minerals in the S-type granites. The highest concentrations of K, Ca, Mg, and P in I-type granites were found in feldspars, amphiboles, and apatite. These elements account for the majority of nutrients derived in their soil profiles. However, it seems that the harsh conditions of the humid tropical environment equalized the effect of the rocks on weathering rates-the difference in chemical index of alteration is more extreme in the dry region.

摘要：本研究借助耦合扫描电子显微镜（Scanning Electron Microscopy, SEM）的能量色散X射线能谱（Energy Dispersive X-ray Spectroscopy, EDS），对不同花岗岩的矿物学特征与化学成分展开分析，以探究花岗岩特征与土壤特性（包括风化模式及土壤肥力状况）之间的关联。本研究基于土壤理化性质开展判别分析（Discriminant Analysis, DA），用于区分巴西东北部不同气候条件下由I型和S型花岗岩发育形成的土壤。判别分析结果显示，有机碳、黏粒与植物养分的高含量是识别I型花岗岩发育土壤剖面的关键变量。与之相对，较高的砂粒含量与铝饱和度则是区分S型花岗岩发育土壤的标志。上述结果与两类花岗岩的矿物学特征密切相关，例如S型花岗岩中富硅矿物占比更高。I型花岗岩中钾（K）、钙（Ca）、镁（Mg）与磷（P）的高含量赋存于长石、角闪石与磷灰石中，这些元素构成了其发育土壤剖面中养分的主要来源。然而，潮湿热带环境的严苛条件似乎抵消了岩石对风化速率的影响——而在干旱区域，化学风化指数的差异更为显著。