Premiers stades d'altération de ponces volcaniques en climat tropical humide (Montagne Pelée, Martinique)

Abstract The early stages (0–2000 years) of soil development on dacito-andesitic pyroclasts in the humid tropics were studied in a chronosequence made of three main deposits, dated 48 BP, 650 BP and 1670 BP, respectively, which cover the massif of Montagne Pelee (Martinique). The initial pumices are essentially glassy, rich in silicon and aluminium, calcium and sodium. The nature of secondary products was investigated through particle-size fractionation, elemental analysis, selective dissolution of amorphous products, differential thermal analysis, XR diffraction and electron microscopy. Within this period, the weathered horizon reached a depth of 30 to 50 cm. The median size of particles decreased from approximately 2 mm (48 BP) to 0.2 mm (1670 BP). The mass of secondary products was estimated to be 1–2% in the youngest soil (48 BP) and approximately 10% in the oldest one (1670 BP). Seventy percent of the secondary products were found in particles finer than 20 μm. These products are aluminium-rich and mainly composed of allophanic products. Low quantities of clay minerals were detected, mainly microspheric halloysite.