Very Early Chemical Alteration of Present-day Basalts in Mount Cameroon

Abstract

Mount Cameroon is located at the edge of the African continental plate and is one of the active volcanoes of the Cameroon line. Volcanic activity has been continuous for several centuries and lavas erupted on several occasions during the 20th century. Climatic conditions are extreme with very high rainfalls (2 to 12 metres per year), high temperatures all year round (26 ~ to 29~ and constant high humidity (>80%). Luxuriant vegetation is present up to altitudes of about 2000 m and this has been shown to have drastic effects on the alteration rate of basalts (Benedetti et al., 1994). Mount Cameroon is therefore an ideal site to determine and quantify chemical mobility during the very first stage in the alteration of basalts. The lava flows are usually massive with variable amounts of vesicles and few phenocrysts. Their compositions range from alkali basalts to basanites and are characterized by high contents of trace elements such as Cs and Rb and major elements such as Na20 and K20, elements that are known to be highly mobile in surface conditions. Of the six eruptions that took place during the 20th century, we selected four massive lavas dated at 1909, 1922, 1959 and 1982. For each of them, we isolated the fresh inner part and the altered surficial part of the lavas. In the case of the 1959 basalt, four slices were made in order to examine in detail potential chemical mobilities from the surface to the inner parts of the lava. All samples were analysed for major and trace elements as well as Sr, Nd and Pb isotopic compositions. As expected for alkaline lavas, trace element concentrations are high: Rb ranges from 37 to 48 ppm, Sr from 912 to 1200 ppm and Ce from 115 to 175 ppm. Trace element contents vary from flow to flow. These differences are correlated and can G6osciences Rennes, CNRS, Universit6 de Rennes 1, Campus de Beaulieu, F-35042 Rennes Cedex, France