Origin of reddening in a paleosol buried by lava flows in Fogo island (Cape Verde)

Abstract

In Fogo island (Cape Verde), a buried paleosol with a reddened topsoil developed on mafic lapilli, intercalated between intrusive carbonatites and an alkaline lava flow, was studied. Significant mineralogical, crystalchemistry and chemical variations were found along the profile. Upwards a decrease of the grain size occurs. Increasing weathering of magnetite to maghemite, and ultimately to hematite is also clearly observed. The Fe3+/Fe2+ ratio in silicate phases increases, along with an increase of illite proportion in the mixed-layer I-Sm. Discrete smectite was detected only in the bottom level related with weak drainage conditions. In the upper level there is a significant gain of Na, Cr, P, Si, and particularly As. The colour variations (towards the paleosurface: dark reddish brown – yellowish brown – red – weak red) are mainly due to the fine fraction of the various levels. Hematite is the main iron phase responsible for the colour and may explain the significant retention of Cr, P, As and LREE in the topsoil. Here hematite is more abundant and occurs as larger-sized particles than in the lower levels. This hematite grain growth is probably due to the thermal effect of the overlying hot volcanic unit. Thus the main alteration processes of the lapilli deposits appear to have been (i) weathering mechanisms in arid environment with the formation of an incipient paleosol, and (ii) thermal metamorphism due to the overlying lava contributing for reddening and cementation of the upper levels. Biological remains are absent due to the paleoenvironmental conditions and/or to the effect of heating.