Ijolite versus carbonatite as sources of fenitization

Ijolite‐carbonatite complexes are ubiquitously surrounded of an aureole of metasomatically altered rocks. The process of alteration is termed fenitization and is generally caused by peralkaline fluids emanating from cooling alkaline, i.e. ijolite and carbonatite magmas. Ijolites and carbonatites normally occur together and attempts to determine the source of the fenitizing fluids may therefore lead to controversial, if not erroneous, conclusions. Mineralogical and chemical data of fenites from Oldoinyo Lengai (Tanzania), Fen (Norway), and Alnö (Sweden) are reviewed in the present paper in order to reveal the main factors controlling the fenitization around ijolite and carbonatite. Despite the overall alkaline nature of the process, variables such as XCO2 of the fluid, activity gradients of SiO2, Al2O3 and CaO, FeO/MgO ratio, fO2 and temperature gradients may differ, producing distinctive patterns of fenitization around the two magmatic sources. The ijolitic‐type fluid has low XCO2, high activities of alkalies, SiO2 and Al2O3, and low activity of CaO. The fO2 evolves along the hm‐mt buffer conditions and the temperature falls gradually with distance from the magmatic source. The carbonatitic‐type fluid has high XCO2, high activities of alkalies and CaO, and low activities of SiO2 and Al2O3. Temperatures and fO2 are initially high, but decrease sharply with distance from the source. Moreover, the CO2‐rich fluid may complex and transport the REE.