Advances in geochemical footprint, iron speciation, and Pb isotopes in the tropical regolith of north-northwestern Amazonia, Brazil

Abstract

A dataset with information on samples of gabbro, charnockite, gneiss, granite, basalt, and sandstone-derived iron lateritic duricrusts, concretions, nodules, and soils was analyzed to address the multi-element chemical composition, background values, scavenger trace elements in Fe oxyhydroxides, parent rock inheritance, and weathering effect on Pb isotopes. Tropical weathering causes high mass loss of SiO2, Ba, Rb, and Sr assigned to the mineral rock framework (quartz, feldspar, and mica). The aqua regia leaching indicates that part of these elements was probably accumulated in authigenic Al-phosphates minerals affecting their mobility in the lateritic terrains. There is a generally Sc and V high accumulation in the lateritic duricrusts with Fe oxyhydroxide mineral framework (goethite, hematite, and amorphous). Th, U, and Pb are also accumulated on the gabbro and granite-derived lateritic duricrusts, Cr on the basalt-derived ones, REE on the gabbro-derived ones, as well as Th and U low to high loss on gneiss-derived ones, thus allowing to track inheritance of parent rocks. The more refined geochemical information obtained from the Pb isotopes related to the Fe oxyhydroxides of the lateritic duricrusts, concretions and soils indicated changes according to the parent rocks since the gabbro-derived lateritic duricrust samples are more radiogenic while the gneiss and granite-derived ones are less radiogenic. The basalt and sandstone-derived lateritic duricrust samples maintain the radiogenic signature of the respective parent rocks. Although homogenization was expected, the mineral, the main and trace elements, and Pb isotope compositions produced by leaching and accumulation in the tropical regolith highlight the parent rock signature and a complex weathering process.