Do decades of tropical rainfall affect the chemical compositions of basaltic lava flows in Mount Cameroon ?
Résumé
To evaluate the effects of tropical rainfall on the compositions of basaltic lavas, we studied 20th century lava flows from Mount Cameroon in Africa. Weathering conditions are extreme because the climate is particularly warm and humid, and vegetation grows extremely quickly on the flows. The high rainfalls and dense vegetation contribute to rapid and intense degradation of the volcanic rocks and should cause significant changes in chemical composition. Such effects need to be quantified to constrain how young a lava flow must be so that its trace element and isotopic composition remains representative of the original magma. Fresh inner parts and altered flowtops of four different lava flows were sampled and analysed for major and trace elements as well as O, U, Sr, Nd and Pb isotopic compositions. Four samples of the 1999 eruption were also analysed to constrain the composition of fresh basalts. Almost all major and trace elements display similar concentrations in inner and outer parts of the same flow. This is notably the case for elements such as K, Rb and Sr, which are highly mobile during weathering. The lack of variation suggests that the overall composition of the lava flows has not been significantly affected. However, some systematic chemical changes are observed: Loss-on-ignition (LOI) and d18O increase slightly from inner parts of flows to near surface samples; Na and, to a lesser extent, U display significant losses in the outer samples. We interpret the Na loss in terms of hydration leading to exchange between Na+ and H+ ions. This process, associated with oxidation of Fe2+ to Fe3+, accounts for the larger loss-on-ignition in the outer parts of flows. A change in U contents is only observed in the 1922 flow, which is covered by dense vegetation. This emphasizes the role that complexation by organic ligands plays in U mobility. While U is not completely immobile, all volcanic rocks are in secular equilibrium ((234U/238U)—1), indicating limited interaction between meteoric waters and basalts. Sr and Nd isotopic compositions remain constant and although 206Pb/204Pb ratios vary from inner to outer parts of the lavas, the changes are not systematic and cannot be attributed to weathering. The absence of significant chemical mobility and substantial isotopic exchange suggests very limited interaction between water and lavas at the centimeter scale, due, most probably, to their relatively young age. This work shows that over a period of about 100 years, no detectable geochemical changes are observed. These results are very promising for petrological and geochemical studies of ocean island basalts located in tropical areas since they provide direct evidence of very limited trace element mobility at the century time scale.