Magnetic properties of some carbonatites from Tanzania, East Africa

The magnetization of fresh natrocarbonatite lavas from Oldoinyo Lengai in Tanzania is dominated by small amounts of singleor pseudo-single-domain grains of a spinel in the solid solution series jacobsite (MnFe204)-magnetite (Fe304). Although this phase may acquire TRM before carbonatite lava crust has ceased being mobile, the Oldoinyo Lengai samples are good palaeomagnetic recorders of the field they cooled in. In comparison, samples from older carbonatites in Tanzania have very different magnetic mineralogies and unstable behaviour of remanent magnetization. There are two possible explanations for the contrast in magnetic properties. Recrystallization of fresh carbonatites during weathering may destroy the original remanence and lead to the production of various authigenic magnetic minerals. Alternatively, the different magnetic mineralogies may derive from distinct types of carbonatite magmas. Some older calcitic carbonatites may have associated magnetic anomalies that could be useful in prospecting for economically valuable minerals often associated with carbonatites.