Detailed geochemical studies of the initial stages of weathering of alkaline rocks: Ilha de São Sebastião, Brazil

Detailed mineral and chemical studies of samples of syenite rocks with thick weathering rinds, found in the alkaline massifs on Ilha de São Sebastião, illustrate the complex interplay between variables controlling initial weathering processes. An array of geochemical information was obtained by optical and electron microscopy, X-ray analyses, mass spectroscopy and optical spectrometry. Sericite and metahalloysite are the principal phases nucleating and forming as the perthitic feldspar rock matrix disintegrates. Incipient feldspar dissolution and the formation of submicrometer X-ray opaque secondary phases were observed in “fresh rock” material using transmission electron microscopy. Pore waters, from which secondary phases nucleate and grow are under the constant influence, as alteration proceeds, of physical (e.g., accessibility of mineral surfaces to percolating fluids) and chemical (e.g. chemistry of minerals providing the dominant solutes) controls. Minor- and trace-element signatures do not vary significantly except for Zn, Rb, Sr, Y, REE and Pb. Lathanide mobilities appear related to their host-mineral stabilities with respect to ambient pore waters.